Artículos de revistas: "Planktonic diazotrophs"

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Berthelot, Hugo, Sophie Bonnet, Olivier Grosso, Véronique Cornet y Aude Barani. "Transfer of diazotroph-derived nitrogen towards non-diazotrophic planktonic communities: a comparative study between Trichodesmium erythraeum, Crocosphaera watsonii and Cyanothece sp." Biogeosciences 13, n.º 13 (13 de julio de 2016): 4005–21. http://dx.doi.org/10.5194/bg-13-4005-2016.

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Abstract. Biological dinitrogen (N2) fixation is the major source of new nitrogen (N) for the open ocean, and thus promotes marine productivity, in particular in the vast N-depleted regions of the surface ocean. Yet, the fate of the diazotroph-derived N (DDN) in marine ecosystems is poorly understood, and its transfer to auto- and heterotrophic surrounding plankton communities is rarely measured due to technical limitations. Moreover, the different diazotrophs involved in N2 fixation (Trichodesmium spp. vs. UCYN) exhibit distinct patterns of N2 fixation and inhabit different ecological niches, thus having potentially different fates in the marine food webs that remain to be explored. Here we used nanometer scale secondary ion mass spectrometry (nanoSIMS) coupled with 15N2 isotopic labelling and flow cytometry cell sorting to examine the DDN transfer to specific groups of natural phytoplankton and bacteria during artificially induced diazotroph blooms in New Caledonia (southwestern Pacific). The fate of the DDN was compared according to the three diazotrophs: the filamentous and colony-forming Trichodesmium erythraeum (IMS101), and the unicellular strains Crocosphaera watsonii WH8501 and Cyanothece ATCC51142. After 48 h, 7–17 % of the N2 fixed during the experiment was transferred to the dissolved pool and 6–12 % was transferred to non-diazotrophic plankton. The transfer was twice as high in the T. erythraeum bloom than in the C. watsonii and Cyanothece blooms, which shows that filamentous diazotrophs blooms are more efficient at promoting non-diazotrophic production in N-depleted areas. The amount of DDN released in the dissolved pool did not appear to be a good indicator of the DDN transfer efficiency towards the non-diazotrophic plankton. In contrast, the 15N-enrichment of the extracellular ammonium (NH4+) pool was a good indicator of the DDN transfer efficiency: it was significantly higher in the T. erythraeum than in unicellular diazotroph blooms, leading to a DDN transfer twice as efficient. This suggests that NH4+ was the main pathway of the DDN transfer from diazotrophs to non-diazotrophs. The three simulated diazotroph blooms led to significant increases in non-diazotrophic plankton biomass. This increase in biomass was first associated with heterotrophic bacteria followed by phytoplankton, indicating that heterotrophs took the most advantage of the DDN in this oligotrophic ecosystem.

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Meunier, Valentine, Sophie Bonnet, Mercedes Camps, Mar Benavides, Jeff Dubosc, Riccardo Rodolfo-Metalpa y Fanny Houlbrèque. "Ingestion of Diazotrophs Makes Corals More Resistant to Heat Stress". Biomolecules 12, n.º 4 (2 de abril de 2022): 537. http://dx.doi.org/10.3390/biom12040537.

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Over the past decade, coral bleaching events have continued to recur and intensify. During bleaching, corals expel millions of their symbionts, depriving the host from its main food source. One mechanism used by corals to resist bleaching consists in exploiting food sources other than autotrophy. Among the food sources available in the reefs, dinitrogen (N2)-fixing prokaryotes or planktonic diazotrophs (hereafter called ‘PD’) have the particularity to reduce atmospheric dinitrogen (N2) and release part of this nitrogen (diazotroph-derived nitrogen or DDN) in bioavailable form. Here, we submitted coral colonies of Stylophora pistillata, fed or not with planktonic diazotrophs, to a temperature stress of up to 31 ± 0.5 °C and measured their physiological responses (photosynthetic efficiency, symbiont density, and growth rates). Heat-unfed colonies died 8 days after the heat stress while heat-PD-fed corals remained alive after 10 days of heat stress. The supply of PD allowed corals to maintain minimal chlorophyll concentration and symbiont density, sustaining photosynthetic efficiency and stimulating coral growth of up to 48% compared to unfed ones. By providing an alternative source of bioavailable nitrogen and carbon, this specific planktonic diazotroph feeding may have a profound potential for coral bleaching recovery.

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Gimenez, Audrey, Melika Baklouti, Sophie Bonnet y Thierry Moutin. "Biogeochemical fluxes and fate of diazotroph-derived nitrogen in the food web after a phosphate enrichment: modeling of the VAHINE mesocosms experiment". Biogeosciences 13, n.º 17 (14 de septiembre de 2016): 5103–20. http://dx.doi.org/10.5194/bg-13-5103-2016.

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Abstract. The VAHINE mesocosm experiment in the oligotrophic waters of the Nouméa lagoon (New Caledonia), where high N2 fixation rates and abundant diazotroph organisms were observed, aimed to assess the role of the nitrogen input through N2 fixation in carbon production and export and to study the fate of diazotroph-derived nitrogen (DDN) throughout the planktonic food web. A 1-D vertical biogeochemical mechanistic model was used in addition to the in situ experiment to enrich our understanding of the dynamics of the planktonic ecosystem and the main biogeochemical carbon (C), nitrogen (N) and phosphate (P) fluxes. The mesocosms were intentionally enriched with ∼ 0.8 µmol L−1 of inorganic P to trigger the development of diazotrophs and amplify biogeochemical fluxes. Two simulations were run, one with and the other without the phosphate enrichment. In the P-enriched simulation, N2 fixation, primary production (PP) and C export increased by 201, 208 and 87 %, respectively, consistent with the trends observed in the mesocosms (+124, +141 and +261 % for N2 fixation, PP and C export, respectively). In total, 5–10 days were necessary to obtain an increase in primary and export productions after the dissolved inorganic phosphate (DIP) enrichment, thereby suggesting that classical methods (short-term microcosms experiments) used to quantify nutrient limitations of primary production may not be relevant. The model enabled us to monitor the fate of fixed N2 by providing the proportion of DDN in each compartment (inorganic and organic) of the model over time. At the end of the simulation (25 days), 43 % of the DDN was found in the non-diazotroph organisms, 33 % in diazotrophs, 16 % in the dissolved organic nitrogen pool, 3 % in the particulate detrital organic pool and 5 % in traps, indicating that N2 fixation was of benefit to non-diazotrophic organisms and contributed to C export.

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Moreira-Coello, Víctor, Beatriz Mouriño-Carballido, Emilio Marañón, Ana Fernández-Carrera, María PÉrez-Lorenzo y Antonio Bode. "Quantifying the overestimation of planktonic N2 fixation due to contamination of 15N2 gas stocks". Journal of Plankton Research 41, n.º 4 (julio de 2019): 567–70. http://dx.doi.org/10.1093/plankt/fbz034.

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AbstractThe 15N2-tracer assay [Montoya et al. (1996) A simple, high-precision, high-sensitivity tracer assay for N2 fixation. Appl. Environ. Microbiol., 62, 986–993.] is the most used method for measuring biological N2 fixation in terrestrial and aquatic environments. The reliability of this technique depends on the purity of the commercial 15N2 gas stocks used. However, Dabundo et al. [(2014) PLoS One, 9, e110335.] reported the contamination of some of these stocks with labile 15N-labeled compounds (ammonium, nitrate and/or nitrite). The contamination of commercial 15N2 gas stocks with 15N-labeled nitrate and 142 ammonium and consequences for nitrogen fixation measurements. Considering that the tracer assay relies on the conversion of isotopically labeled 15N2 into organic nitrogen, this contamination may have led to overestimated N2 fixation rates. We conducted laboratory and field experiments in order to (i) test the susceptibility of 15N contaminants to assimilation by non-diazotroph organisms and (ii) determine the potential overestimation of the N2 fixation rates estimated in the field. Our findings indicate that the contaminant 15N-compounds are assimilated by non-diazotrophs organisms, leading to an overestimation of N2 fixation rates in the field up to 16-fold under hydrographic conditions of winter mixing.

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Bonnet, Sophie, Melika Baklouti, Audrey Gimenez, Hugo Berthelot y Ilana Berman-Frank. "Biogeochemical and biological impacts of diazotroph blooms in a low-nutrient, low-chlorophyll ecosystem: synthesis from the VAHINE mesocosm experiment (New Caledonia)". Biogeosciences 13, n.º 15 (10 de agosto de 2016): 4461–79. http://dx.doi.org/10.5194/bg-13-4461-2016.

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Abstract. In marine ecosystems, biological N2 fixation provides the predominant external source of nitrogen (N; 140 ± 50 Tg N yr−1), contributing more than atmospheric and riverine inputs to the N supply. Yet the fate and magnitude of the newly fixed N, or diazotroph-derived N (hereafter named DDN) in marine ecosystems is poorly understood. Moreover, whether the DDN is preferentially and directly exported out of the photic zone, recycled by the microbial loop and/or transferred into larger organisms remains unclear. These questions were investigated in the framework of the VAHINE (VAriability of vertical and tropHIc transfer of diazotroph derived N in the south wEst Pacific) project. Triplicate large volume ( ∼ 50 m3) mesocosms were deployed in the tropical south-west Pacific coastal ocean (New Caledonia). The mesocosms were intentionally fertilized with ∼ 0.8 µM dissolved inorganic phosphorus (DIP) at the start of the experiment to stimulate diazotrophy. A total of 47 stocks, fluxes, enzymatic activities and diversity parameters were measured daily inside and outside the mesocosms by the 40 scientists involved in the project. The experiment lasted for 23 days and was characterized by two distinct and successive diazotroph blooms: a dominance of diatom-diazotroph associations (DDAs) during the first half of the experiment (days 2–14) followed by a bloom of unicellular cyanobacterial lineage C (UCYN-C during the second half of the experiment (days 15–23). These conditions provided a unique opportunity to compare the DDN transfer and export efficiency associated with different diazotrophs. Here we summarize the major experimental and modelling results obtained during the project and described in the VAHINE special issue, in particular those regarding the evolution of the main standing stocks, fluxes and biological characteristics over the 23-day experiment, the contribution of N2 fixation to export fluxes, the DDN released to dissolved pool and its transfer to the planktonic food web (bacteria, phytoplankton, zooplankton). We then apply our Eco3M modelling platform to further infer the fate of DDN in the ecosystem and the role of N2 fixation on productivity, food web structure and carbon export. Recommendations for future work are finally provided in the conclusion section.

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Johnson, Claire, Lindsay L. Dubbs y Michael Piehler. "Reframing the contribution of pelagic Sargassum epiphytic N2 fixation". PLOS ONE 18, n.º 8 (1 de agosto de 2023): e0289485. http://dx.doi.org/10.1371/journal.pone.0289485.

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Though nitrogen fixation by epiphytic diazotrophs on pelagic Sargassum has been recognized for decades, it has been assumed to contribute insignificantly to the overall marine nitrogen budget. This six-year study reframes this concept through long-term measurements of Sargassum community nitrogen fixation rates, and by extrapolating mass-specific rates to a theoretical square meter portion of Sargassum mat allowing for comparison of these rates to those of other marine and coastal diazotrophs. On 24 occasions from 2015 to 2021, rates of nitrogen fixation were measured using whole fronds of Sargassum collected from the western edge of the Gulf Stream off Cape Hatteras, North Carolina. Across all dates, mass-specific rates ranged from 0 to 37.77 μmol N g-1 h-1 with a mean of 4.156 μmol N g-1 h-1. Extrapolating using a mat-specific density of Sargassum, these rates scale to a range of 0 to 30,916 μmol N m-2 d-1 and a mean of 3,697 μmol N m-2 d-1. Quantifying this community’s rates of nitrogen fixation over several years captured the sometimes-extreme variability in rates, characteristic of marine diazotrophs, which has not been reported in the literature to date. When these measurements are considered alongside estimates of the density of pelagic Sargassum, rates of nitrogen fixation by Sargassum’s epiphytic diazotrophs rival that of their coastal macrophyte and planktonic counterparts. Given Sargassum’s wide and expanding geographic range, the results of this study suggest this community may contribute reactive nitrogen on a meaningful, basin-wide scale, which merits further study.

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Church, Matthew J., Cindy M. Short, Bethany D. Jenkins, David M. Karl y Jonathan P. Zehr. "Temporal Patterns of Nitrogenase Gene (nifH) Expression in the Oligotrophic North Pacific Ocean". Applied and Environmental Microbiology 71, n.º 9 (septiembre de 2005): 5362–70. http://dx.doi.org/10.1128/aem.71.9.5362-5370.2005.

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ABSTRACT Dinitrogen (N2)-fixing microorganisms (diazotrophs) play important roles in ocean biogeochemistry and plankton productivity. In this study, we examined the presence and expression of specific planktonic nitrogenase genes (nifH) in the upper ocean (0 to 175 m) at Station ALOHA in the oligotrophic North Pacific Ocean. Clone libraries constructed from reverse-transcribed PCR-amplified mRNA revealed six unique phylotypes. Five of the nifH phylotypes grouped with sequences from unicellular and filamentous cyanobacteria, and one of the phylotypes clustered with γ-proteobacteria. The cyanobacterial nifH phylotypes retrieved included two sequence types that phylogenetically grouped with unicellular cyanobacteria (termed groups A and B), several sequences closely related (97 to 99%) to Trichodesmium spp. and Katagnymene spiralis, and two previously unreported phylotypes clustering with heterocyst-forming nifH cyanobacteria. Temporal patterns of nifH expression were evaluated using reverse-transcribed quantitative PCR amplification of nifH gene transcripts. The filamentous and presumed unicellular group A cyanobacterial phylotypes exhibited elevated nifH transcription during the day, while members of the group B (closely related to Crocosphaera watsonii) unicellular phylotype displayed greater nifH transcription at night. In situ nifH expression by all of the cyanobacterial phylotypes exhibited pronounced diel periodicity. The γ-proteobacterial phylotype had low transcript abundance and did not exhibit a clear diurnal periodicity in nifH expression. The temporal separation of nifH expression by the various phylotypes suggests that open ocean diazotrophic cyanobacteria have unique in situ physiological responses to daily fluctuations of light in the upper ocean.

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Fernández, Ana, Rocío Graña, Beatriz Mouriño-Carballido, Antonio Bode, Manuel Varela, J. Francisco Domínguez-Yanes, José Escánez, Demetrio de Armas y Emilio Marañón. "Community N2 fixation and Trichodesmium spp. abundance along longitudinal gradients in the eastern subtropical North Atlantic". ICES Journal of Marine Science 70, n.º 1 (21 de agosto de 2012): 223–31. http://dx.doi.org/10.1093/icesjms/fss142.

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AbstractFernández, A., Graña, R., Mouriño-Carballido, B., Bode, A., Varela, M., Domínguez-Yanes, J. F., Escánez, J., de Armas, D., and Marañón, E. 2013. Community N2 fixation and Trichodesmium spp. abundance along longitudinal gradients in the eastern subtropical North Atlantic. – ICES Journal of Marine Science, 70:223–231. We have determined planktonic community N2 fixation, Trichodesmium abundance, the concentration and vertical diffusive flux of phosphate, and satellite-derived estimates of atmospheric concentration of dust along two longitudinal transects in the eastern subtropical North Atlantic during November 2007 and from April–May 2008. Trichodesmium abundance was particularly low (<3 trichome l−1) during the spring 2008 cruise, when low sea surface temperatures were recorded and vertical stratification was less marked. However, community N2 fixation was always measurable, albeit low compared with other regions of the tropical Atlantic. The average, vertically-integrated N2 fixation rate was 1.20 ± 0.48 µmol N m−2 d−1 in autumn 2007 and 8.31 ± 3.31 µmol N m−2 d−1 in spring 2008. The comparison of these rates of diazotrophy with the observed Trichodesmium abundances suggests that other, presumably unicellular, diazotrophs must have contributed significantly to community N2 fixation, at least during the spring 2008 cruise. Satellite data of atmospheric dust concentration suggested similar rates of atmospheric deposition during the two surveys. In contrast, vertical diffusive fluxes of phosphate were 5-fold higher in spring than in autumn (14.2 ± 12.1 µmol P m−2 d−1 and 2.8 ± 2.6 µmol P m−2 d−1, respectively), which may have stimulated N2 fixation. These findings agree with the growing view that N2 fixation is a more widespread process than the distribution of Trichodesmium alone may suggest. Our data also suggest a role for phosphorus supply in controlling the local variability of diazotrophic activity in a region subject to relatively high atmospheric inputs of iron.

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Paul, A. J., E. P. Achterberg, L. T. Bach, T. Boxhammer, J. Czerny, M. Haunost, K. G. Schulz, A. Stuhr y U. Riebesell. "No observed effect of ocean acidification on nitrogen biogeochemistry in a summer Baltic Sea plankton community". Biogeosciences Discussions 12, n.º 20 (30 de octubre de 2015): 17507–41. http://dx.doi.org/10.5194/bgd-12-17507-2015.

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Abstract. Nitrogen fixation by filamentous cyanobacteria supplies significant amounts of new nitrogen (N) to the Baltic Sea. This balances N loss processes such as denitrification and anammox and forms an important N source supporting primary and secondary production in N-limited post-spring bloom plankton communities. Laboratory studies suggest that filamentous diazotrophic cyanobacteria growth and N2-fixation rates are sensitive to ocean acidification with potential implications for new N supply to the Baltic Sea. In this study, our aim was to assess the effect of ocean acidification on diazotroph growth and activity as well as the contribution of diazotrophically-fixed N to N supply in a natural plankton assemblage. We enclosed a natural plankton community in a summer season in the Baltic Sea near the entrance to the Gulf of Finland in six large-scale mesocosms (volume ~ 55 m3) and manipulated fCO2 over a range relevant for projected ocean acidification by the end of this century (average treatment fCO2: 365–1231 μatm). The direct response of diazotroph growth and activity was followed in the mesocosms over a 47 day study period during N-limited growth in the summer plankton community. Diazotrophic filamentous cyanobacteria abundance throughout the study period and N2-fixation rates (determined only until day 21 due to subsequent use of contaminated commercial 15N-N2 gas stocks) remained low. Thus estimated new N inputs from diazotrophy were too low to relieve N limitation and stimulate a summer phytoplankton bloom. Instead regeneration of organic N sources likely sustained growth in the plankton community. We could not detect significant CO2-related differences in inorganic or organic N pools sizes, or particulate matter N : P stoichiometry. Additionally, no significant effect of elevated CO2 on diazotroph activity was observed. Therefore, ocean acidification had no observable impact on N cycling or biogeochemistry in this N-limited, post-spring bloom plankton assemblage in the Baltic Sea.

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Stukel, M. R., V. J. Coles, M. T. Brooks y R. R. Hood. "Top-down, bottom-up and physical controls on diatom-diazotroph assemblage growth in the Amazon River plume". Biogeosciences 11, n.º 12 (19 de junio de 2014): 3259–78. http://dx.doi.org/10.5194/bg-11-3259-2014.

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Abstract. The nutrient-rich waters of the Amazon River plume (ARP) support dense blooms of diatom-diazotroph assemblages (DDAs) that introduce large quantities of new nitrogen to the planktonic ecosystem and, unlike other nitrogen-fixers, are likely to directly fuel vertical carbon flux. To investigate the factors controlling DDA blooms, we develop a five phytoplankton (cyanobacteria, diatoms, unicellular microbial diazotrophs, DDAs, and Trichodesmium), two zooplankton model and embed it within a 1/6° resolution physical model of the tropical and subtropical Atlantic. The model generates realistic DDA blooms in the ARP and also exhibits basin-wide primary production, nitrogen fixation, and grazing rates consistent with observed values. By following ARP water parcels with synthetic Lagrangian drifters released at the river mouth we are able to assess the relative impacts of grazing, nutrient supply, and physical forcing on DDA bloom formation. DDA bloom formation is stimulated in the nitrogen-poor and silica-rich water of the ARP by decreases in grazing pressure when mesozooplankton (which co-occur in high densities with coastal diatom blooms) concentrations decrease. Bloom termination is driven primarily by silica limitation of the DDAs. In agreement with in situ data, this net growth niche for DDAs exists in a salinity range from ∼20–34 PSU, although this co-occurrence is coincidental rather than causative. Because net growth rates are relatively modest, bloom formation in ARP water parcels depends critically on the time spent in this ideal habitat, with high DDA biomass only occurring when water parcels spent >23 days in the optimal habitat niche.

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Stukel, M. R., V. J. Coles, M. T. Brooks y R. R. Hood. "Top-down, bottom-up and physical controls on diatom-diazotroph assemblage growth in the Amazon River Plume". Biogeosciences Discussions 10, n.º 8 (23 de agosto de 2013): 13931–76. http://dx.doi.org/10.5194/bgd-10-13931-2013.

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Abstract. The nutrient-rich waters of the Amazon River Plume (ARP) support dense blooms of diatom-diazotroph assemblages (DDA) that introduce large quantities of new nitrogen to the planktonic ecosystem and, unlike other nitrogen-fixers, are likely to directly fuel vertical carbon flux. To investigate the factors controlling DDA blooms, we develop a five phytoplankton (cyanobacteria, diatoms, unicellular microbial diazotrophs, DDA, and Trichodesmium), two zooplankton model and embed it within a 1/6° resolution physical model of the tropical and subtropical Atlantic. The model generates realistic DDA blooms in the ARP and also exhibits basin-wide primary production, nitrogen fixation, and grazing rates consistent with observed values. By following ARP water parcels with synthetic Lagrangian drifters released at the river mouth we are able to assess the relative impacts of grazing, nutrient supply, and physical forcing on DDA bloom formation. DDA bloom formation is stimulated in the silica-rich water of the ARP by decreases in grazing pressure when mesozooplankton (which co-occur in high densities with coastal diatom blooms) concentrations decrease. Bloom termination is driven primarily by silica limitation of the DDA. In agreement with in situ data, this net growth niche for DDA exists in a salinity range from ~ 20–34 PSU, although this co-occurrence is coincidental rather than causative. Because net growth rates are relatively modest, bloom formation in ARP water parcels depends critically on the time spent in this ideal habitat, with high DDA biomass only occurring when water parcels spent > 23 days in the optimal habitat niche.

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Paul, Allanah J., Eric P. Achterberg, Lennart T. Bach, Tim Boxhammer, Jan Czerny, Mathias Haunost, Kai-Georg Schulz, Annegret Stuhr y Ulf Riebesell. "No observed effect of ocean acidification on nitrogen biogeochemistry in a summer Baltic Sea plankton community". Biogeosciences 13, n.º 13 (7 de julio de 2016): 3901–13. http://dx.doi.org/10.5194/bg-13-3901-2016.

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Abstract. Nitrogen fixation by filamentous cyanobacteria supplies significant amounts of new nitrogen (N) to the Baltic Sea. This balances N loss processes such as denitrification and anammox, and forms an important N source supporting primary and secondary production in N-limited post-spring bloom plankton communities. Laboratory studies suggest that filamentous diazotrophic cyanobacteria growth and N2-fixation rates are sensitive to ocean acidification, with potential implications for new N supply to the Baltic Sea. In this study, our aim was to assess the effect of ocean acidification on diazotroph growth and activity as well as the contribution of diazotrophically fixed N to N supply in a natural plankton assemblage. We enclosed a natural plankton community in a summer season in the Baltic Sea near the entrance to the Gulf of Finland in six large-scale mesocosms (volume ∼ 55 m3) and manipulated fCO2 over a range relevant for projected ocean acidification by the end of this century (average treatment fCO2: 365–1231 µatm). The direct response of diazotroph growth and activity was followed in the mesocosms over a 47 day study period during N-limited growth in the summer plankton community. Diazotrophic filamentous cyanobacteria abundance throughout the study period and N2-fixation rates (determined only until day 21 due to subsequent use of contaminated commercial 15N-N2 gas stocks) remained low. Thus estimated new N inputs from diazotrophy were too low to relieve N limitation and stimulate a summer phytoplankton bloom. Instead, regeneration of organic N sources likely sustained growth in the plankton community. We could not detect significant CO2-related differences in neither inorganic nor organic N pool sizes, or particulate matter N : P stoichiometry. Additionally, no significant effect of elevated CO2 on diazotroph activity was observed. Therefore, ocean acidification had no observable impact on N cycling or biogeochemistry in this N-limited, post-spring bloom plankton assemblage in the Baltic Sea.

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Bonnet, S., O. Grosso y T. Moutin. "Planktonic dinitrogen fixation along a longitudinal gradient across the Mediterranean Sea during the stratified period (BOUM cruise)". Biogeosciences 8, n.º 8 (19 de agosto de 2011): 2257–67. http://dx.doi.org/10.5194/bg-8-2257-2011.

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Abstract. This study provides extensive data on planktonic N2 fixation rates across the whole Mediterranean Sea. They show that N2 fixation occurs in Mediterranean waters during the stratification period, with a clear decreasing trend from the oligotrophic western basin (10–76 μmol m−2 d−1) to the ultra oligotrophic eastern basin (0–0.4 μmol m−2 d−1). Highest rates are measured in the less oligotrophic western basin, between the surface and 75 m-depth, where 45 to 75 % of N2 fixation are found within the picoplanktonic fraction (<3 μm). While the biogeochemical impact of N2 fixation in the eastern basin seems negligible, N2 fixation is able to sustain up to 35 % of new primary production during the stratified period in the western basin. These data disagree with indirect estimates of N2 fixation based on geochemical tracers and nutrient budgets, which indicates that N2 fixation increases with increasing N:P ratios and decreasing stable N isotopic signature of particulate organic nitrogen and NO3− from west to east. These results finally point out the need to assess N2 fixation at a higher temporal resolution in order to better understand the diazotrophs' dynamic under contrasted biogeochemical conditions.

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Carlotti, François, Marc Pagano, Loïc Guilloux, Katty Donoso, Valentina Valdés, Olivier Grosso y Brian P. V. Hunt. "Meso-zooplankton structure and functioning in the western tropical South Pacific along the 20th parallel south during the OUTPACE survey (February–April 2015)". Biogeosciences 15, n.º 23 (10 de diciembre de 2018): 7273–97. http://dx.doi.org/10.5194/bg-15-7273-2018.

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Abstract. The western tropical South Pacific (WTSP) is one of the most understudied oceanic regions in terms of the planktonic food web, despite supporting some of the largest tuna fisheries in the world. In this stratified oligotrophic ocean, nitrogen fixation may play an important role in supporting the plankton food web and higher trophic level production. In the austral summer (February–April) of 2015, the OUTPACE (Oligotrophy to UlTra-oligotrophy PACific Experiment) project conducted a comprehensive survey of 4000 km along 20∘ S, from New Caledonia to Tahiti, to determine the role of N2 fixation on biogeochemical cycles and food web structure in this region. Here, we characterize the zooplankton community and plankton food web processes at 15 short-duration stations (8 h each) to describe the large-scale variability across trophic gradients from oligotrophic waters around Melanesian archipelagoes (MAs) to ultra-oligotrophic waters of the South Pacific gyre (GY). Three long-duration stations (5 days each) enabled a more detailed analysis of processes and were positioned (1) in offshore northern waters of New Caledonia (MA), (2) near Niue Island (MA), and (3) in the subtropical Pacific gyre (GY) near the Cook Islands. At all stations, meso-zooplankton was sampled with a bongo net with 120 µm mesh size to estimate abundance, biomass, community taxonomy and size structure, and size fractionated δ15N. Subsequently, we estimated zooplankton carbon demand, grazing impact, excretion rates, and the contribution of diazotroph-derived nitrogen (DDN) to zooplankton biomass. The meso-zooplankton community showed a general decreasing trend in abundance and biomass from west to east, with a clear drop in the GY waters. Higher abundance and biomass corresponded to higher primary production associated with complex mesoscale circulation in the Coral Sea and between 170–180∘ W. The taxonomic structure showed a high degree of similarity in terms of species richness and abundance distribution across the whole region, with, however, a moderate difference in the GY region, where the copepod contribution to meso-zooplankton increased. The calculated ingestion and metabolic rates allowed us to estimate that the top–down (grazing) and bottom–up (excretion of nitrogen and phosphorous) impact of zooplankton on phytoplankton was potentially high. Daily grazing pressure on phytoplankton stocks was estimated to remove 19 % to 184 % of the total daily primary production and 1.5 % to 22 % of fixed N2. The top–down impact of meso-zooplankton was higher in the eastern part of the transect, including GY, than in the Coral Sea region and was mainly exerted on nano- and micro-phytoplankton. The regeneration of nutrients by zooplankton excretion was high, suggesting a strong contribution to regenerated production, particularly in terms of N. Daily NH4+ excretion accounted for 14.5 % to 165 % of phytoplankton needs for N, whereas PO43- excretion accounted for only 2.8 % to 34 % of P needs. From zooplankton δ15N values, we estimated that the DDN contributed to up to 67 % and 75 % to the zooplankton biomass in the western and central parts of the MA regions, respectively, but strongly decreased to an average of 22 % in the GY region and down to 7 % in the easternmost station. Thus, the highest contribution of diazotrophic microorganisms to zooplankton biomass occurred in the region of highest N2 fixation rates and when Trichodesmium dominated the diazotrophs community (MA waters). Our estimations of the fluxes associated with zooplankton were highly variable between stations and zones but very high in most cases compared to literature data, partially due to the high contribution of small forms. The highest values encountered were found at the boundary between the oligotrophic (MA) and ultra-oligotrophic regions (GY). Within the MA zone, the high variability of the top–down and bottom–up impact was related to the high mesoscale activity in the physical environment. Estimated zooplankton respiration rates relative to primary production were among the highest cited values at similar latitudes, inducing a high contribution of migrant zooplankton respiration to carbon flux. Despite the relatively low biomass values of planktonic components in quasi-steady state, the availability of micro- and macronutrients related to physical mesoscale patterns in the waters surrounding the MA, the fueling by DDN, and the relatively high rates of plankton production and metabolism estimated during OUTPACE may explain the productive food chain ending with valuable fisheries in this region.

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15

Bonnet, S., H. Berthelot, K. Turk-Kubo, S. Fawcett, E. Rahav, S. l'Helguen y I. Berman-Frank. "Dynamics of N2 fixation and fate of diazotroph-derived nitrogen in a low nutrient low chlorophyll ecosystem: results from the VAHINE mesocosm experiment (New Caledonia)". Biogeosciences Discussions 12, n.º 23 (10 de diciembre de 2015): 19579–626. http://dx.doi.org/10.5194/bgd-12-19579-2015.

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Abstract. N2 fixation rates were measured daily in large (~ 50 m3) mesocosms deployed in the tropical South West Pacific coastal ocean (New Caledonia) to investigate the spatial and temporal dynamics of diazotrophy and the fate of diazotroph-derived nitrogen (DDN) in a low nutrient, low chlorophyll ecosystem. The mesocosms were intentionally fertilized with ~ 0.8 μM dissolved inorganic phosphorus (DIP) to stimulate diazotrophy. Bulk N2 fixation rates were replicable between the three mesocosms, averaged 18.5 ± 1.1 nmol N L−1 d−1 over the 23 days, and increased by a factor of two during the second half of the experiment (days 15 to 23) to reach 27.3 ± 1.0 nmol N L−1 d−1. These rates are higher than the upper range reported for the global ocean, indicating that the waters surrounding New Caledonia are particularly favourable for N2 fixation. During the 23 days of the experiment, N2 fixation rates were positively correlated with seawater temperature, primary production, bacterial production, standing stocks of particulate organic carbon, nitrogen and phosphorus, and alkaline phosphatase activity, and negatively correlated with DIP concentrations, DIP turnover time, nitrate, and dissolved organic nitrogen and phosphorus concentrations. The fate of DDN was investigated during the bloom of the unicellular diazotroph, UCYN-C, that occurred during the second half of the experiment. Quantification of diazotrophs in the sediment traps indicates that ~ 10 % of UCYN-C from the water column were exported daily to the traps, representing as much as 22.4 ± 5.5 % of the total POC exported at the height of the UCYN-C bloom. This export was mainly due to the aggregation of small (5.7 ± 0.8 μm) UCYN-C cells into large (100–500 μm) aggregates. During the same time period, a DDN transfer experiment based on high-resolution nanometer scale secondary ion mass spectrometry (nanoSIMS) coupled with 15N2 isotopic labelling revealed that 16 ± 6 % of the DDN was released to the dissolved pool and 21 ± 4 % was transferred to non-diazotrophic plankton, mainly picoplankton (18 ± 4 %) followed by diatoms (3 ± 2 %) within 24 h of incubation. This is consistent with the observed dramatic increase in picoplankton and diatom abundances, primary production, bacterial production and standing stocks of particulate organic carbon, nitrogen and phosphorus during the second half of the experiment in the mesocosms. These results offer insights into the fate of DDN during a bloom of UCYN-C in low nutrient, low chlorophyll ecosystems.

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16

Caffin, Mathieu, Hugo Berthelot, Véronique Cornet-Barthaux, Aude Barani y Sophie Bonnet. "Transfer of diazotroph-derived nitrogen to the planktonic food web across gradients of N2 fixation activity and diversity in the western tropical South Pacific Ocean". Biogeosciences 15, n.º 12 (21 de junio de 2018): 3795–810. http://dx.doi.org/10.5194/bg-15-3795-2018.

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Abstract. Biological dinitrogen (N2) fixation provides the major source of new nitrogen (N) to the open ocean, contributing more than atmospheric deposition and riverine inputs to the N supply. Yet the fate of the diazotroph-derived N (DDN) in the planktonic food web is poorly understood. The main goals of this study were (i) to quantify how much of DDN is released to the dissolved pool during N2 fixation and how much is transferred to bacteria, phytoplankton and zooplankton, and (ii) to compare the DDN release and transfer efficiencies under contrasting N2 fixation activity and diversity in the oligotrophic waters of the western tropical South Pacific (WTSP) Ocean. We used nanometre-scale secondary ion mass spectrometry (nanoSIMS) coupled with 15N2 isotopic labelling and flow cytometry cell sorting to track the DDN transfer to plankton, in regions where the diazotroph community was dominated by either Trichodesmium or by UCYN-B. After 48 h, ∼ 20–40 % of the N2 fixed during the experiment was released to the dissolved pool when Trichodesmium dominated, while the DDN release was not quantifiable when UCYN-B dominated; ∼ 7–15 % of the total fixed N (net N2 fixation + release) was transferred to non-diazotrophic plankton within 48 h, with higher transfer efficiencies (15 ± 3 %) when UCYN-B dominated as compared to when Trichodesmium dominated (9 ± 3 %). The pico-cyanobacteria Synechococcus and Prochlorococcus were the primary beneficiaries of the DDN transferred (∼ 65–70 %), followed by heterotrophic bacteria (∼ 23–34 %). The DDN transfer in bacteria was higher (34 ± 7 %) in the UCYN-B-dominating experiment compared to the Trichodesmium-dominating experiments (24 ± 5 %). Regarding higher trophic levels, the DDN transfer to the dominant zooplankton species was less efficient when the diazotroph community was dominated by Trichodesmium (∼ 5–9 % of the DDN transfer) than when it was dominated by UCYN-B (∼ 28 ± 13 % of the DDN transfer). To our knowledge, this study provides the first quantification of DDN release and transfer to phytoplankton, bacteria and zooplankton communities in open ocean waters. It reveals that despite UCYN-B fix N2 at lower rates compared to Trichodesmium in the WTSP, the DDN from UCYN-B is much more available and efficiently transferred to the planktonic food web than the DDN originating from Trichodesmium.

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17

Bonnet, Sophie, Hugo Berthelot, Kendra Turk-Kubo, Sarah Fawcett, Eyal Rahav, Stéphane L'Helguen y Ilana Berman-Frank. "Dynamics of N2 fixation and fate of diazotroph-derived nitrogen in a low-nutrient, low-chlorophyll ecosystem: results from the VAHINE mesocosm experiment (New Caledonia)". Biogeosciences 13, n.º 9 (4 de mayo de 2016): 2653–73. http://dx.doi.org/10.5194/bg-13-2653-2016.

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Abstract. N2 fixation rates were measured daily in large (∼ 50 m3) mesocosms deployed in the tropical southwest Pacific coastal ocean (New Caledonia) to investigate the temporal variability in N2 fixation rates in relation with environmental parameters and study the fate of diazotroph-derived nitrogen (DDN) in a low-nutrient, low-chlorophyll ecosystem. The mesocosms were fertilized with ∼ 0.8 µM dissolved inorganic phosphorus (DIP) to stimulate diazotrophy. Bulk N2 fixation rates were replicable between the three mesocosms, averaged 18.5 ± 1.1 nmol N L−1 d−1 over the 23 days, and increased by a factor of 2 during the second half of the experiment (days 15 to 23) to reach 27.3 ± 1.0 nmol N L−1 d−1. These later rates measured after the DIP fertilization are higher than the upper range reported for the global ocean. During the 23 days of the experiment, N2 fixation rates were positively correlated with seawater temperature, primary production, bacterial production, standing stocks of particulate organic carbon (POC), nitrogen (PON) and phosphorus (POP), and alkaline phosphatase activity, and negatively correlated with DIP concentrations, DIP turnover time, nitrate, and dissolved organic nitrogen and phosphorus concentrations. The fate of DDN was investigated during a bloom of the unicellular diazotroph UCYN-C that occurred during the second half of the experiment. Quantification of diazotrophs in the sediment traps indicates that ∼ 10 % of UCYN-C from the water column was exported daily to the traps, representing as much as 22.4 ± 5.5 % of the total POC exported at the height of the UCYN-C bloom. This export was mainly due to the aggregation of small (5.7 ± 0.8 µm) UCYN-C cells into large (100–500 µm) aggregates. During the same time period, a DDN transfer experiment based on high-resolution nanometer-scale secondary ion mass spectrometry (nanoSIMS) coupled with 15N2 isotopic labeling revealed that 16 ± 6 % of the DDN was released to the dissolved pool and 21 ± 4 % was transferred to non-diazotrophic plankton, mainly picoplankton (18 ± 4 %) followed by diatoms (3 ± 2 %). This is consistent with the observed dramatic increase in picoplankton and diatom abundances, primary production, bacterial production, and standing stocks of POC, PON, and POP in the mesocosms during the second half of the experiment. These results offer insights into the fate of DDN during a bloom of UCYN-C in low-nutrient, low-chlorophyll ecosystems.

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18

Doblin, Martina A., Katherina Petrou, Sutinee Sinutok, Justin R. Seymour, Lauren F. Messer, Mark V. Brown, Louiza Norman et al. "Nutrient uplift in a cyclonic eddy increases diversity, primary productivity and iron demand of microbial communities relative to a western boundary current". PeerJ 4 (25 de abril de 2016): e1973. http://dx.doi.org/10.7717/peerj.1973.

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The intensification of western boundary currents in the global ocean will potentially influence meso-scale eddy generation, and redistribute microbes and their associated ecological and biogeochemical functions. To understand eddy-induced changes in microbial community composition as well as how they control growth, we targeted the East Australian Current (EAC) region to sample microbes in a cyclonic (cold-core) eddy (CCE) and the adjacent EAC. Phototrophic and diazotrophic microbes were more diverse (2–10 times greater Shannon index) in the CCE relative to the EAC, and the cell size distribution in the CCE was dominated (67%) by larger micro-plankton $(\geq 20\lrm{\mu }\mathrm{m})$, as opposed to pico- and nano-sized cells in the EAC. Nutrient addition experiments determined that nitrogen was the principal nutrient limiting growth in the EAC, while iron was a secondary limiting nutrient in the CCE. Among the diazotrophic community, heterotrophicNifHgene sequences dominated in the EAC and were attributable to members of the gamma-, beta-, and delta-proteobacteria, while the CCE contained both phototrophic and heterotrophic diazotrophs, includingTrichodesmium, UCYN-A and gamma-proteobacteria. Daily sampling of incubation bottles following nutrient amendment captured a cascade of effects at the cellular, population and community level, indicating taxon-specific differences in the speed of response of microbes to nutrient supply. Nitrogen addition to the CCE community increased picoeukaryote chlorophyllaquotas within 24 h, suggesting that nutrient uplift by eddies causes a ‘greening’ effect as well as an increase in phytoplankton biomass. After three days in both the EAC and CCE, diatoms increased in abundance with macronutrient (N, P, Si) and iron amendment, whereas haptophytes and phototrophic dinoflagellates declined. Our results indicate that cyclonic eddies increase delivery of nitrogen to the upper ocean to potentially mitigate the negative consequences of increased stratification due to ocean warming, but also increase the biological demand for iron that is necessary to sustain the growth of large-celled phototrophs and potentially support the diversity of diazotrophs over longer time-scales.

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19

Drira, Zaher, Dorra Chaari, Asma Hamza, Malika Bel Hassen, Marc Pagano y Habib Ayadi. "Diazotrophic cyanobacteria signatures and their relationship to hydrographic conditions in the Gulf of Gabes, Tunisia". Journal of the Marine Biological Association of the United Kingdom 97, n.º 1 (13 de enero de 2016): 69–80. http://dx.doi.org/10.1017/s0025315415002210.

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Changes in the planktonic cyanobacteria structure, composition and diversity were followed over three consecutive years (2005–2006–2007) in the Gulf of Gabes (Eastern Mediterranean Sea, Tunisia). Cyanobacteria abundances, biomasses and cell lengths were measured together with selected environmental variables (pH, salinity, temperature and nutrients). The space and time variations of the cyanobacteria in relation to the environmental factors showed a close relationship between these plankton communities and the hydrographic structure of the water column. Cyanobacteria developed over semi-mixed conditions (May–June 2006) and during the thermal stratification (July 2005). The cyanobacterial abundance and biomass was evident between 20 and 35 m in inshore stations and between 20 and 25 m in deeper stations during the semi-mixing conditions and stratification. This thermocline level coincided with the euphotic layer (21.85 ± 3.76 m) allowing access of light radiation. The cyanobacteria bloom occurred during May–June 2006 when the N/P ratio (<10) was clearly below the accepted standard molar ratio of N/P = 16/1. Commonalities among cyanobacterial genera include being highly competitive for low concentrations of inorganic P (DIP) and the ability to acquire organic P compounds. Our study showed that both diazotrophic (N2-fixing) cyanobacteria such as Anabaena sp., Chroococcus sp., Trichodesmium erythraeum, Spirulina sp. and Spirulina subsalsa and non-diazotrophic cyanobacteria such as Pseudoanabaena sp. and Microcystis display a great flexibility in the N sources which allow formation of blooms.

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20

Pahlow, Markus, Chia-Te Chien, Lionel A. Arteaga y Andreas Oschlies. "Optimality-based non-Redfield plankton–ecosystem model (OPEM v1.1) in UVic-ESCM 2.9 – Part 1: Implementation and model behaviour". Geoscientific Model Development 13, n.º 10 (2 de octubre de 2020): 4663–90. http://dx.doi.org/10.5194/gmd-13-4663-2020.

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Abstract. Uncertainties in projections of marine biogeochemistry from Earth system models (ESMs) are associated to a large degree with the imperfect representation of the marine plankton ecosystem, in particular the physiology of primary and secondary producers. Here, we describe the implementation of an optimality-based plankton–ecosystem model (OPEM) version 1.1 with variable carbon : nitrogen : phosphorus (C:N:P) stoichiometry in the University of Victoria ESM (UVic; Eby et al., 2009; Weaver et al., 2001) and the behaviour of two calibrated reference configurations, which differ in the assumed temperature dependence of diazotrophs. Predicted tracer distributions of oxygen and dissolved inorganic nutrients are similar to those of an earlier fixed-stoichiometry formulation in UVic (Nickelsen et al., 2015). Compared to the classic fixed-stoichiometry UVic model, OPEM is closer to recent satellite-based estimates of net community production (NCP), despite overestimating net primary production (NPP), can better reproduce deep-ocean gradients in the NO3-:PO43- ratio and partially explains observed patterns of particulate C:N:P in the surface ocean. Allowing diazotrophs to grow (but not necessarily fix N2) at similar temperatures as other phytoplankton results in a better representation of surface Chl and NPP in the Arctic and Antarctic oceans. Deficiencies of our calibrated OPEM configurations may serve as a magnifying glass for shortcomings in global biogeochemical models and hence guide future model development. The overestimation of NPP at low latitudes indicates the need for improved representations of temperature effects on biotic processes, as well as phytoplankton community composition, which may be represented by locally varying parameters based on suitable trade-offs. The similarity in the overestimation of NPP and surface autotrophic particulate organic carbon (POC) could indicate deficiencies in the representation of top-down control or nutrient supply to the surface ocean. Discrepancies between observed and predicted vertical gradients in particulate C:N:P ratios suggest the need to include preferential P remineralisation, which could also benefit the representation of N2 fixation. While OPEM yields a much improved distribution of surface N* (NO3--16⋅PO43-+2.9 mmol m−3), it still fails to reproduce observed N* in the Arctic, possibly related to a misrepresentation of the phytoplankton community there and the lack of benthic denitrification in the model. Coexisting ordinary and diazotrophic phytoplankton can exert strong control on N* in our simulations, which questions the interpretation of N* as reflecting the balance of N2 fixation and denitrification.

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21

Bopp, Laurent, Olivier Aumont, Lester Kwiatkowski, Corentin Clerc, Léonard Dupont, Christian Ethé, Thomas Gorgues, Roland Séférian y Alessandro Tagliabue. "Diazotrophy as a key driver of the response of marine net primary productivity to climate change". Biogeosciences 19, n.º 17 (9 de septiembre de 2022): 4267–85. http://dx.doi.org/10.5194/bg-19-4267-2022.

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Abstract. The impact of anthropogenic climate change on marine net primary production (NPP) is a reason for concern because changing NPP will have widespread consequences for marine ecosystems and their associated services. Projections by the current generation of Earth system models have suggested decreases in global NPP in response to future climate change, albeit with very large uncertainties. Here, we make use of two versions of the Institut Pierre-Simon Laplace Climate Model (IPSL-CM) that simulate divergent NPP responses to similar high-emission scenarios in the 21st century and identify nitrogen fixation as the main driver of these divergent NPP responses. Differences in the way N fixation is parameterised in the marine biogeochemical component PISCES (Pelagic Interactions Scheme for Carbon and Ecosystem Studies) of the IPSL-CM versions lead to N-fixation rates that are either stable or double over the course of the 21st century, resulting in decreasing or increasing global NPP, respectively. An evaluation of these two model versions does not help constrain future NPP projection uncertainties. However, the use of a more comprehensive version of PISCES, with variable nitrogen-to-phosphorus ratios as well as a revised parameterisation of the temperature sensitivity of N fixation, suggests only moderate changes in globally averaged N fixation in the 21st century. This leads to decreasing global NPP, in line with the model-mean changes of a recent multi-model intercomparison. Lastly, despite contrasting trends in NPP, all our model versions simulate similar and significant reductions in planktonic biomass. This suggests that projected plankton biomass may be a more robust indicator than NPP of the potential impact of anthropogenic climate change on marine ecosystems across models.

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22

Rahav, E., B. Herut, A. Levi, M. R. Mulholland y I. Berman-Frank. "Springtime contribution of dinitrogen fixation to primary production across the Mediterranean Sea". Ocean Science 9, n.º 3 (14 de mayo de 2013): 489–98. http://dx.doi.org/10.5194/os-9-489-2013.

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Abstract. Dinitrogen (N2) fixation rates were measured during early spring across the different provinces of Mediterranean Sea surface waters. N2 fixation rates, measured using 15N2 enriched seawater, were lowest in the eastern basin and increased westward with a maximum at the Strait of Gibraltar (0.10 to 2.35 nmol N L−1 d−1, respectively). These rates were 3–7 fold higher than N2 fixation rates measured previously in the Mediterranean Sea during summertime and we estimated that methodological differences alone did not account for the seasonal changes we observed. Higher contribution of N2 fixation to primary production (4–8%) was measured in the western basin compared to the eastern basin (∼2%). Our data indicates that these differences between basins may be attributed to changes in N2-fixing planktonic communities and that heterotrophic diazotrophy may play a significant role in the eastern Mediterranean while autotrophic diazotrophy has a more dominant role in the western basin.

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23

Bird, Clare, Joaquín Martinez Martinez, Anthony G. O'Donnell y Michael Wyman. "Spatial Distribution and Transcriptional Activity of an Uncultured Clade of Planktonic Diazotrophic γ-Proteobacteria in the Arabian Sea". Applied and Environmental Microbiology 71, n.º 4 (abril de 2005): 2079–85. http://dx.doi.org/10.1128/aem.71.4.2079-2085.2005.

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ABSTRACT The spatial distribution of an uncultured clade of marine diazotrophic γ-proteobacteria in the Arabian Sea was investigated by the development of a specific primer pair to amplify an internal fragment of nifH by PCR. These organisms were most readily detected in highly oligotrophic surface waters but could also be found in deeper waters below the nutricline. nifH transcripts originating from this clade were detected in oligotrophic surface waters and, in addition, in the deeper and the more productive near-coastal waters. The nifH sequences most closely related to the unidentified marine bacterial group are from environmental clones amplified from the Atlantic and Pacific Oceans. These findings suggest that these γ-proteobacteria are widespread and likely to be an important component of the heterotrophic diazotrophic microbial community of the tropical and subtropical oceans.

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24

Falcón, Luisa I., Frank Cipriano, Andrei Y. Chistoserdov y Edward J. Carpenter. "Diversity of Diazotrophic Unicellular Cyanobacteria in the Tropical North Atlantic Ocean". Applied and Environmental Microbiology 68, n.º 11 (noviembre de 2002): 5760–64. http://dx.doi.org/10.1128/aem.68.11.5760-5764.2002.

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ABSTRACT We present data on the genetic diversity and phylogenetic affinities of N2-fixing unicellular cyanobacteria in the plankton of the tropical North Atlantic Ocean. Our dinitrogenase gene (nifH) sequences grouped together with a group of cyanobacteria from the subtropical North Pacific; another subtropical North Pacific group was only distantly related. Most of the 16S ribosomal DNA sequences from our tropical North Atlantic samples were closely allied with sequences from a symbiont of the diatom Climacodium frauenfeldianum. These findings suggest a complex pattern of evolutionary and ecological divergence among unicellular cyanobacteria within and between ocean basins.

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25

Meyer, J., C. R. Löscher, S. C. Neulinger, A. F. Reichel, A. Loginova, C. Borchard, R. A. Schmitz, H. Hauss, R. Kiko y U. Riebesell. "Changing nutrient stoichiometry affects phytoplankton production, DOP build up and dinitrogen fixation – a mesocosm experiment in the eastern tropical North Atlantic". Biogeosciences Discussions 12, n.º 13 (3 de julio de 2015): 9991–10029. http://dx.doi.org/10.5194/bgd-12-9991-2015.

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Abstract. Ocean deoxygenation due to climate change may alter redox-sensitive nutrient cycles in the marine environment. The productive eastern tropical North Atlantic (ETNA) upwelling region may be particularly affected when the relatively moderate oxygen minimum zone (OMZ) deoxygenates further and microbially-driven nitrogen (N) loss processes are promoted. Consequently, water masses with a low N : P ratio could reach the euphotic layer, possibly influencing primary production in those waters. Previous mesocosm studies in the oligotrophic Atlantic Ocean identified N availability as controlling of primary production, while a possible co-limitation of nitrate and phosphate (P) could not be ruled out. To better understand the impact of changing N : P ratios on primary production and on N2 fixation in the ETNA surface ocean, we conducted land-based mesocosm experiments with natural plankton communities and applied a broad range of N : P ratios (2.67–48). Silicate was supplied at 15 μmol L−1 in all mesocosms. We monitored nutrient drawdown, bloom formation, biomass build up and diazotrophic feedback in response to variable nutrient stoichiometry. Our results confirmed N to be limiting to primary production. We found that excess P was channeled through particulate organic matter (POP) into the dissolved organic matter (DOP) pool. In mesocosms with low P availability, DOP was utilized while N2 fixation increased, suggesting a link between those two processes. Interestingly this observation was most pronounced in mesocosms where inorganic N was still available, indicating that bioavailable N does not necessarily has to have a negative impact on N2 fixation. We observed a shift from a mixed cyanobacterial/proteobacterial dominated active diazotrophic community towards diazotrophic diatom symbionts of the Richelia-Rhizosolenia symbiosis. We hypothesize that a potential change in nutrient stoichiometry in the ETNA might lead to a general shift within the diazotrophic community, potentially modifying primary productivity.

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26

Kodama, Taketoshi, Tsuyoshi Watanabe, Yukiko Taniuchi, Akira Kuwata y Daisuke Hasegawa. "Micro-size plankton abundance and assemblages in the western North Pacific Subtropical Gyre under microscopic observation". PLOS ONE 16, n.º 4 (26 de abril de 2021): e0250604. http://dx.doi.org/10.1371/journal.pone.0250604.

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While primary productivity in the oligotrophic North Pacific Subtropical Gyre (NPSG) is changing, the micro-size plankton community has not been evaluated in the last 4 decades, prompting a re-evaluation. We collected samples over three years (2016–2018) from depths of 10 to 200 m (n = 127), and the micro-size plankton were identified and counted to understand the heterogeneity of micro-size plankton community structure. The assemblages were consistent to the those of 4 decades ago. Dinophyceae (dinoflagellates) were the most numerically abundant, followed by Cryptophyceae and Bacillariophyceae (diatoms). The other micro-size plankton classes (Cyanophyceae, Haptophyceae, Dictyochophyceae, Euglenophyceae, and Prasinophyceae) were not always detected, whereas only Trichodesmium spp. was counted in the Cyanophyceae. Other unidentified autotrophic and heterotrophic flagellates were also significantly present, and their numeric abundance was higher than or at the same level as was that of the Dinophyceae. In the Dinophyceae, Gymnodiniaceae and Peridiniales were abundant. The chlorophyll a concentration and these class-level assemblages suggested micro-size plankton is not a major primary producer in this area. We applied generalized additive models (GAMs) and principal coordination analyses (PCoAs) to evaluate the habitats of every plankton group and the heterogeneity of the assemblages. The GAMs suggested that every classified plankton abundance showed a similar response to salinity, and we observed differences in habitats in terms of temperature and nitrate concentrations. Based on the PCoAs, we observed unique communities at the 200 m depth layer compared with those at the other sampling layers. The site scores of PCoAs indicated that the micro-size plankton assemblages are most heterogeneous at the 10 m depth layer. At such depth, diazotrophic Cyanophyceae (Trichodesmium spp.) are abundant, particularly in less-saline water. Therefore, nitrogen fixation may contribute to the heterogeneity in the abundance and assemblages in the western NPSG.

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27

Zehr, Jonathan P., Mark T. Mellon y Sabino Zani. "New Nitrogen-Fixing Microorganisms Detected in Oligotrophic Oceans by Amplification of Nitrogenase (nifH) Genes". Applied and Environmental Microbiology 64, n.º 9 (1 de septiembre de 1998): 3444–50. http://dx.doi.org/10.1128/aem.64.9.3444-3450.1998.

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ABSTRACT Oligotrophic oceanic waters of the central ocean gyres typically have extremely low dissolved fixed inorganic nitrogen concentrations, but few nitrogen-fixing microorganisms from the oceanic environment have been cultivated. Nitrogenase gene (nifH) sequences amplified directly from oceanic waters showed that the open ocean contains more diverse diazotrophic microbial populations and more diverse habitats for nitrogen fixers than previously observed by classical microbiological techniques. Nitrogenase genes derived from unicellular and filamentous cyanobacteria, as well as from the α and γ subdivisions of the class Proteobacteria, were found in both the Atlantic and Pacific oceans. nifH sequences that cluster phylogenetically with sequences from sulfate reducers or clostridia were found associated with planktonic crustaceans. Nitrogenase sequence types obtained from invertebrates represented phylotypes distinct from the phylotypes detected in the picoplankton size fraction. The results indicate that there are in the oceanic environment several distinct potentially nitrogen-fixing microbial assemblages that include representatives of diverse phylotypes.

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28

Lin, Senjie, Sheri Henze, Pernilla Lundgren, Birgitta Bergman y Edward J. Carpenter. "Whole-Cell Immunolocalization of Nitrogenase in Marine Diazotrophic Cyanobacteria, Trichodesmiumspp." Applied and Environmental Microbiology 64, n.º 8 (1 de agosto de 1998): 3052–58. http://dx.doi.org/10.1128/aem.64.8.3052-3058.1998.

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ABSTRACT The mechanism by which planktonic marine cyanobacteria of the genusTrichodesmium fix N2 aerobically during photosynthesis without heterocysts is unknown. As an aid in understanding how these species protect nitrogenase, we have developed an immunofluorescence technique coupled to light microscopy (IF-LM) with which intact cyanobacteria can be immunolabeled and the distribution patterns of nitrogenase and other proteins can be described and semiquantified. Chilled ethanol was used to fix the cells, which were subsequently made permeable to antibodies by using dimethyl sulfoxide. Use of this technique demonstrated that about 3 to 20 cells (mean ± standard deviation, 9 ± 4) consecutively arranged in a Trichodesmium trichome were labeled with the nitrogenase antibody. The nitrogenase-containing cells were distributed more frequently around the center of the trichome and were rarely found at the ends. On average 15% of over 300 randomly encountered cells examined contained nitrogenase. The percentage of nitrogenase-containing cells (nitrogenase index [NI]) in an exponential culture was higher early in the light period than during the rest of the light-dark cycle, while that for a stationary culture was somewhat constant at a lower level throughout the light-dark cycle. The NI was not affected by treatment of the cultures with the photosynthetic inhibitor dichloro 1,3′-dimethyl urea or with low concentrations of ammonium (NH4Cl). However, incubation of cultures with 0.5 μM NH4Cl over 2 days reduced the NI. The IF technique combined with 14C autoradiography showed that the CO2 fixation rate was lower in nitrogenase-containing cells. The results of the present study suggest that (i) the IF-LM technique may be a useful tool for in situ protein localization in cyanobacteria, (ii) cell differentiation occurs inTrichodesmium and only a small fraction of cells in a colony have the potential to fix nitrogen, (iii) the photosynthetic activity (CO2 uptake) is reduced if not absent in N2-fixing cells, and (iv) variation in the NI may be a modulator of nitrogen-fixing activity.

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Janson, Sven, Pirzada J. A. Siddiqui, Anthony E. Walsby, Kristen M. Romans, Edward J. Carpenter y Birgitta Bergman. "CYTOMORPHOLOGICAL CHARACTERIZATION OF THE PLANKTONIC DIAZOTROPHIC CYANOBACTERIA TRICHODESMIUM SPP. FROM THE INDIAN OCEAN AND CARIBBEAN AND SARGASSO SEAS1". Journal of Phycology 31, n.º 3 (junio de 1995): 463–77. http://dx.doi.org/10.1111/j.0022-3646.1995.00463.x.

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30

Moutin, Thierry, Andrea Michelangelo Doglioli, Alain de Verneil y Sophie Bonnet. "Preface: The Oligotrophy to the UlTra-oligotrophy PACific Experiment (OUTPACE cruise, 18 February to 3 April 2015)". Biogeosciences 14, n.º 13 (6 de julio de 2017): 3207–20. http://dx.doi.org/10.5194/bg-14-3207-2017.

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Abstract. The overall goal of OUTPACE (Oligotrophy to UlTra-oligotrophy PACific Experiment) was to obtain a successful representation of the interactions between planktonic organisms and the cycle of biogenic elements in the western tropical South Pacific Ocean across trophic and N2 fixation gradients. Within the context of climate change, it is necessary to better quantify the ability of the oligotrophic ocean to sequester carbon through biological processes. OUTPACE was organized around three main objectives, which were (1) to perform a zonal characterization of the biogeochemistry and biological diversity of the western tropical South Pacific during austral summer conditions, (2) to study the production and fate of organic matter (including carbon export) in three contrasting trophic regimes (increasing oligotrophy) with a particular emphasis on the role of dinitrogen fixation, and (3) to obtain a representation of the main biogeochemical fluxes and dynamics of the planktonic trophic network. The international OUTPACE cruise took place between 18 February and 3 April 2015 aboard the RV L'Atalante and involved 60 scientists (30 onboard). The west–east transect covered ∼ 4000 km from the western part of the Melanesian archipelago (New Caledonia) to the western boundary of the South Pacific gyre (French Polynesia). Following an adaptive strategy, the transect initially designed along the 19° S parallel was adapted along-route to incorporate information coming from satellite measurements of sea surface temperature, chlorophyll a concentration, currents, and diazotroph quantification. After providing a general context and describing previous work done in this area, this introductory paper elucidates the objectives of OUTPACE, the implementation plan of the cruise and water mass and climatological characteristics and concludes with a general overview of the other papers that will be published in this special issue.

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31

Cairns, Johannes, Sebastián Coloma, Kaarina Sivonen y Teppo Hiltunen. "Evolving interactions between diazotrophic cyanobacterium and phage mediate nitrogen release and host competitive ability". Royal Society Open Science 3, n.º 12 (diciembre de 2016): 160839. http://dx.doi.org/10.1098/rsos.160839.

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Interactions between nitrogen-fixing (i.e. diazotrophic) cyanobacteria and their viruses, cyanophages, can have large-scale ecosystem effects. These effects are mediated by temporal alterations in nutrient availability in aquatic systems owing to the release of nitrogen and carbon sources from cells lysed by phages, as well as by ecologically important changes in the diversity and fitness of cyanobacterial populations that evolve in the presence of phages. However, ecological and evolutionary feedbacks between phages and nitrogen-fixing cyanobacteria are still relative poorly understood. Here, we used an experimental evolution approach to test the effect of interactions between a common filamentous, nitrogen-fixing cyanobacterium ( Nodularia sp.) and its phage on cellular nitrogen release and host properties. Ecological, community-level effects of phage-mediated nitrogen release were tested with a phytoplankton bioassay. We found that cyanobacterial nitrogen release increased significantly as a result of viral lysis, which was associated with enhanced growth of phytoplankton species in cell-free filtrates compared with phage-resistant host controls in which lysis and subsequent nutrient release did not occur after phage exposure. We also observed an ecologically important change among phage-evolved cyanobacteria with phage-resistant phenotypes, a short-filamentous morphotype with reduced buoyancy compared with the ancestral long-filamentous morphotype. Reduced buoyancy might decrease the ability of these morphotypes to compete for light compared with longer, more buoyant filaments. Together, these findings demonstrate the potential of cyanobacteria–phage interactions to affect ecosystem biogeochemical cycles and planktonic community dynamics.

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32

Meyer, J., C. R. Löscher, S. C. Neulinger, A. F. Reichel, A. Loginova, C. Borchard, R. A. Schmitz, H. Hauss, R. Kiko y U. Riebesell. "Changing nutrient stoichiometry affects phytoplankton production, DOP accumulation and dinitrogen fixation – a mesocosm experiment in the eastern tropical North Atlantic". Biogeosciences 13, n.º 3 (11 de febrero de 2016): 781–94. http://dx.doi.org/10.5194/bg-13-781-2016.

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Abstract. Ocean deoxygenation due to climate change may alter redox-sensitive nutrient cycles in the marine environment. The productive eastern tropical North Atlantic (ETNA) upwelling region may be particularly affected when the relatively moderate oxygen minimum zone (OMZ) deoxygenates further and microbially driven nitrogen (N) loss processes are promoted. Consequently, water masses with a low nitrogen to phosphorus (N : P) ratio could reach the euphotic layer, possibly influencing primary production in those waters. Previous mesocosm studies in the oligotrophic Atlantic Ocean identified nitrate availability as a control of primary production, while a possible co-limitation of nitrate and phosphate could not be ruled out. To better understand the impact of changing N : P ratios on primary production and N2 fixation in the ETNA surface ocean, we conducted land-based mesocosm experiments with natural plankton communities and applied a broad range of N : P ratios (2.67–48). Silicic acid was supplied at 15 µmol L−1 in all mesocosms. We monitored nutrient drawdown, biomass accumulation and nitrogen fixation in response to variable nutrient stoichiometry. Our results confirmed nitrate to be the key factor determining primary production. We found that excess phosphate was channeled through particulate organic matter (POP) into the dissolved organic matter (DOP) pool. In mesocosms with low inorganic phosphate availability, DOP was utilized while N2 fixation increased, suggesting a link between those two processes. Interestingly this observation was most pronounced in mesocosms where nitrate was still available, indicating that bioavailable N does not necessarily suppress N2 fixation. We observed a shift from a mixed cyanobacteria–proteobacteria dominated active diazotrophic community towards a diatom-diazotrophic association of the Richelia-Rhizosolenia symbiosis. We hypothesize that a potential change in nutrient stoichiometry in the ETNA might lead to a general shift within the diazotrophic community, potentially influencing primary productivity and carbon export.

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33

Österholm, Julia, Rafael V. Popin, David P. Fewer y Kaarina Sivonen. "Phylogenomic Analysis of Secondary Metabolism in the Toxic Cyanobacterial Genera Anabaena, Dolichospermum and Aphanizomenon". Toxins 12, n.º 4 (11 de abril de 2020): 248. http://dx.doi.org/10.3390/toxins12040248.

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Cyanobacteria produce an array of toxins that pose serious health risks to humans and animals. The closely related diazotrophic genera, Anabaena, Dolichospermum and Aphanizomenon, frequently form poisonous blooms in lakes and brackish waters around the world. These genera form a complex now termed the Anabaena, Dolichospermum and Aphanizomenon (ADA) clade and produce a greater array of toxins than any other cyanobacteria group. However, taxonomic confusion masks the distribution of toxin biosynthetic pathways in cyanobacteria. Here we obtained 11 new draft genomes to improve the understanding of toxin production in these genera. Comparison of secondary metabolite pathways in all available 31 genomes for these three genera suggests that the ability to produce microcystin, anatoxin-a, and saxitoxin is associated with specific subgroups. Each toxin gene cluster was concentrated or even limited to a certain subgroup within the ADA clade. Our results indicate that members of the ADA clade encode a variety of secondary metabolites following the phylogenetic clustering of constituent species. The newly sequenced members of the ADA clade show that phylogenetic separation of planktonic Dolichospermum and benthic Anabaena is not complete. This underscores the importance of taxonomic revision of Anabaena, Dolichospermum and Aphanizomenon genera to reflect current phylogenomic understanding.

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34

Zani, Sabino, Mark T. Mellon, Jackie L. Collier y Jonathan P. Zehr. "Expression of nifH Genes in Natural Microbial Assemblages in Lake George, New York, Detected by Reverse Transcriptase PCR". Applied and Environmental Microbiology 66, n.º 7 (1 de julio de 2000): 3119–24. http://dx.doi.org/10.1128/aem.66.7.3119-3124.2000.

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ABSTRACT A modified nested reverse transcriptase PCR (RT-PCR) method was used to detect the expression of nitrogenase genes in meso-oligotrophic Lake George, New York. Net (>20-μm pore size) plankton samples collected from two sites (Dome Island and Hague Marina) were extracted for total RNA and genomic DNA to determine the identity of diazotrophic organisms that were present and those that were actively expressing nitrogenase genes. Phylogenetic analysis of individual sequences cloned from PCR amplifications showed that there were phylogenetically diverse groups of bacteria that possessed a nifH gene, including representatives of unicellular and filamentous cyanobacteria, the α- and γ-subdivisions of the division Proteobacteria (α- and γ-proteobacteria), and a previously undefined group of bacteria. The phylotypes cloned from RT-PCR amplifications, which were actively expressing nifH transcripts, clustered with the unicellular and filamentous cyanobacteria, α-proteobacteria, and the novel bacterial cluster. No bacterial sequences were found which clustered with sequences from cluster II (alternative nitrogenases), III (nitrogenases in strict anaerobes), or IV (nifH-like sequences). These results indicate that there were several distinct groups of nitrogen-fixing microorganisms in the net plankton from both sampling sites and that most of the groups had representative phylotypes that were actively expressing nitrogenase genes.

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35

Garg, Ritu, Manja Luckner, Jürgen Berger, Katharina Hipp, Gerhard Wanner, Karl Forchhammer y Iris Maldener. "Changes in Envelope Structure and Cell–Cell Communication during Akinete Differentiation and Germination in Filamentous Cyanobacterium Trichormus variabilis ATCC 29413". Life 12, n.º 3 (16 de marzo de 2022): 429. http://dx.doi.org/10.3390/life12030429.

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Planktonic freshwater filamentous cyanobacterium Trichormus variabilis ATCC 29413 (previously known as Anabaena variabilis) can differentiate heterocysts and akinetes to survive under different stress conditions. Whilst heterocysts enable diazotrophic growth, akinetes are spore-like resting cells that make the survival of the species possible under adverse growth conditions. Under suitable environmental conditions, they germinate to produce new vegetative filaments. Several morphological and physiological changes occur during akinete formation and germination. Here, using scanning electron microscopy (SEM), we found that the mature akinetes had a wrinkled envelope, and the surface of the envelope smoothened as the cell size increased during germination. Thereupon, the akinete envelope ruptured to release the short emerging filament. Focused ion beam–scanning electron microscopy (FIB/SEM) tomography of immature akinetes revealed the presence of cytoplasmic granules, presumably consisting of cyanophycin or glycogen. In addition, the akinete envelope architecture of different layers, the exopolysaccharide and glycolipid layers, could be visualized. We found that this multilayered envelope helped to withstand osmotic stress and to maintain the structural integrity. Furthermore, by fluorescence recovery after photobleaching (FRAP) measurements, using the fluorescent tracer calcein, we found that intercellular communication decreased during akinete formation as compared with the vegetative cells. In contrast, freshly germinating filaments restored cell communication.

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36

Kulizin, Pavel Vladimirovich, Ekaterina Leonidovna Vodeneeva y Alexander Gennadievich Okhapkin. "Phytoplankton of some rivers in the south of broadleaved forests subzone of the Middle Volga basin in a long-term perspective". Samara Journal of Science 10, n.º 2 (1 de junio de 2021): 45–53. http://dx.doi.org/10.17816/snv2021102106.

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The paper provides information on the composition, structure and dynamics of phytoplankton of three morphologically different tributaries of the Cheboksary reservoir (the Vetluga, the Kerzhenets and the Vishnya rivers). The species composition of algae includes 826 species and intraspecific taxa, the flora was formed by diatom, green and euglena algae. It reflects the watercourses particularities of the southern taiga zone. Based on long-term studies of phytoplankton a change in the composition of algoflora by more than half was noticed, especially in the groups of Charophyta, Cryptophyta, Miozoa, Ochrophyta and Euglenophyta. An increase in quantitative indicators of algocoenoses from oligotrophic to oligotrophic-mesotrophic level in the middle reaches of rivers by mesotrophic-eutrophic level in estuarine areas is shown. In a fifty-year research series a tendency of phytoplankton reduction in size by more than 3,5 times has been established. It reflects the impact of eutrophication and gradual warming of the climate. The period of abnormally hot summer of 2010 led to increased vegetation in the middle reaches of rivers of diazotrophic blue-green algae and dinophytes. Since the early 2000s, gradual penetration and naturalization of both planktonic invasive algae species and representatives of benthic algocoenoses have been noticed. The mentioned changes reflect the nature of phytoplankton rearrangements as the essential component of the biota and emphasize the importance of long-term research with a possibility to predict negative consequences.

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37

Buitenhuis, E. T., M. Vogt, R. Moriarty, N. Bednaršek, S. C. Doney, K. Leblanc, C. Le Quéré et al. "MAREDAT: towards a World Ocean Atlas of MARine Ecosystem DATa". Earth System Science Data Discussions 5, n.º 2 (23 de noviembre de 2012): 1077–106. http://dx.doi.org/10.5194/essdd-5-1077-2012.

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Abstract. We present a summary of biomass data for 11 Plankton Functional Types (PFTs) plus phytoplankton pigment data, compiled as part of the MARine Ecosystem biomass DATa (MAREDAT) initiative. The goal of the MAREDAT initiative is to provide global gridded data products with coverage of all biological components of the global ocean ecosystem. This special issue is the first step towards achieving this. The PFTs presented here include picophytoplankton, diazotrophs, coccolithophores, Phaeocystis, diatoms, picoheterotrophs, microzooplankton, foraminifers, mesozooplankton, pteropods and macrozooplankton. All variables have been gridded onto a World Ocean Atlas (WOA) grid (1° × 1° × 33 vertical levels × monthly climatologies). The data show that (1) the global total heterotrophic biomass (2.0–6.4 Pg C) is at least as high as the total autotrophic biomass (0.5–2.6 Pg C excluding nanophytoplankton and autotrophic dinoflagellates), (2) the biomass of zooplankton calcifiers (0.9–2.3 Pg C) is substantially higher than that of coccolithophores (0.01–0.14 Pg C), (3) patchiness of biomass distribution increases with organism size, and (4) although zooplankton biomass measurements below 200 m are rare, the limited measurements available suggest that Bacteria and Archaea are not the only heterotrophs in the deep sea. More data will be needed to characterize ocean ecosystem functioning and associated biogeochemistry in the Southern Hemisphere and below 200 m. Microzooplankton database: doi:10.1594/PANGAEA.779970.

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Richardson, Katherine y Jørgen Bendtsen. "Photosynthetic oxygen production in a warmer ocean: the Sargasso Sea as a case study". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 375, n.º 2102 (7 de agosto de 2017): 20160329. http://dx.doi.org/10.1098/rsta.2016.0329.

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Photosynthetic O 2 production can be an important source of oxygen in sub-surface ocean waters especially in permanently stratified oligotrophic regions of the ocean where O 2 produced in deep chlorophyll maxima (DCM) is not likely to be outgassed. Today, permanently stratified regions extend across approximately 40% of the global ocean and their extent is expected to increase in a warmer ocean. Thus, predicting future ocean oxygen conditions requires a better understanding of the potential response of photosynthetic oxygen production to a warmer ocean. Based on our own and published observations of water column processes in oligotrophic regions, we develop a one-dimensional water column model describing photosynthetic oxygen production in the Sargasso Sea to quantify the importance of photosynthesis for the downward flux of O 2 and examine how it may be influenced in a warmer ocean. Photosynthesis is driven in the model by vertical mixing of nutrients (including eddy-induced mixing) and diazotrophy and is found to substantially increase the downward O 2 flux relative to physical–chemical processes alone. Warming (2°C) surface waters does not significantly change oxygen production at the DCM. Nor does a 15% increase in re-mineralization rate (assuming Q 10 = 2; 2°C warming) have significant effect on net sub-surface oxygen accumulation. However, changes in the relative production of particulate (POM) and dissolved organic material (DOM) generate relatively large changes in net sub-surface oxygen production. As POM/DOM production is a function of plankton community composition, this implies plankton biodiversity and food web structure may be important factors influencing O 2 production in a warmer ocean. This article is part of the themed issue ‘Ocean ventilation and deoxygenation in a warming world’.

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39

Gómez, Fernando. "Symbioses of Ciliates (Ciliophora) and Diatoms (Bacillariophyceae): Taxonomy and Host–Symbiont Interactions". Oceans 1, n.º 3 (24 de julio de 2020): 133–55. http://dx.doi.org/10.3390/oceans1030010.

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The nature of the plankton symbioses between ciliates and diatoms has been investigated from the tropical South Atlantic Ocean, and Mediterranean and Caribbean Seas. The obligate symbioses of the diatoms Chaetoceros dadayi or C. tetrastichon with the tintinnid Eutintinnus spp., and Chaetoceros coarctatus with the peritrich ciliate Vorticella oceanica are the most widespread, and the consortium of Chaetoceros densus and Vorticella sp. have been rediscovered. Facultative symbioses between Eutintinnus lususundae and Chaetoceros peruvianus, Hemiaulus spp., and Thalassionema sp. are less frequent, often containing three or four partners because Hemiaulus can also harbor the diazotrophic cyanobacteria Richelia intracellularis. Another three-partner consortium is the peritrich ciliate Zoothamnium pelagicum, ectobiont bacteria, and the diatom Licmophora sp. The predominantly oligotrophic conditions of tropical seas do not favor the survival of large diatoms, but large species of Coscinodiscus and Palmerina in facultative symbiosis with Pseudovorticella coscinodisci have a competitive advantage over other diatoms (i.e., reduction of sinking speed and diffusive boundary layer). Symbioses allow sessile peritric ciliates to extend their distribution in the pelagic environment, permit boreal-polar related diatoms such as C. coarctatus or Fragilariopsis doliolus to inhabit tropical seas, and help large diatoms to extend their survival under unfavorable conditions.

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40

Landolfi, A., H. Dietze, W. Koeve, R. Mather y R. Sanders. "Organic nutrients as sources of N and P to the upper layers of the North Atlantic subtropical gyre along 24.5° N". Biogeosciences Discussions 7, n.º 3 (31 de mayo de 2010): 4001–44. http://dx.doi.org/10.5194/bgd-7-4001-2010.

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Abstract. There is a longstanding discussion on how the macronutrient requirement of the export production in the North Atlantic subtropical gyre is sustained. In this study we asses the role of dissolved organic nitrogen (DON) and phosphorous (DOP) as sources of new nutrients into the North Atlantic subtropical gyre at 24.5° N. We define, based on measurements of DON, DOP, phytoplankton community structure, stable nitrogen isotopic signals, surface mixed layer depth and ocean color as viewed from space, four regions characterized by different nutrient supply regimes. Within these regions, two distinct loci of N2 fixation occur associated with different plankton assemblages and separated by a region in which N2 fixation occurs at levels insufficient to leave its distinctive isotopic fingerprint on the isotopic composition of PON. Here, the phosphorus supply pathways to the mixed plankton assemblage appear to be different. In the wester oligotrophic gyre (70–46° W), the lateral advection of DOP supplies the missing P that, together with, shallow mixed layer, almost permanent stratification and high water temperatures, stimulate diazotrophic growth, which augment TON local accumulation. In the eastern oligotophic gyre (46–30° W), DOP cannot support the P demand as it is exhausted on its way from productive areas. This is inferred from DOP turnover rates, estimated form enzymatic clevage rates, which are shorter (11 ± 8 months) than transit timescales, estimated from a 3-D circulation model (>4 yr). A stronger seasonal cycle in chlorophyll and mixed layer depth, favour some nutrient injections from below. Here additional N sources come from the advected DON which has a turnover-time of 6.7 ± 3 yr, instead fast remineralization and little DOP export are needed to maintain the P requirements. We conclude from these observations that organic nutrient utilisation patterns drive diverse phytoplankton assemblages and oceanic nitrogen fixation gradients.

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Häder, Donat-P. y Kunshan Gao. "Aquatic Productivity under Multiple Stressors". Water 15, n.º 4 (20 de febrero de 2023): 817. http://dx.doi.org/10.3390/w15040817.

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Aquatic ecosystems are responsible for about 50% of global productivity. They mitigate climate change by taking up a substantial fraction of anthropogenically emitted CO2 and sink part of it into the deep ocean. Productivity is controlled by a number of environmental factors, such as water temperature, ocean acidification, nutrient availability, deoxygenation and exposure to solar UV radiation. Recent studies have revealed that these factors may interact to yield additive, synergistic or antagonistic effects. While ocean warming and deoxygenation are supposed to affect mitochondrial respiration oppositely, they can act synergistically to influence the migration of plankton and N2-fixation of diazotrophs. Ocean acidification, along with elevated pCO2, exhibits controversial effects on marine primary producers, resulting in negative impacts under high light and limited availability of nutrients. However, the acidic stress has been shown to exacerbate viral attacks on microalgae and to act synergistically with UV radiation to reduce the calcification of algal calcifiers. Elevated pCO2 in surface oceans is known to downregulate the CCMs (CO2 concentrating mechanisms) of phytoplankton, but deoxygenation is proposed to enhance CCMs by suppressing photorespiration. While most of the studies on climate-change drivers have been carried out under controlled conditions, field observations over long periods of time have been scarce. Mechanistic responses of phytoplankton to multiple drivers have been little documented due to the logistic difficulties to manipulate numerous replications for different treatments representative of the drivers. Nevertheless, future studies are expected to explore responses and involved mechanisms to multiple drivers in different regions, considering that regional chemical and physical environmental forcings modulate the effects of ocean global climate changes.

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42

Buitenhuis, E. T., M. Vogt, R. Moriarty, N. Bednaršek, S. C. Doney, K. Leblanc, C. Le Quéré et al. "MAREDAT: towards a world atlas of MARine Ecosystem DATa". Earth System Science Data 5, n.º 2 (12 de julio de 2013): 227–39. http://dx.doi.org/10.5194/essd-5-227-2013.

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Abstract. We present a summary of biomass data for 11 plankton functional types (PFTs) plus phytoplankton pigment data, compiled as part of the MARine Ecosystem biomass DATa (MAREDAT) initiative. The goal of the MAREDAT initiative is to provide, in due course, global gridded data products with coverage of all planktic components of the global ocean ecosystem. This special issue is the first step towards achieving this. The PFTs presented here include picophytoplankton, diazotrophs, coccolithophores, Phaeocystis, diatoms, picoheterotrophs, microzooplankton, foraminifers, mesozooplankton, pteropods and macrozooplankton. All variables have been gridded onto a World Ocean Atlas (WOA) grid (1° × 1° × 33 vertical levels × monthly climatologies). The results show that abundance is much better constrained than their carbon content/elemental composition, and coastal seas and other high productivity regions have much better coverage than the much larger volumes where biomass is relatively low. The data show that (1) the global total heterotrophic biomass (2.0–4.6 Pg C) is at least as high as the total autotrophic biomass (0.5–2.4 Pg C excluding nanophytoplankton and autotrophic dinoflagellates); (2) the biomass of zooplankton calcifiers (0.03–0.67 Pg C) is substantially higher than that of coccolithophores (0.001–0.03 Pg C); (3) patchiness of biomass distribution increases with organism size; and (4) although zooplankton biomass measurements below 200 m are rare, the limited measurements available suggest that Bacteria and Archaea are not the only important heterotrophs in the deep sea. More data will be needed to characterise ocean ecosystem functioning and associated biogeochemistry in the Southern Hemisphere and below 200 m. Future efforts to understand marine ecosystem composition and functioning will be helped both by further archiving of historical data and future sampling at new locations. Microzooplankton database: doi:10.1594/PANGAEA.779970 All MAREDAT databases: http://www.pangaea.de/search?&q=maredat

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43

Riemann, Lasse, Eyal Rahav, Uta Passow, Hans-Peter Grossart, Dirk de Beer, Isabell Klawonn, Meri Eichner, Mar Benavides y Edo Bar-Zeev. "Planktonic Aggregates as Hotspots for Heterotrophic Diazotrophy: The Plot Thickens". Frontiers in Microbiology 13 (6 de abril de 2022). http://dx.doi.org/10.3389/fmicb.2022.875050.

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Biological dinitrogen (N2) fixation is performed solely by specialized bacteria and archaea termed diazotrophs, introducing new reactive nitrogen into aquatic environments. Conventionally, phototrophic cyanobacteria are considered the major diazotrophs in aquatic environments. However, accumulating evidence indicates that diverse non-cyanobacterial diazotrophs (NCDs) inhabit a wide range of aquatic ecosystems, including temperate and polar latitudes, coastal environments and the deep ocean. NCDs are thus suspected to impact global nitrogen cycling decisively, yet their ecological and quantitative importance remain unknown. Here we review recent molecular and biogeochemical evidence demonstrating that pelagic NCDs inhabit and thrive especially on aggregates in diverse aquatic ecosystems. Aggregates are characterized by reduced-oxygen microzones, high C:N ratio (above Redfield) and high availability of labile carbon as compared to the ambient water. We argue that planktonic aggregates are important loci for energetically-expensive N2 fixation by NCDs and propose a conceptual framework for aggregate-associated N2 fixation. Future studies on aggregate-associated diazotrophy, using novel methodological approaches, are encouraged to address the ecological relevance of NCDs for nitrogen cycling in aquatic environments.

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44

Bednarz, Vanessa N., Jeroen A. J. M. van de Water, Renaud Grover, Jean-François Maguer, Maoz Fine y Christine Ferrier-Pagès. "Unravelling the Importance of Diazotrophy in Corals – Combined Assessment of Nitrogen Assimilation, Diazotrophic Community and Natural Stable Isotope Signatures". Frontiers in Microbiology 12 (24 de junio de 2021). http://dx.doi.org/10.3389/fmicb.2021.631244.

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There is an increasing interest in understanding the structure and function of the microbiota associated with marine and terrestrial organisms, because it can play a major role in host nutrition and resistance to environmental stress. Reef-building corals live in association with diazotrophs, which are microbes able to fix dinitrogen. Corals are known to assimilate diazotrophically-derived nitrogen (DDN), but it is still not clear whether this nitrogen source is derived from coral-associated diazotrophs and whether it substantially contributes to the coral’s nitrogen budget. In this study, we aimed to provide a better understanding of the importance of DDN for corals using a holistic approach by simultaneously assessing DDN assimilation rates (using 15N2 tracer technique), the diazotrophic bacterial community (using nifH gene amplicon sequencing) and the natural δ15N signature in Stylophora pistillata corals from the Northern Red Sea along a depth gradient in winter and summer. Overall, our results show a discrepancy between the three parameters. DDN was assimilated by the coral holobiont during winter only, with an increased assimilation with depth. Assimilation rates were, however, not linked to the presence of coral-associated diazotrophs, suggesting that the presence of nifH genes does not necessarily imply functionality. It also suggests that DDN assimilation was independent from coral-associated diazotrophs and may instead result from nitrogen derived from planktonic diazotrophs. In addition, the δ15N signature presented negative values in almost all coral samples in both seasons, suggesting that nitrogen sources other than DDN contribute to the nitrogen budget of corals from this region. This study yields novel insight into the origin and importance of diazotrophy for scleractinian corals from the Northern Red Sea using multiple proxies.

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45

Meunier, Valentine, Sophie Bonnet, Mar Benavides, Andreas Ravache, Olivier Grosso, Christophe Lambert y Fanny Houlbrèque. "Diazotroph-Derived Nitrogen Assimilation Strategies Differ by Scleractinian Coral Species". Frontiers in Marine Science 8 (28 de julio de 2021). http://dx.doi.org/10.3389/fmars.2021.692248.

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Reef-building corals generally thrive in nutrient-poor tropical waters, where among other elements, nitrogen (N) availability often limits primary productivity. In addition to their close association with endosymbiotic dinoflagellates of the family Symbiodiniaceae, enabling an effective use and retention of dissolved inorganic nitrogen (DIN), scleractinian corals have developed strategies to acquire new N: (1) They can ingest N-rich sediment particles and preys (from picoplankton to macro-zooplankton) via heterotrophy, including diazotrophs [plankton fixing dinitrogen (N2) and releasing part of this nitrogen—Diazotroph-Derived N (DDN)—in seawater], a pathway called “heterotrophic nutrition on diazotrophs”; (2) Symbiotic diazotrophs located in the coral holobiont have the molecular machinery to fix N2, a pathway called “symbiotic N2 fixation”. Here we used the 15N2 isotopic labeling in a series of incubations to investigate the relative contribution of each of these DDN transfer pathways in three worldwide distributed coral species: Acropora muricata, Galaxea fascicularis, and Pocillopora damicornis. We show that N provision via “symbiotic N2 fixation” is negligible compared to that obtained via “heterotrophic nutrition on diazotrophs,” with DDN assimilation rates about a thousand times lower for P. damicornis and G. fascicularis, or assimilation rates via “symbiotic N2 fixation” almost nil for A. muricata. Through heterotrophic feeding on planktonic diazotrophs, only G. fascicularis and P. damicornis can successfully obtain N and fulfill a large part of their N requirements (DDN asimilation rates: 0.111 ± 0.056 and 0.517 ± 0.070 μg N cm–2 h–1 in their Symbiodiniaceae, respectively). Whereas this contribution is again negligible for A. muricata. They also largely consume the picoplankton that likely benefit from this DDN (Prochlorococcus and Synechococcus cells; respectively, 2.56 ± 1.57 104 and 2.70 ± 1.66 104 cell h–1 cm–2 for G. fascicularis; 3.02 ± 0.19 105 and 1.14 ± 0.79 104 cell h–1 cm–2 for P. damicornis). The present study confirms the different dependencies of the three tested species regarding heterotrophy, with P. damicornis and G. fascicularis appearing highly efficient at capturing plankton, while A. muricata, considered as mainly autotroph, does not rely on these food resources to meet its N and energy needs.

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Wang, Seaver, Weiyi Tang, Erwan Delage, Scott Gifford, Hannah Whitby, Aridane G. González, Damien Eveillard, Hélène Planquette y Nicolas Cassar. "Investigating the microbial ecology of coastal hotspots of marine nitrogen fixation in the western North Atlantic". Scientific Reports 11, n.º 1 (9 de marzo de 2021). http://dx.doi.org/10.1038/s41598-021-84969-1.

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AbstractVariation in the microbial cycling of nutrients and carbon in the ocean is an emergent property of complex planktonic communities. While recent findings have considerably expanded our understanding of the diversity and distribution of nitrogen (N2) fixing marine diazotrophs, knowledge gaps remain regarding ecological interactions between diazotrophs and other community members. Using quantitative 16S and 18S V4 rDNA amplicon sequencing, we surveyed eukaryotic and prokaryotic microbial communities from samples collected in August 2016 and 2017 across the Western North Atlantic. Leveraging and significantly expanding an earlier published 2015 molecular dataset, we examined microbial community structure and ecological co-occurrence relationships associated with intense hotspots of N2 fixation previously reported at sites off the Southern New England Shelf and Mid-Atlantic Bight. Overall, we observed a negative relationship between eukaryotic diversity and both N2 fixation and net community production (NCP). Maximum N2 fixation rates occurred at sites with high abundances of mixotrophic stramenopiles, notably Chrysophyceae. Network analysis revealed such stramenopiles to be keystone taxa alongside the haptophyte diazotroph host Braarudosphaera bigelowii and chlorophytes. Our findings highlight an intriguing relationship between marine stramenopiles and high N2 fixation coastal sites.

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47

Geisler, Eyal, Anne Bogler, Eyal Rahav y Edo Bar-Zeev. "Direct Detection of Heterotrophic Diazotrophs Associated with Planktonic Aggregates". Scientific Reports 9, n.º 1 (26 de junio de 2019). http://dx.doi.org/10.1038/s41598-019-45505-4.

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48

Mériguet, Zoé, Marion Vilain, Alberto Baudena, Chloé Tilliette, Jérémie Habasque, Anne Lebourges-Dhaussy, Nagib Bhairy, Cécile Guieu, Sophie Bonnet y Fabien Lombard. "Plankton community structure in response to hydrothermal iron inputs along the Tonga-Kermadec arc". Frontiers in Marine Science 10 (4 de octubre de 2023). http://dx.doi.org/10.3389/fmars.2023.1232923.

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The Western Tropical South Pacific (WTSP) basin has been identified as a hotspot of atmospheric dinitrogen fixation due to the high dissolved iron ([DFe]) concentrations (up to 66 nM) in the photic layer linked with the release of shallow hydrothermal fluids along the Tonga-Kermadec arc. Yet, the effect of such hydrothermal fluids in structuring the plankton community remains poorly studied. During the TONGA cruise (November-December 2019), we collected micro- (20-200 μm) and meso-plankton (>200 μm) samples in the photic layer (0-200 m) along a west to east zonal transect crossing the Tonga volcanic arc, in particular two volcanoes associated with shallow hydrothermal vents (< 500 m) in the Lau Basin, and both sides of the arc represented by Melanesian waters and the South Pacific Gyre. Samples were analyzed by quantitative imaging (FlowCam and ZooScan) and then coupled with acoustic observations, allowing us to study the potential transfer of phytoplankton blooms to higher planktonic trophic levels. We show that micro- and meso-plankton exhibit high abundances and biomasses in the Lau Basin and, to some extent, in Melanesian waters, suggesting that shallow hydrothermal inputs sustain the planktonic food web, creating productive waters in this otherwise oligotrophic region. In terms of planktonic community structure, we identified major changes with high [DFe] inputs, promoting the development of a low diversity planktonic community dominated by diazotrophic cyanobacteria. Furthermore, in order to quantify the effect of the shallow hydrothermal vents on chlorophyll a concentrations, we used Lagrangian dispersal models. We show that chlorophyll a concentrations were significantly higher inside the Lagrangian plume, which came into contact with the two hydrothermal sites, confirming the profound impact of shallow hydrothermal vents on plankton production.

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49

Suikkanen, Sanna, Laura Uusitalo, Sirpa Lehtinen, Maiju Lehtiniemi, Pirkko Kauppila, Katja Mäkinen y Harri Kuosa. "Diazotrophic cyanobacteria in planktonic food webs". Food Webs, junio de 2021, e00202. http://dx.doi.org/10.1016/j.fooweb.2021.e00202.

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50

Fernández-Urruzola, Igor, Antonio Bode, Natalie Loick-Wilde, Wolfgang Schneider, Dhugal Lindsay y Ruben Escribano. "Trophic ecology of midwater zooplankton along a productivity gradient in the Southeast Pacific". Frontiers in Marine Science 10 (15 de febrero de 2023). http://dx.doi.org/10.3389/fmars.2023.1057502.

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The environmental regulation of planktonic food web structure is a key determinant for the efficiency of energy transfer through trophic levels, with direct implications for the amount of energy that is available to top-consumers and deep-sea communities. Yet, the complex trophic interplay between the different components of plankton communities under contrasting environmental conditions remains unresolved. Here, we present a comprehensive field study based on compound-specific isotope analysis of amino acids that aims to explore depth-resolved changes in the trophic strategies of various size-classes of mesozooplankton from distinct habitats across the Southeast Pacific. According to the δ15N values of the so-called source amino acids such as phenylalanine, we first identified a clear shift in nitrogen sources for biological production, from nitrate in the productive upwelling waters of the Humboldt Current System to increased inputs of diazotroph-N within the ultra-oligotrophic South Pacific gyre. These shifts in the productivity and in the source of N at the base of the food web were not accompanied by significant changes in the trophic position (TP) of mesozooplankton, which were mainly omnivores (TP = 2.8 ± 0.3, n = 65) in all ecoregions. However, although the planktonic food web length remained relatively unaltered along the productivity gradient, the microbial contribution to mid-trophic level consumers appeared to be more important at relatively high productivities, likely due to more intense remineralization processes in a late successional stage of the upwelling throughout the coastal-transition zone. Altogether, this research work will contribute to a better understanding of the food web functioning in one of the least explored marine regions of the world’s oceans.

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