Littérature scientifique sur le sujet « Algal overgrowth »

One of the most prominent coral diseases in Hawai‘i is growth anomaly (GA) of the skeleton. GA reduces key biological functions of Montipora capitata, but the effect of this disease on the ecological competitiveness of the coral has not been investigated. This study evaluated the effect of the GA on the susceptibility of an endemic Hawaiian coral M. capitata to algal overgrowth at Wai‘ōpae, Hawai‘i. Field surveys and photo analyses were conducted to collect epizootiological data. A relationship was found between GA severity and turf algal growth at the colony level; however, this relationship was weaker than expected. Despite the fact that GA lesions can provide areas of protuberant exposed coral skeleton, turf algal overgrowth was no more prevalent on GA-affected colonies than on unaffected colonies. As algal overgrowth is ephemeral compared to the progression of GA, we suggest that filamentous algae takes advantage of ideal environmental conditions regardless of the availability of new surfaces to invade presented by GA lesions.

Observations of coral–algal competition can provide valuable information about the state of coral reef ecosystems. Here, we report contact rates and apparent competition states for six shallow lagoonal reefs in Fiji. A total of 81.4% of examined coral perimeters were found to be in contact with algae, with turf algae (54.7%) and macroalgae of the genus Lobophora (16.8%) representing the most frequently observed contacts. Turf algae competitiveness was low, with 21.8% of coral–turf contacts being won by the algae (i.e. overgrowth or bleaching of coral tissue). In contrast, Lobophora competitiveness against corals was high, with 62.5% of contacts being won by the alga. The presence of epiphytic algae on Lobophora was associated with significantly greater algal competitiveness against corals, with 75.8% and 21.1% of interactions recorded as algal wins in the presence and absence of epiphytes, respectively. Sedimentation rate, herbivorous fish biomass, and coral colony size did not have a significant effect on Lobophora–coral interactions. This research indicates a novel and important role of epiphytes in driving the outcome of coral–algal contacts.

Microbes play a key role in reef dynamics, mediating the competition between scleractinian corals and benthic algae; however, major shifts in bacterial communities among coral species in response to increases in the abundance of algae are not well understood. We investigated the taxonomic composition of coral-associated microbial communities under algae-overgrowth conditions using 16S rRNA gene sequencing. The results showed that non-algal (i.e., healthy) tissue (HH) had lower bacterial abundance and diversity than tissue collected from the coral–algae interface boundary (HA) and areas of algae growth (AA). Specifically, the HA and AA samples had higher relative abundances of Saprospiraceae, Rhodobacteraceae, and Alteromonadaceae. Compared with Platygyra sp. and Montipora sp., the physiological response of Pocillopora sp. was more intense under algae-induced stress based on microbial gene function prediction. Our results indicate that algal pressure can significantly alter the microbial community structure and function of coral ecosystems. Our data thus provide new insight into the relationship between corals and their microbiome under environmental stress.

Although many coral reefs have shifted from coral-to-algal dominance, the consequence of such a transition for coral–algal interactions and their underlying mechanisms remain poorly understood. At the microscale, it is unclear how diffusive boundary layers (DBLs) and surface oxygen concentrations at the coral–algal interface vary with algal competitors and competitiveness. Using field observations and microsensor measurements in a flow chamber, we show that coral (massive Porites ) interfaces with thick turf algae, macroalgae, and cyanobacteria, which are successful competitors against coral in the field, are characterized by a thick DBL and hypoxia at night. In contrast, coral interfaces with crustose coralline algae, conspecifics, and thin turf algae, which are poorer competitors, have a thin DBL and low hypoxia at night. Furthermore, DBL thickness and hypoxia at the interface with turf decreased with increasing flow speed, but not when thick turf was upstream. Our results support the importance of water-mediated transport mechanisms in coral–algal interactions. Shifts towards algal dominance, particularly dense assemblages, may lead to thicker DBLs, higher hypoxia, and higher concentrations of harmful metabolites and pathogens along coral borders, which in turn may facilitate algal overgrowth of live corals. These effects may be mediated by flow speed and orientation.

SynopsisFouling communities typical of shallow water inshore sites were found at three locations in the Moray Firth. At each, an initial background cover of solitary tubeworms and barnacles was overgrown by secondary fouling organisms. On the piles of Nigg jetty, overgrowth consisted of mussels in the depth range 0–6 m and hydroids, sponges, soft corals and anemones from 6–26 m. Buoys in the approaches to Cromarty Firth were completely covered by a mixture of algae and mussels. Sunlit areas of the float cleaned annually bore a diverse algal cover, whereas uncleaned shaded areas and the freely hanging chain had three-year-old mussels up to 7 cm long. Mussel fouling extended down the chain to within 1 m of the seabed at 26 m depth. Concrete anchor blocks on the seabed were covered with solitary tubeworms and hydroids. Steel piled platforms in the Beatrice Field were completely fouled after four years. Mussels and seaweeds were abundant from 0–5 m. In the depth range 8–35 m the background calcareous layer was overgrown by soft corals up to 10 cm long and hydroids. From 35 m to the seabed at 46 m, soft overgrowth was provided mainly by hydroids and ascidians, with only a few small corals.

ABSTRACTHarmful algal blooms, caused by massive and exceptional overgrowth of microalgae and cyanobacteria, are a serious environmental problem worldwide.In the present study, we looked forBacillusstrains with sufficiently strong anticyanobacterial activity to be used as biocontrol agents. Among 24 strains,Bacillus amyloliquefaciensFZB42 showed the strongest bactericidal activity againstMicrocystis aeruginosa, with a kill rate of 98.78%. The synthesis of the anticyanobacterial substance did not depend on Sfp, an enzyme that catalyzes a necessary processing step in the nonribosomal synthesis of lipopeptides and polyketides, but was associated with thearogene cluster that is involved in the synthesis of thesfp-independent antibiotic bacilysin. Disruption ofbacB, the gene in the cluster responsible for synthesizing bacilysin, or supplementation with the antagonistN-acetylglucosamine abolished the inhibitory effect, but this was restored when bacilysin synthesis was complemented. Bacilysin caused apparent changes in the algal cell wall and cell organelle membranes, and this resulted in cell lysis. Meanwhile, there was downregulated expression ofglmS,psbA1,mcyB, andftsZ—genes involved in peptidoglycan synthesis, photosynthesis, microcystin synthesis, and cell division, respectively. In addition, bacilysin suppressed the growth of other harmful algal species. In summary, bacilysin produced byB. amyloliquefaciensFZB42 has anticyanobacterial activity and thus could be developed as a biocontrol agent to mitigate the effects of harmful algal blooms.

The reef health worldwide is seriously threatened by a multitude of factors such as abnormally elevated and low ocean temperatures, high UV radiations, changes in salinity, pollution and increasing incidence of diseases. Under adverse circumstances the equilibrium between the partners of the coral holobiont may be compromised and can lead to coral bleaching events. Bleaching refers to the loss in the coloration of the coral colony induced by the dissociation of the symbiosis between corals and their symbiotic algae. The extreme or unexpected environmental fluctuations could be very stressful for sessile marine organisms such as corals, causing important cell damage since corals lack any developed physiological regulatory system. One mechanism of reaction to deleterious conditions is the rapid increase of the induction of a set of stress proteins called Heat shock proteins (Hsps). Under normal cellular physiological conditions the Hsps mainly function as molecular chaperones and they are involved in a multitude of proteome-maintenance functions that regulate protein homeostasis in directing the folding and assembly of other proteins. They also are involved in the intracellular protein transport and in the degradation of damaged proteins. An up-regulation of the expression of Hsps constitutes an emergency response and confers tolerance to harsh conditions. This study highlights the modulation of the expression of a vital but scarcely investigated group of Hsps, the mitochondrial Hsp60 which are essential for the vitality of the cell and whose induction represent one of the first reaction to stress. The overall objective of my dissertation is to elucidate the major aspects of Hsp60 modulation in various taxa of corals as a result of their exposure to different abiotic and biotic stress factors. In the first study we investigated for the first time the effectiveness of the Hsp60 as indicator of biotic stress and competitive interaction in the coral Acropora muricata, focusing on two biological interactions such as a coral disease, the Skeleton eroding band (SEB) caused by the protozoan Halofolliculina corallasia and the algal overgrowth. The two different biological stresses trigger diverse responses on Hsp60 level. No detectable effect on Hsp60 modulation appeared in colonies subjected to algal overgrowth. On the contrary, corals displayed a robust up-regulation of Hsp60 in the fragments sampled just above the SEB dark band indicating that the aggressive behavior of the protozoan causes cellular damage also in coral portions neighboring and along the advancing front of the infection. Portions of coral sampled distant to the SEB band showed a Hsp60 level comparable to that observed in healthy colonies. We propose Hsp60 expression as a promising tool to evaluate physiological stress caused by coral disease in reef corals. In the second study we examined the different modulation of Hsp60 in the coral Seriatopora caliendrum subjected to salinity stress, since that corals are generally considered stenohaline and osmoconformers. We analyzed the Hsp60 expression profiles of the coral polyps under three salinity scenarios (hypersalinity of 45 ppt, hyposalinity of 27 ppt and extreme hyposalinity of 15 ppt) during the time course of a 2 days period. Experiments were conducted at the Civic Aquarium of Milan using a flow-through aquaria system. S. caliendrum responds differently to hyper- and hyposaline conditions at morphological and cellular levels and the response of corals to osmotic stress reflects the severity and duration of the disturbance. The Western blot analysis showed for each salinity a similar strong up-regulation of Hsp60 after the first 6 h of stress, but subsequently Hsp60 exhibited for each salinity treatment specific patterns of expression. In hypersalinity condition a negative trend of Hsp60 expression was observed, but the colonies showed a morphological appearance similar to healthy control colonies, suggesting a possible metabolic acclimation of corals to the stress. In S. caliendrum exposed to moderate hyposalinity, Hsp60 exhibited marked oscillation and the level of Hsp60 generally remained high over time indicating that cellular damages in the animal host were in progress. In extreme hyposalinity condition, a considerable gradual down-regulation of Hsp60 was detected until the end of the experiment. This was accompanied by extreme degradation and necrosis of coral tissues. Finally, we focused on the responses of Hsp60 to thermal stresses, initially analyzing the susceptibility of three coral genera (Montipora, Acropora and Seriatopora) to a severe heat stress of 36°C for 12 h. Despite the Hsp60 trend appeared similar, each genus displayed a different persistence of the Hsp60 signal, and so a different threshold of tolerance and resistance. Secondly, the sensitive S. caliendrum was subjected to a cold shock of 21°C, a moderate heat shock of 29°C and a severe heat shock of 34°C. The modulation of the Hsp60 at lowered temperatures are similar to those involved in very elevated temperature stress with an up-regulation after 6 h followed by a down-regulation when the cellular damage become irreparable. This is accompanied by the appearance of bleaching events. The mild heat shock of 29°C did not significantly affect the normal Hsp60 oscillatory pattern. With this study we proposed the application of the mitochondrial Hsp60 and the analysis of its modulation as an useful and accurate biomarker, to assess the effect of several types of stress in scleractinian corals, and to diagnose coral health prior that the coral bleaching occurred.

Epitaxial growth of semi-polar (11-22) AlGaN layers with different Al composition has been performed by using the metal organic chemical vapour deposition (MOCVD) technique. A two-step overgrowth technique has been developed to both improve the crystal quality and address the cracking issue simultaneously for the AlGaN growth. Comprehensive studies on the structural and optical properties of the AlGaN overgrowth layers have been performed. Semi-polar (11-22) AlGaN layers with various Al compositions have been obtained with two different approaches. The first one is the standard AlGaN growth on the planar (10-10) m-plane sapphire substrates with either AlN or GaN buffer layer. Semi-polar AlGaN multiple quantum wells (MQWs) with various QW thickness grown on this standard template show no clear blueshift in optical emission wavelength with increasing excitation power, indicating the absence of quantum-confined Stark effect (QCSE). However, this method results in inadequate crystal quality and serious wafer cracking in the epilayers. To address both issues, another approach - a two-step overgrowth technique - has been developed. With such overgrowth technique, we have achieved thick (> 2 μm) and crack-free semi-polar (11-22) AlGaN layers grown on the top of nearly but not yet fully-coalesced GaN overgrown on micro-rod arrayed templates. These overgrowth layers with the Al composition up to 55.3% exhibit the best crystal quality ever achieved compared to other reports. A comprehensive investigation on the reduction of defects including the basal-plane stacking faults (BSFs) and strain relaxation has been carried out on the AlGaN overgrowth layers. Detailed X-ray diffraction measurements in on-axis and off-axis planes for different AlGaN structures indicate significantly reduced dislocation density in the AlGaN overgrowth samples. The BSF density is also found to be greatly reduced with the overgrowth technique by performing both the (11-22) reciprocal space mapping (RSM) and the low temperature (LT) photoluminescence (PL) measurements. The in-plane and out-of-plane strain in the overgrown AlGaN layers have been calculated by adopting a triclinic unit cell model, exhibiting anisotropic strain along two primary in-plane directions and even compressive instead of tensile in-plane strain. The great strain relaxation has been attributed to the residual voids formed within the underlying GaN during the overgrowth process. It has been found that large lattice tilts between the AlGaN and the GaN are present, which also confirms dramatic strain relaxation along the in-plane direction. Furthermore, the optical properties of the AlGaN samples with Al composition ranging from 32.0% to 55.3% have been investigated systematically. Both the near-band-edge (NBE) and BSFs-related emission are studied by LT PL and room temperature cathodoluminescence (CL) measurements. The energy separation between the two emission peaks increases with increasing Al composition, which is mainly ascribed to the compositional discrepancy within BSF regions and enhanced QCSE also within the BSF regions. Temperature-dependent PL measurements have also been performed to study the exciton localization effect in both emission regions, showing the localization depth of the BSFs-related emission is larger than that of the NBE in each sample. More importantly, a detailed comparison study has been performed on these semi-polar AlGaN samples and their c-plane AlGaN counterparts with similar Al composition. All the semi-polar AlGaN samples show a clear reduction in exciton localization depth and PL linewidth compared with their c-plane counterparts. These results presented demonstrate that semi-polar (11-22) AlGaN may be more favourable to be employed towards deep UV laser diodes than c-plane AlGaN.

Magister Scientiae (Biodiversity and Conservation Biology)
To further investigate the strength of the association and the relative advantages of the association to both organisms, several manipulation experiments were set up. A cage experiment set up in the shallow subtidal zone showed that the coralline survived equally well without the winkle and did therefore not require the winkle or its empty shell for survival. A second controlled laboratory aquarium experiment was designed under both fluorescent (rich in blue light) and incandescent light (rich in red light) to ascertain whether the coralline had a preference for O. sinensis over the similar O. tigrina. This experiment was inconclusive as no recruitment was obtained under either of the light regimes. A third laboratory experiment was designed to determine whether the extra coralline weight had any possible advantage to the winkle, particularly against predation from the rock lobster Jasus lalandii. Results suggested that there were no apparent advantages to the winkle bearing the extra coralline load as adult O. sinensis bearing the coralline alga (3.7 ± 2.2 winkles 24hr-1) were equally prone to predation than those lacking the coralline (2.3 ± 1.9 winkles 24hr-1) (p = 0.184). Observations suggested instead that the convoluted nature of the coralline may indeed have promoted predation. We ultimately deduced that the high occurrence of the coralline on the shells of O. sinensis was probably due to the substantial overlap in the niches of the two organisms. This conclusion was supported by the high densities of juvenile O. sinensis combined with the high percent cover abundance of the coralline in intertidal rockpools. Understanding sexual reproduction in coralline algae as well as the life cycle of the winkle, ultimately provided insight into the postulated life cycle of this coralline-winkle association.
South Africa

To further investigate the strength of the association and the relative advantages of the association to both organisms, several manipulation experiments were set up. A cage experiment set up in the shallow subtidal zone showed that the coralline survived equally well without the winkle and did therefore not require the winkle or its empty shell for survival. A second controlled laboratory aquarium experiment was designed under both fluorescent (rich in blue light) and incandescent light (rich in red light) to ascertain whether the coralline had a preference for O. sinensis over the similar O. tigrina. This experiment was inconclusive as no recruitment was obtained under either of the light regimes. A third laboratory experiment was designed to determine whether the extra coralline weight had any possible advantage to the winkle, particularly against predation from the rock lobster Jasus lalandii. Results suggested that there were no apparent advantages to the winkle bearing the extra coralline load as adult O. sinensis bearing the coralline alga (3.7 ±
2.2 winkles 24hr-1) were equally prone to predation than those lacking the coralline (2.3 ±
1.9 winkles 24hr-1) (p = 0.184). Observations suggested instead that the convoluted nature of the coralline may indeed have promoted predation. We ultimately deduced that the high occurrence of the coralline on the shells of O. sinensis was probably due to the substantial overlap in the niches of the two organisms. This conclusion was supported by the high densities of juvenile O. sinensis combined with the high percent cover abundance of the coralline in intertidal rockpools. Understanding sexual reproduction in coralline algae as well as the life cycle of the winkle, ultimately provided insight into the postulated life cycle of this coralline-winkle association...

Abstract The ocean has been exploited for a much shorter duration than land, making it possible to assess current stocks against historical baselines, inferred by modeling catches over the past 400 years. Present numbers range from 33% (whales) to less than 5% (turtles) of their historical maxima. Whales have recently increased as a result of the 1982 moratorium on whaling, but many are still considered endangered. Although official records suggest that fish catch has plateaued since 1990, a thorough assessment indicates that catch likely reached a maximum in the early 1990s and has been declining ever since, primarily because of stock depletion. As on land, large species are removed first and the smaller primary consumers are removed only after initial depletion. This process is known as fishing down the food chain. Ramifying effects on the food web are particularly clear for corals, which suffer from algal overgrowth in locations of heavy fishing. Despite declines, few marine species are known to have become extinct, including just two mammal species.