Dissertations / Theses on the topic 'Denitrification'

Nitrate reductase (Nar) of F. canadensis is membrane-bound. Glucose is the most effective reductant to support nitrate uptake, and methyl and benzyl viologens are good electron donors to Nar both in intact cells and in membrane fractions. Nitrate uptake depends upon nitrate reduction, and requires the presence of active Nar. Nitrate transport depends upon the transmembrane pH gradient.
Oxygen reversibly inhibits nitrate uptake, and the minimal air saturation for this inhibition is about 2-4%. Oxygen inhibits denitrification at the level of nitrate transport rather than its reduction. The reduction of both nitric oxide (NO) and nitrous oxide by F. canadensis is relatively tolerant to oxygen, and its nitrite reductase (Nir) is much more sensitive to oxygen than the other reductases. Neither copper- nor heme-type Nir DNA probes from Pseudomonas species hybridized with the total DNA of F. canadensis, indicating that F. canadensis Nir may possess unique properties.
F. canadensis keeps the NO concentration very low under normal conditions. However, ionophores (carbonyl cyanide m-chlorophenylhydrazone (CCCP), carbonyl cyanide p-trifluoromethoxylphenylhydrazone (FCCP), and nigericin), high concentrations of nitrite, and low pH stimulate net NO production during reduction of nitrite. NO consumption by F. canadensis inhibited by several inhibitors. They are azide, cyanide, CCCP, FCCP, nigericin, sulfide, hydroxylamine, carbon monoxide, diethyldithiocarbamate, and Triton X-100. NO is toxic to Nor (nitric oxide reductase) only at concentrations $>$67 nM.
Studies on chloramphenicol inhibition of denitrification enzyme activity indicate that chloramphenicol inhibits denitrification at the levels of nitrate reduction and NO consumption in F. canadensis.

Denitrification is a process used by many bacterial species to support anaerobic respiration, where, faced with a lack of oxygen, energy is instead created from available nitrates. Arable soils with high nitrogen content, and commonly-used fertilisers, encourage this process. Unfortunately, the ultimate impact on the environment is negative since nitrous oxide gas, which emerges as a bi-product, escapes into the atmosphere where it presents a 300-fold greater danger for global warming than carbon dioxide. The aim of this thesis is find a way to estimate the level of nitrous oxide which may escape into the atmosphere from denitrifying soil. Traditionally, the chain of chemical reactions followed in the denitrification process is modelled using Michaelis-Menten kinetics. We begin this thesis by reviewing existing work, discussing some of its limitations and proposing various alterations. Later, we present a preliminary model of the oxygen distribution within a soil with the aim of identifying anaerobic micro-sites where bacteria can denitrify. Our first models consist of a solitary circle of oxygen-absorbing soil residing beneath ground level in an environment saturated with oxygen. We show that normal respiration occurs inside the circle except within a core anaerobic region where denitrification occurs. We extend the oxygen distribution model by generalising to multiple oxygen-absorbing regions. The model is then considered from two viewpoints. We either think of the model as an aggregated soil where each circle represents an individual aggregate surrounded by air. Or we think of the model as a solid non-aggregated soil, where each circle represents a high respiration area. For both of these viewpoints results are found for realistic parameters and levels of denitrification within the soil can be estimated.

Three membrane bioreactors, a low flux filter (LFF), a diafilter (DF), and an ion-exchange (IE) membrane bioreactor were used to treat water polluted with 50 ppm-N nitrate. The three systems were compared in terms of removal efficiency of nitrate, operational complexity, and overall quality of the treated water. In the low flux filter (LFF) membrane bioreactor an hemo-dialysis hollow fiber module was used and operated continuously for 29 days with a constant flux of permeate. The performance of the system was constant during the span of the experiment, which demonstrated that when the module was operated under constant low flux of permeate, the membrane filtration process was not affected by fouling. The removal rate of the LFF was 100% since the treated effluent did not contain nitrate or nitrite. The volumetric denitrification rate was 240 g-N day-1 m-3, which is within the range of denitrification rates obtained in tubular membrane modules. The treated effluent contained acetate, the carbon source of the biological process, and other inorganic nutrients, which showed that operating this ultrafiltration module at controlled flux did not improve the retention of these substances in the bioreactor. The same hemo-dialysis hollow fiber module employed in the LFF system was used in the diafilter (DF) membrane bioreactor. In the DF system, however, the membrane module was used as a contactor that separated the treated water and the bioreactor system, which allowed the transfer of solutes through the membrane porous structure and supported the growth of a biofilm on the membrane surface. The nitrate removal rate of the DF system increased from 76% to 91% during the 17 days assay. Unfortunately, this improvement could be attributed to microbial contamination of the water circuit because significant concentrations of the carbon source, acetate, nutrients, and nitrate were found in the treated effluent. The volumetric denitrification rate of the system was 200 g-N day-1 m-3, and the surface denitrification rate was lower than values previously reported for contactor membrane bioreactors. The results hereby presented do not evidence any advantage of operating the Filtral 20 ® membrane module as a contactor instead of as a filter such as in the LFF system. On the other hand, the third system herein presented, the IE membrane bioreactor, demonstrated several advantages of a contactor configuration but with a non-porous ion exchange membrane module in place of the Filtral 20 ®. As in a contactor system, the anion membrane provided a surface for biofilm growth, facilitated the transport of nitrate, and prevented mixing of treated water and bioreactor medium. Compared to the two previous systems, the most remarkable result of the IE was the reduction of secondary pollution in the treated water. The concentrations of phosphate and ethanol were zero and less than 1% of the concentration in the bioreactor, respectively. In addition, the IE system was less complex than the two other systems because the ion exchange membrane is non-porous. Therefore, unlike with porous contactors, it was not necessary to control the flux of treated water that could be lost through the bioreactor. The average surface denitrification rate of the IE system was 7.0 g-N day-1 m-2, which is higher than what had been reported for other contactor denitrification systems. However, because of the low surface to volume ratio of the membrane module that was used, the volumetric denitrification rate of the IE system was low, equivalent to 65 g-N day-1 m-3.
Master of Science

A study is reported which demonstrates that electron transport to the reductase reactions of denitrification in the bacterium Thiosphaera pantotropha can occur aerobically. Use of dark-type electrodes has demonstrated that the N₂O reductase enzyme of this organism is active under aerobic conditions, and that O₂ and N₂O reduction can occur simultaneously. The reduction of NO₃^- to N₂ gas, even under aerobic conditions, is shown to proceed via NO as an intermediate. It is concluded that the reaction of NO with O₂ must be sufficiently slow that it does not effectively compete with the reduction of NO to N₂O. The ability of T. pantotropha to catalyse aerobic NO₃^- reduction, the first step of the aerobic denitrification process, is shown to correlate with the expression of a NO₃^- reductase enzyme that is located in the periplasm. This periplasmic enzyme is expressed, and is active, under both aerobic and anaerobic conditions. A membrane bound NO₃^- reductase is also expressed, but only under anaerobic conditions, by this organism. This latter reductase resembles the NO₃^- reductase of Paracoccus denitrificans in respect of both its catalytic properties and the inhibition of activity in intact cells under aerobic conditions. Mutants of T. pantotropha that lack the membrane bound NO₃^- reductase, and not only retain but overproduce the periplasmic enzyme, have been obtained via Tn5 mutagenesis. The periplasmic NO₃^- reductase identified in T. pantotropha bears catalytic and structural similarities to an enzyme previously characterised in some strains of Rhodobacter capsulatus. The ability of strains of R. capsulatus to reduce NO to N₂O is reported together with evidence that there is a discrete NO reductase in this organism. The electron transport pathway to NO reductase has been elucidated. The first identification of a denitrifying strain of R. capsulatus is reported.

Nitrate is practically ubiquitous in waters abstracted for municipal potable water production in Europe due to decades of intensive agricultural practice. Ion exchange is principally selected to target abstracted waters with elevated nitrate concentrations. However, the cost associated with disposal of the waste stream has re-ignited interest in destructive rather concentrative technologies. This thesis explores the potential of membrane bioreactor (MBR) technology for the removal of nitrate from potable water. Two configurations are considered: an MBR to replace ion-exchange completely; and an MBR to treat the ion-exchange waste stream in-situ for re-use. For the replacement MBR, permeate quality can be affected by nitrite accumulation, micro-organism and carbon breakthrough. However, at steady-state and provided substrate addition was controlled, permeate quality was consistently high. Selection of an appropriate substrate was observed to improve permeability by a factor of three. Permeability was sustained within the MBR by adopting a dead-end filtration strategy having identified a relationship between filtered volume, flux and suspended solids concentration. Provided the filtered volume within a single filtration cycle did not exceed a set volume, the accumulated deposit was reversible. For the ion-exchange waste stream MBR, organic carbon breakthrough was considerable. However, the impact upon resin capacity was apparently limited when permeate was re-used for resin regeneration. Salt shocking did not induce permeability decline although some denitrification capacity was lost. Cost evaluation demonstrated that operating ion- exchange in parallel with MBR regenerant treatment was more cost effective than ion exchange with direct disposal.

Nitrogen and phosphorus are known to cause eutrophic conditions in lakes and rivers, resulting ultimately in deteriorating water quality in these natural systems. Nitrate poses a threat to the ecosystem and aquatic life, and also has an adverse impact on human health when present in water in large concentrations. Regulatory bodies such as the Federal EPA and state agencies are imposing increasingly stringent effluent standards on point sources to protect and preserve natural water bodies. Technologies using biological nutrient removal processes are being incorporated into the waste treatment scheme at most wastewater treatment plants in an attempt to limit nutrient discharge. The use of completely-submerged anoxic rotating biological contactors (RBCs) to remove NO₃-N is a relatively new concept, although RBCs have been used for removal of ammonia and biochemical oxygen demand (BOD) for some time. In this study, HDPE disks (10” x 9”) obtained from the Greater Peoria Sanitary District (GPSD) were used as RBC media and mounted on a shaft rotating at 1 rpm in two 20-liter enclosed reactors. At a flowrate of 45 liters per day, synthetic wastewater containing sodium citrate as the carbon source and nitrate as the electron acceptor was used as influent. The duration of each experiment was about 30 days, during which, overall nitrate removal and denitrification rate constants were estimated under different experimental conditions. Factors affecting startup growth were also identified.

Groundwater (GW) accounting for most of the freshwater available around the World, finding sustainable techniques to depollute it is of crucial importance for safe drinking water supply. The extensive use of fertilizers in the agriculture, as well as other anthropogenic activities, are contributing to the excessive nitrate levels in some aquifers. These levels need to be reduced to obtain potable water. Bioelectrochemical systems (BES), using microorganisms to catalyze a desired electrochemical reaction, recently proved to be a very promising technology for water remediation. Groundwater denitrification using Microbial Electrolysis Cell (MEC) needs to be improved for further scaled-up on-site system. The advantages conferred by fluidized bed reactor (FBR), as well as the outstanding electrochemical properties of reduced graphene oxide (rGO), are two potential enhancements of such bioelectrochemical denitrification system that were investigated in this thesis. Some essential parameters could be determined during the preliminary steps' experiments. The fluidization trials gave us a clear insight that Coconut-based Activated Carbon (CAC) particles were resistant carrier particles, nicely fluidized within a 39.27cm3 circular cathodic chamber for a flow rate ranging between 450ml/min to 590ml/min. For the same flow rate of 500ml/min, we could obtain CAC particles fluidization for the upstream fluidized configuration, and still bed particles for the fixed bed downstream configuration, which would be very useful for later unbiased comparison. The denitrifying bacteria showed during their enrichment, a nitrate removal rate of up to 1.986ppm NO3-N/h in serum bottles, with an average of 0.38ppm NO2-N/h accumulation. The parallel running of fixed bed versus fluidized bed denitrifying reactor in order to compare their denitrification performances, was planned, but could not be performed due to COVID-19. The graphene oxide (GO) batch experiments showed a good biocompatibility between GO/rGO and our autotrophic denitrifying bacteria. A change of morphology within about 20 hours was observed, probably suggesting the reduction of GO to rGO by the bacteria. During a first test, the presence of GO led to a 2.7 folds less efficient denitrification performance as compared with the GO/rGO-free condition, likely due to the competition between nitrate and GO for being reduced. However, the denitrification rate in presence of GO/rGO increased up to 1.873ppm NO3-N/h after the second pulse of groundwater and flush with H2/CO2 gas, which is almost 2.3 folds higher than initially in the same condition. This suggests that GO needs some time to get fully reduced to rGO, and the denitrification rate might reach the same or higher levels as in the GO/rGO-free conditions, when GO is fully reduced. Improved denitrification would indicate that rGO facilitates the electron transfer between bacteria and nitrate, as it can be expected from its electrochemical properties previously studied. This would be worth being investigated in the scope of a longer experience.
Grundvatten (GW) som står för det mesta av det sötvatten som finns tillgängligt runt om i världen och att hitta hållbara tekniker för att förorena det är av avgörande betydelse för en trygg dricksvattenförsörjning. Den omfattande användningen av gödselmedel i jordbruket, liksom andra antropogena aktiviteter, bidrar till de överdrivna nitratnivåerna i vissa vattenfiskare. Dessa nivåer måste sänkas för att erhålla dricksvatten. Bioelektrokemiska system (BES), med användning av mikroorganismer för att katalysera en önskad elektrokemisk reaktion, visade sig nyligen vara en mycket lovande teknik för sanering av vatten. Grundvatten denitrifikation med hjälp av Microbial Electrolysis Cell (MEC) måste förbättras för att ytterligare skala upp systemet på plats. Fördelarna med fluidiserad bäddreaktor (FBR) såväl som de enastående elektrokemiska egenskaperna hos reducerad grafenoxid (rGO) är två potentiella förbättringar av ett sådant bioelektrokemiskt denitrifikationssystem som undersöktes i denna avhandling. Vissa väsentliga parametrar kan bestämmas under de preliminära stegens experiment. Fluidiseringsstudierna gav oss en klar insikt om att kokosnötbaserade aktiverade kolpartiklar (CAC) -partiklar var resistenta bärarpartiklar, trevligt fluidiserade i en cirkulär katodisk kammare på 39,27 cm3 för en flödeshastighet mellan 450ml/min till 590ml/min. För samma flödeshastighet på 500ml/min kunde vi få CAC-partikelfluidisering för uppströms fluidiserad konfiguration och stillbäddspartiklar för den fixerade bädden nedströms konfiguration, vilket skulle vara mycket användbart för senare opartisk jämförelse. De denriffriserande bakterierna visade under deras anrikning en nitratborttagningshastighet av upp till 1,986 ppm NO3-N/h i serumflaskor, med ett genomsnitt på 0,38 ppm NO2-N / h ackumulering. Den parallella körningen av denitrifierande reaktorn med fast bädd kontra fluidiserad bädd för att jämföra deras denitrifikationsprestanda planerades, men kunde inte utföras på grund av COVID-19. Diagramexperimenten av grafenoxid (GO) visade en god biokompatibilitet mellan GO/rGO och våra autotrofiska denitrifierande bakterier. En förändring av morfologin inom cirka 20 timmar observerades, vilket antagligen antydde att bakterierna minskade GO till rGO. Under ett första test ledde närvaron av GO till 2,7 gånger mindre effektiv denitrifikationsprestanda jämfört med GO/rGO-fritt tillstånd, troligtvis på grund av konkurrensen mellan nitrat och GO för att ha minskat. Denitrifikationsgraden i närvaro av GO/rGO ökade emellertid upp till 1,873 ppm NO3-N/h efter den andra grundvattenspulsen och spolades med H2/CO2-gas, vilket är nästan 2,3 gånger högre än ursprungligen i samma tillstånd. Detta antyder att GO behöver lite tid för att helt reduceras till rGO, och denitrifikationsgraden kan nå samma eller högre nivåer som i GO/rGO-fria förhållanden, när GO är helt reducerad. Förbättrad denitrifikation skulle indikera att rGO underlättar elektronöverföring mellan bakterier och nitrat, som det kan förväntas av dess elektrokemiska egenskaper som tidigare studerats. Detta skulle vara värt att undersökas inom ramen för en längre upplevelse.

Denitrification is the sequential reduction of NO3- to N2, through a number of intermediary steps, one of which is N2O, a potent green house gas. N2O has a global warming potential over 300 times greater than that of CO2 over a 100 a year period (IPCC, 2007). Soils are a significant source of N2O through the microbially mediated process of denitrification. The rhizosphere is a potentially important source of N2O as rhizodeposited carbon from plant roots can support a larger and more active microbial biomass. Despite this little is known about the effects of low molecular weight carbon (LMWC) on the production of N2O or N2 or how the production of these gases can vary with the small scale variation in carbon quantity found in the rhizosphere. Studies using a soil microcosm revealed that not all LMWC compounds were able to stimulate the production of N2O. Of the three compounds studied the addition of glucose or glutamine to soil resulted in a greater production of N2O than occurred in the control, while the addition of citric acid did not. An experimental system was developed that would allow the creation of a carbon gradient over 6 cm and along which N2O and N2 could be quantified. This allowed an insight into the potential for small scale variation in denitrification. Similar spatial variation in N2O and N2 concentrations were found in both glucose and glutamine treated soil, while no spatial variation in these gasses was found in citric acid treated soil. Peak N2 concentrations occurred closer to the carbon source than peak N2O concentrations, potentially as a result of the higher C : N ratio. This was associated with a shift in the bacterial community, and in glucose treated soil with an increase in the proportion of bacteria containing nosZ or nirK. Despite this spatial patterns in N2O production remained similar even in experiments where the community did not change. The bacterial community did however exert an influence by affecting the magnitude of N2O and N2 produced. The results from this thesis suggest that denitrification has the potential to vary in the rhizosphere as a result of changes in carbon concentrations, carbon compounds and the associated changes in the microbial community.

Methane (CH4) and Nitrous Oxide (N2O) are both potent greenhouse gases which contribute to global warming by an estimated value of 20% and 6% respectively in addition to which their relative concentrations within the atmosphere are also on the increase at a rate of 0.8% and 0.3% yr-1 respectively over the past few decades (IPCC, 2007; Verma et al., 2006; Moiser et al., 1998). With the combined contribution of both CH4 and N2O constitute in relation to global warming, highlighting the importance of research into their reduction. Through the investigation of the process of methane oxidation and denitrification, both of which are bacterial processes which can lead to a reduction in the relative concentrations of both CH4 and N2O respectively within the atmosphere. Highlighting the importance of the research carried out within this thesis, in relation to the investigation of the potential coupling between methanotrophic and denitrifying bacteria, known as methanotrophic dependent denitrification (MDD) to occur within the environment. The initial experimental setup was designed to test whether soil derived and model denitrifying bacteria; Pseudomonas nitroreducens, Pseudomonas citronellolis and Paracoccus denitrificans, were able denitrify to N2 on the presumptive methanotrophic carbon exudates sodium acetate and sodium formate as their sole carbon source (Costa et a; 2000; Hanson & Hanson 1996; Rhee & Fuhs 1978). All of which was carried out in pure cultures, under ≤ 0.4 % O2 v/v headspace. 15N-labelling was carried out in order to obtain a more complete picture in relation to the production of N2, and the creation of a N2O:N2 product ratio for and whether there was any difference in the utilisation between the two carbon sources. The presented data demonstrated that Pseudomonas nitroreducens and Pseudomonas citronellolis were able to produce N2 on sodium acetate but not on sodium formate. This was followed by assessing how different oxygen headspace conditions 10, 3, 2, 1 and 0.4 % O2 v/v would affect the exudation of acetate by Type II soil derived methanotrophic bacteria Methylocystis parvus, Methylocystis rosea and Methylocystis trichosporium. The results demonstrated that the methanotrophic bacteria (i) exudate acetate (ii) the presence of the acetate exudate within the media was only detectable under microaerobic conditions of ≤ 2 % available O2 v/v within the headspace (iii) the concentration of acetate exudate present within the media increased as the available O2 v/v decreased from 2 % to ≤ 0.4 % O2 v/v available within the headspace. This was then followed by a series of experiments designed to assess the ability of Pseudomonas nitroreducens and Pseudomonas citronellolis denitrify to N2 on 0.22 μm filter sterilized acetate exudate bacteria from M. parvus, M. iv rosea and M. trichosporium grown under a ≤ 0.4 % O2 v/v headspace as their sole carbon source when also grown under the same ≤ 0.4 % O2 v/v headspace conditions. The results demonstrated that Pseudomonas nitroreducens and Pseudomonas citronellolis were able to denitrify on filter sterilized acetate exudate from the type II soil derived methanotrophic bacteria, under which the greatest concentration of acetate production was under a ≤ 0.4 % O2 headspace, demonstrating that the amount of acetate which is exuded by the Type II methanotrophic bacteria is great enough to support denitrification to N2. This was followed by a co-culture experimental setup in which both the methanotrophic and denitrifying bacteria were grown simultaneously in the same closed system, in which no bacterial mixing occurred.

Bacterial respiration generates the energy required for bacterial growth. Respiration is not only limited to oxygen but could be fuelled with nitrate in anaerobic environments. Upon signal reception bacteria adjust their respiratory pathway in short time by effectively regulating respiratory gene expression and subsequently engineering the complete removal of nitrite, nitric oxide and nitrous oxide from the cytoplasm. Comparison of anaerobic and aerobic gene expression data in continuous cultures of Paracoccus denitrificans revealed a majority of highly expressed genes were co-regulated by CPR/FNR type transcriptional regulators. Motif analysis of the upstream region showed similar patterns recognizable by FNR. P. denitrificans expresses three FNR type regulators that could potentially compete for cognate site binding. Three mutant strains of fnrP, nnrR and narR were used to investigate the transcriptional expression of genes involved in respiration. It was demonstrated that the transcriptional factor FnrP positively regulated the transcription of nar, nor and nap and repressed the expression of nos operon. NnrR positively regulated the nir and nor operons and inhibited the expression of the nar and nos operons, in the latter case due to substrate unavailability. Finally, NarR positively enhanced the expression of the nar operon during the initial stage of anaerobicity. Additionally the expression of the nir and nos operons was repressed in the ΔnarR strain suggesting that NarR may compete for the promoter binding sites and possibly repress the expression of those genes. Additionally, sub-optimal pH inhibited growth and repressed the expression of nirS, norB and nosZ resulting in detectable nitrous oxide emissions. Therefore, the transcription factors FnrP, NnrR and NarR compete for the binding sites upstream of the denitrification operons in a way that optimizes the metabolic rates of denitrification and subsequently eliminates the accumulation of toxic denitrification intermediates. Therefore a new model of regulation of denitrification in P. denitrificans is proposed and discussed.

Homogeneous soil slurries were employed for testing the regulating factors for aerobic denitrification. The presence of nitrite, at a relatively high concentration, was a strong inducer of aerobic nitrous oxide production, during which no dinitrogen evolution was measured. High concentrations of nitrite also appeared to inhibit reduction of nitrous oxide under more suitable denitrifying conditions (low oxygen), which resulted in a high denitrifier nitrous oxide-to-dinitrogen ratio. In contrast, dinitrogen production was efficient in near-anoxic slurries when nitrate or low concentrations of nitrite were present. Nitrous oxide and dinitrogen production in soil slurries exhibited various responses with the addition of different carbon compounds. Simple sugars (glucose and mannitol) induced the lowest nitrous oxide production whereas more complex substrates (glutamic acid and butyrate) induced more nitrous oxide under oxic conditions. In addition, no dinitrogen production occurred when slurries were incubated with more than 2% oxygen in the headspace, except when supplemented with butyrate. In addition to soil slurries a culture-based approach was adopted to investigate whether bacterial cells extracted from soil exhibited any aerobic denitrification activity. During the respiratory depletion of oxygen these extracted cells only initiated denitrification when oxygen concentration fell below 10 &'956;M, and once anoxic denitrification was highly efficient with little intermediates accumulating. However, anoxic cells, containing a fully functional denitrification pathway, appeared to sustain denitrification where re-exposed to oxygen. The resulting denitrification was highly perturbed in that nitrous oxide was the dominant product. The results suggest that aerobic denitrification is a possibility in soils and that nitrite might be a replacement. Dynamic changes in oxygen could lead to higher soil nitrous oxide production following an anoxic phase.

Denitrification was investigated during 1985 and 1986 in a stagnogleyic brown earth (Macmerry) and a stagnogley (Winton), both soil types widely used for cereal production in south-east Scotland. The crop was winter barley. Much of the work was devoted to the evaluation and attempted field application of the acetylene-inhibition technique. This involves blocking the terminal reduction of nitrous oxide to elemental nitrogen by acetylene concentrations in excess of 0.1% by volume; total denitrification loss is then estimated by measuring the resultant enhanced nitrous oxide flux at the soil surface. For prolonged periods during the growing season relatively high concentrations of nitrous oxide in the soil atmosphere indicated significant rates of denitrification. It proved impossible, however, to obtain an adequate distribution of acetylene throughout the profile, resulting in improbably low denitrification estimates. This finding casts doubt on any reported measurements using the acetylene- inhibition technique in wet soils with poor structure. The method was applied with more success on a freely-drained Darvel soil. An alternative technique was investigated for the gleyed soils: laboratory incubation of intact cores in an atmosphere containing acetylene. This produced increased estimates of denitrification but was found to introduce other uncertainties. It was concluded that further development was required before the technique could be used to give reliable quantitative results. Measured gaseous diffusion constants and nitrous oxide concentrations at depth were employed in the Fick?s Law calculation of nitrous oxide flux; calculated losses during 1986 ranged from 0.2-0.5 kg N /ha for direct-drilled Winton plots to 3.6-7.4 kg N /ha for normally-ploughed M acmerry soil. These were minimum values for total gaseous loss, since losses as elemental nitrogen were not recorded. A mathematical model of denitrification was developed. It depends on numerical solution of the differential equations governing the simultaneous steady-state diffusion and reduction of oxygen, nitrate, and nitrous oxide in a spherical m icro-environm ent (an aggregate where aggregates are present - an "effective aggregate" in a structureless soil). A model aggregated soil is pictured as an assembly of spherical aggregates with log-normally distributed aggregated radius and reductive potential; the radius of the effective aggregate in a structureless soil is determined by the density of air-filled pores. Intra?aggregate diffusion constants are calculated by a method which amounts to an assumption of parallel diffusion through all possible serial combinations of intra-aggregate pores. Reductive potentials for the various reactions considered in the model are assumed to be proportional to one another. The model predicts an approximately linear relationship between the denitrification rate of an assembly of soil aggregates and its anaerobic fraction calculated according to the method of Smith (1980); such a relationship has in fact been observed (Parkin and Tiedje, 1984). Model results illustrate the importance of soil structure: a model clay soil continues to denitrify at moisture potentials much lower (more negative) than a model sand. Calculated whole-soil denitrification rates range from 0 g N/ha/d to 5.2 kg N/ha/d. Reported field measurements range from 0 g N/ha/d to 4.5 kg N/ha/d.

BiOWiSHTM – Aqua is a blend of preserved multi-bacteria culture with the capability of denitrification. If an anaerobic nitrate rich activation procedure is used instead of the standard aerobic activation procedure, the denitrification rate is increased by 28 percent under the conditions of 30°C, 1C:1N, 200mg/L of carbon, and 200mg/L nitrogen.

This study was undertaken to investigate denitrification and nitrous oxide production in sediments and the key environmental factors influencing these within selected river systems of the LOIS (Land-Ocean Interaction Study) area in North-East England and southern Scotland. Seasonal and spatial trends were evident in both environmental and denitrification data measured monthly for 1.5 years along the Swale-Ouse system from source to tidal limits. Denitrification, measured in sediment cores using acetylene inhibition and expressed by unit area of sediment, increased with distance from source down to freshwater tidal limits. Results from a supplementary survey of the freshwater tidal reaches of the Yorkshire Ouse showed a decrease from the tidal limits. Denitrification activity showed a spring (March to May) peak, particularly in the lowland sites. The highest rate (883±134 µmol N m(^-2) h(^-1)) was measured on the River Wiske, a highly eutrophic lowland tributary to the Swale. A high degree of colinearity was evident between environmental variables, although a significant relationship between denitrification, nitrate and temperature was found through multiple regression. For comparison, measurements were made in the less populated Tweed river system. The seasonal and spatial trends evident in both the environmental and denitrification data from the River Tweed, under a more limited sampling programme, were generally consistent with those observed in the Swale-Ouse system. An intensive field investigation of 50 river sites showed that both potential denitrification rate and N(_2)O production in sediment slurries were positively correlated with nitrate water concentration, sediment water content and percentage of fine (<100 µm) sediment particles. An experimental study investigating the kinetic parameters for denitrification, found that sediment cores taken along the Swale-Ouse exhibited a saturation type curve with added nitrate. Apparent affinity and estimates of apparent maximum velocity for mixed populations of denitrifying bacteria showed an increase on moving downstream and were highest on the Wiske.

Thesis: M. Eng., Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2014.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 37-38).
This study investigates the occurrence of denitrifying soil bacteria in a bioretention system located in Singapore and containing a saturated anoxic zone intended to facilitate denitrification. Soil samples were collected from six depths within the rain garden, four of which were within the saturated anoxic zone. These samples were analyzed using endpoint PCR, targeting total bacterial 16S rRNA or a denitrification gene (nosZ) in order to determine presence or absence of denitrifying bacteria. Three dilutions were used to produce semiquantitative results for the abundance of denitrifying bacteria in a sample relative to samples from other depths. The highest numbers of nosZ amplicons per gram of soil were observed in the deeper levels of the saturated anoxic zone as well as within the root zone of the rain garden. Subsurface water samples from the saturated anoxic zone were also analyzed for oxidation-reduction potential, dissolved oxygen, and nitrogen and phosphorus species. Concentrations of nitrate and nitrite were below the detection limit for most samples, indicating consumption by denitrifying bacteria and high rates of removal for long detention times. Ammonia and phosphorus concentrations are of potential concern because they appear to increase within the saturated anoxic zone.
by Riana Larissa Kernan.
M. Eng.

Thesis (MTech (Chemical Engineering))--Cape Peninsula University of Technology, 2016.
Anthropogenic activities that utilise cyanide in various chemical forms have resulted in the disposal of cyanide-contaminated effluents into drainage systems that ultimately reach wastewater treatment plants (WWTP), without prior treatment. Cyanides (CN) and soluble salts could potentially inhibit biological processes in WWTP, which are responsible for the removal of contaminants from incoming wastewaters. The removal of nitrogenous compounds from such waters in processes such as nitrification and denitrification is among the core biological processes used to treat wastewaters in WWTP. Electroplating and mining industries are among the perpetrators of cyanide contamination of WWTP. The presence of these hazardous contaminants results in the alteration of metabolic functions of the microbial populations that are utilised in WWTP, thus rendering the wastewater treatment process ineffective. In this study, bacterial isolates that were able to carry out nitrification and aerobic denitrification under high salinity cyanogenic conditions were isolated from poultry slaughterhouse effluent. These strains were referred to as I, H and G. The isolated bacterial species were found to be able to oxidise ammonium nitrogen (NH4-N) in the presence of free cyanide (CN-) under halophilic conditions. Isolates I, H and G were identified using the 16S rDNA gene and were identified to be Enterobacter sp., Yersinia sp. and Serratia sp., respectively. Furthermore, Response Surface Methodology was used to optimise the physicochemical conditions suitable for the proliferation of the isolates for free-cyanide degradation, nitrification and aerobic denitrification.

This thesis investigates the mechanism of denitrification o f the Arctic lower stratosphere and the impact o f denitrification on ozone loss using the SLIMCAT chemical transport model. The development of a new microphysical model for the simulation of growth and sedimentation of large nitric acid trihydrate particles is also described. Model simulations of Arctic denitrification were carried out using thermodynamic equilibrium schemes based on the sedimentation of either nitric acid trihydrate or ice using different meteorological analyses. The severity and extent of denitrification in ice-based model runs was found to be highly sensitive to the meteorological analyses used whereas nitric acid trihydrate denitrification schemes exhibited considerably less sensitivity. The response of thermodynamic equilibrium and microphysical NAT-based denitrification to meteorological conditions has been studied in a series of short idealised simulations. It was found that microphysical denitrification was considerably more sensitive to the relative orientation of the polar vortex flow and the region of cold temperatures. A concentric vortex and cold region are required to promote the long particle growth times required for strong denitrification in the microphysical model. Reduced rates of denitrification were evident in the microphysical model at the highest altitudes. Results from the microphyical denitrification scheme were compared with in-situ and remote observations of denitrification for two recent cold Arctic winters. There was remarkable agreement between model and observations of both the magnitude and location of denitrification despite the simple volume-averaged nucleation rate used in the model. The limited range of observations did not allow further constraints to be placed on the microphysical model. Denitrification was found to enhance Arctic ozone loss by up to 30% during 1999/2000. Sensitivity studies o f the impact of denitrification on Arctic ozone loss were performed using thermodynamic nitric acid trihydrate denitrification schemes. Cumulative ozone depletion was found to increase non-linearly with increasing denitrification. Enhanced recovery of chlorine radicals to hydrogen chloride in strongly denitrified model runs offset reduced recovery to chlorine nitrate, limiting the impact of denitrification to the equivalent of 20 days additional ozone loss.

Haloarchaea are extremophiles, generally thriving at high temperatures and salt concentrations, thus, with limited access to oxygen. As a strategy to maintain a respiratory metabolism, many halophilic archaea are capable of denitrification. Among them are members of the genus Haloferax, which are abundant in saline/hypersaline environments. Based on the haloarchaeal genomes analysed, the genes involved in denitrification are grouped into three gene clusters (nar, nir-nor, nos) coding for denitrification enzymes NarGHI, NirK, qNor and NosZ. In case of incomplete denitrifiers, some of the genes or clusters are absent. Amon all haloarchaea analysed, three reported denitrifiers, H. mediterranei, H. denitrificans and H. volcanii were characterized with respect to their denitrification phenotype using a semi-automatic incubation system. Out of the species tested, only H. mediterranei was able to consistently reduce all available N-oxyanions to N2, while the other two released significant amounts of NO and N2 O, which affect tropospheric and stratospheric chemistries respectively. Also, H. mediterranei showed a well-orchestrated system of gene expression during denitrification, being Nar and Nos, both transcriptionally activated by hypoxia (and probably nitrate), while Nir and Nor expression require the presence of nitric oxide (and possibly nitrite) as well as Nos. The prevalence and magnitude of hypersaline ecosystems are on the rise due to climate change and anthropogenic activity. Thus, the biology of halophilic denitrifiers is inherently interesting, due to their contribution to the global nitrogen cycle, and potential application in bioremediation.

Thesis (Ph. D.)--Ohio State University, 2004.
Document formatted into pages; contains xiii, 234 p. Includes bibliographical references. Abstract available online via OhioLINK's ETD Center; full text release delayed at author's request until 2005 June 1.

Bioreactors are a low-cost water treatment technology to mitigate nutrient runoff from agricultural areas. Bioreactors are woodchip-filled trenches installed in the soil, which convert nitrate to dinitrogen, a harmless gas in the atmosphere. Nitrate removal and greenhouse gas production of five on-farm bioreactors were monitored for the first time in Queensland, to reduce the nutrient runoff to the Great Barrier Reef. The bioreactors effectively removed nitrate, with negligible emissions of greenhouse gases. This research expanded the knowledge on bioreactors installed in Subtropical and Tropical climates and contributed to the development of guidelines for the application of bioreactors on Queensland farms.

Thesis (M.S.)--Ohio State University, 2004.
Advisor: Timothy Granata, Dept. of Civil and Environmental Engineering and Geodetic Science. Includes bibliographical references (leaves 83-88). Available online via OhioLINK's ETD Center

In the last decades, nitrate pollution has become a major threat to groundwater quality. The consequences include health concerns and environmental impacts. Nitrate contamination is mainly derived from agricultural practices, such as the application of manure as fertilizer. The Osona area (NE Spain) is one of the areas vulnerable to nitrate pollution from agricultural sources. Nitrate is derived from intensive farming activities and the high nitrate content results in a loss of water availability for domestic uses. The most important natural nitrate attenuation process is denitrification. Denitrifying bacteria are generally heterotrophic and use carbon compounds as the electron donor. Nevertheless, a limited number of bacteria are able to carry out chemolithotrophic denitrification, and to utilize inorganic compounds. Several field studies have suggested by means of geochemical and/or isotopic data that denitrification in some aquifers is controlled by pyrite oxidation. However, the feasibility of pyrite-driven denitrification has been questioned several times in laboratory studies. This thesis is concerned with the role of pyrite in denitrification and its potential use as a bioremediation strategy. Earlier studies showed the occurrence of denitrification processes in a small area located in the northern part of the Osona region and suggested that sulfide oxidation had an important role in natural attenuation. Therefore, the first part of this thesis deals with the characterization of the denitrification processes occurring in the Osona aquifer and their spatial and temporal variations. Denitrification processes linked to pyrite oxidation were identified in some zones of the studied area by means of multi-isotopic methods integrated with classical hydrogeological methods. Nitrate removal from groundwater can be accomplished by the enhancement of in situ biological denitrification. One such bioremediation strategy is biostimulation, which involves the addition of suitable electron donors and/or energy sources to stimulate indigenous denitrifying microorganisms. The second part of this thesis is devoted to clarify the role of pyrite as electron donor for denitrification and to evaluate the feasibility of a bioremediation strategy based on pyrite addition to stimulate native denitrifying bacteria. Nitrate consumption in experiments amended with pyrite and inoculated with “Thiobacillus denitrificans” demonstrated that this bacterium is able to reduce nitrate using pyrite as the electron donor. The efficiency in nitrate removal and the nitrate reduction rate depended on the initial nitrate concentration, pH and pyrite grain size. High nitrate removal efficiency was attained in long-term flow-through experiments under laboratory conditions similar to those found in slow-moving, nitrate-contaminated groundwater. In addition, biostimulation experiments performed with sediments and groundwater from the Osona aquifer showed that the addition pyrite stimulated the activity of the indigenous microbial community and enhanced the nitrate removal. Furthermore, the long-term efficiency of the process was demonstrated. Hence, biostimulation with pyrite could be considered to remediate nitrate contamination in groundwater in future water management strategies, although further research is needed, especially at field scale. It is critical for the success of the bioremediation strategy that not only the processes but also the microbial populations and their changes induced by the bioremediation treatment be well understood. The addition of pyrite resulted in an increase in the proportion of denitrifying bacteria and both autotrophic and heterotrophic denitrifiers were stimulated. Bacterial populations closely related to the “Xanthomonadaceae” might probably be the dominant autotrophic denitrifiers that used pyrite as the electron donor in the biostimulated experiments. The N and O isotopic enrichment factors associated with the pyrite‐driven denitrification were computed and used to recalculate the extent of the natural nitrate attenuation in the Osona aquifer. This refinement becomes useful to predict the evolution of the contaminant in the aquifer, and it should be taken into account for potential implementation of induced remediation techniques. The isotopic approach was proved to be an excellent tool to identify and quantify natural denitrification processes in the field, and to monitor the efficacy of bioremediation strategies in the laboratory. In order to improve the long-term performance of potential bioremediation strategies based on pyrite-driven denitrification, it is necessary to know the contribution of attached and free-phase denitrifying bacteria to this process in aquifers. The last part of this thesis addresses the ability of “T. denitrificans” to grow and colonize pyrite surfaces. In the colonization experiments, attachment onto pyrite surface was required for at lest a small number of the cells in order to accomplish pyrite-driven denitrification. Nevertheless, both attached and planktonic cells probably contributed to the overall denitrification. However, the details of the relative roles of the two phases and the specific mechanisms remain to be addressed.
A les darreres dècades, la contaminació de nitrat de l’aigua subterrània ha esdevingut un dels principals problemes que afecten la qualitat dels recursos hídrics subterranis. La presència de nitrat a l’aigua subterrània està relacionada, principalment, amb pràctiques agrícoles, com per exemple, l’ús intensiu de purins com a fertilitzant orgànic. La comarca d’Osona (Catalunya) és una de les àrees declarades vulnerables a la contaminació per nitrat d’origen agrícola. El nitrat procedeix principalment de la intensa activitat agrícola i ramadera. El principal procés d’atenuació natural del nitrat és la desnitrificació. La majoria dels bacteris desnitrificants són heteròtrofs i usen compostos orgànics com a font d’electrons. Tanmateix, també existeixen bacteris desnitrificants autòtrofs que són capaços d’usar compostos inorgànics, com a donants d’electrons. Estudis de camp previs han demostrat a partir de dades geoquímiques i/o isotòpiques que en alguns aqüífers la desnitrificació està controlada per l’oxidació de pirita. Tot i així, hi ha estudis de laboratori que qüestionen la viabilitat de la desnitrificació lligada a l’oxidació de pirita (compost inorgànic donant d’electrons). En aquesta tesi s’estudia el paper de la pirita en la desnitrificació i la seva possible aplicació com a estratègia de bioremediació d’aigües contaminades amb nitrat. Estudis previs demostraren l’ocurrència de processos de desnitrificació a Osona, en una petita àrea situada en el sector nord de la zona d’estudi. Els resultats d’aquests estudis suggereixen que l’oxidació de sulfurs hi juga un paper molt important en l’atenuació natural. A la primera part de la tesi s’exposa la caracterització dels processos de desnitrificació que tenen lloc en l’aqüífer d’Osona, així com de les seves variacions temporals i espacials. S’han usat mètodes multi-isotòpics integrats amb mètodes hidrogeològics clàssics, que han servit per identificar l’existència de processos de desnitrificació lligats a l’oxidació de pirita en algunes zones de l’àrea d’estudi. L’eliminació del nitrat de l’aigua subterrània pot aconseguir-se mitjançant estratègies que incentiven la desnitrificació biològica in situ. Una de les estratègies de bioremediació in situ és la bioestimulació, que consisteix en afegir donants d’electrons o fonts d’energia apropiades per tal d’estimular l’activitat de bacteris desnitrificants existents en el medi. En la segona part de la tesi s’esbrina el paper de la pirita com a potencial donant d’electrons en els procés de desnitrificació i s’avalua la viabilitat d’una estratègia de bioremediació, basada en l’addició de pirita per estimular l’activitat de bacteris desnitrificants autòctons. Experiments amb pirita i inoculats amb “Thiobacillus denitrificans” han demostrat que aquest bacteri és capaç de reduir el nitrat utilitzant la pirita com a donant d’electrons. A més a més, s’ha determinat que l’eficiència i la velocitat de reducció del nitrat depèn de la seva concentració inicial, del pH i de la mida de gra de la pirita. En experiments de flux continu, de llarga durada que simulen el flux d’aigües subterrànies contaminades amb nitrat, s’han aconseguit altes eficiències en l’eliminació del nitrat. A més a més, s’han realitzat experiments de bioestimulación amb pirita usant aigua subterrània i sediments de l’aqüífer d’Osona. Aquests experiments han demostrat que afegint pirita s’aconsegueix d’estimular l’activitat dels bacteris desnitrificants autòctons i, per tant, induir i/o augmentar la desnitrificació. També, s’ha provat l’alta eficiència d’aquest procés a llarg termini. Per tant, l’ús de la pirita podria tenir-se en compte com a estratègia per a l’eliminació de nitrat en properes mesures de gestió dels recursos hídrics. Tanmateix, cal més investigació, i especialment, a escala de camp. Per aconseguir l’èxit amb la bioremediació cal conèixer amb detall els processos i les poblacions microbiològiques que intervenen en els canvis poblacionals associats al tractament de bioremediació. Els resultats dels experiments de bioestimulació han demostrat que els bacteris desnitrificants autòtrofs dominants en el sistema són probablement poblacions relacionades amb Xanthomonadaceae. L’addició de pirita puposa un increment en la proporció de bacteris desnitrificants i produeix l’estimulació tant dels desnitrificants autòtrofs com dels heteròtrofs. A més a més, s’han calculat els factors d’enriquiment isotòpic de N i O associats al procés de desnitrificació per oxidació de pirita. Aquests factors d’enriquiment s’han usat per recalcular el grau d’atenuació natural del nitrat que té lloc en l’aqüífer d’Osona. Aquest recàlcul pot ser d’utilitat a l’hora de predir l’evolució de la contaminació en l’aqüífer i cal tenir-lo en compte a l’hora d’implementar possibles tècniques de remediació induïda. Per tant, la metodologia isotòpica ha demostrat ser una eina excel∙lent per identificar i quantificar processos de desnitrificació natural i per monitoritzar l’eficàcia d’estratègies de bioremediació en el laboratori. Per millorar el rendiment i la durabilitat de potencials estratègies de bioremediació basades en la desnitrificació i oxidació de pirita és necessari conèixer la contribució relativa dels bacteris desnitrificants planctònics i dels bacteris adherits als sediments. Així, a l’última part de la tesi l’interès s’enfoca en l’avaluació de l’habilitat de “T. denitrificans” en colonitzar i créixer sobre la superfície de la pirita. Els resultats experimentals de colonització suggereixen que per aconseguir la desnitrificació amb pirita com a donant d’electrons, cal que una part dels bacteris s’adhereixi a la superfície de la pirita. No obstant, sembla que ambdós bacteris, adherits i planctònics, contribueixen al procés global. Tanmateix, queda pendent d’examinar el paper específic de cada tipus en la desnitrificació.

The reservoir size and pathway rates of the nitrogen (N) cycle have been deeply modified by the human enhancement of N fixation, atmospheric emissions, and climate warming, doubling the reactive nitrogen (Nr) available in the biosphere. Denitrification transforms nitrate into nitrogenous gas and thus removes Nr back to the atmospheric reservoir. Across ecosystems, there is still rather limited knowledge of the denitrification rates and their relationships with environmental factors and the N-transforming guilds, particularly, for the abundant cold and N-poor freshwater systems. The main goal of this thesis was to improve the current knowledge of denitrification in mountain lakes. In particular, we studied eleven pristine oligotrophic mountain lakes that have been affected by a high N deposition. The selected lakes showed a gradient of dissolved inorganic nitrogen in the water due to different lake productivity. We focused on spatial rather than in temporal variation, sampling during the ice-free period. Within lakes, we focused on the sediments because of their known higher denitrification rates than the water column. Specifically, we studied sediments near the deepest point of the lake, lithic biofilms from littoral cobbles, and littoral sediments from beds of isoetid and elodeid macrophytes, helophyte (Carex rostrata) belts, and rocky areas. We aimed to measure the denitrification activity as similar as possible to in situ conditions, with this aim we used little disturbed sediment cores and in situ temperature for the denitrification rate measurements. We characterized the environment by including proximal (benthic) and more distal (lake) descriptors to capture potential drivers acting at different spatial scales. Denitrification is part of the N cycle, and other processes facilitate (e.g. nitrification) or compete (e.g. DNRA) with this pathway. Using molecular tools, we quantified the guilds involved in the main N-transformation pathways in benthic habitats (article I) and related them to the denitrification rates (article IV). We have also developed a method appropriate for estimating denitrification rates in any aquatic system with retrievable sediment cores (article II). Finally, we showed the interest of quantifying the temperature dependence of the process at different degrees of substrate limitation but within the range — or a beat above — the in situ substrate levels, instead of the typical quantification at substrate saturation (article III). Following, there is a summary of the main findings. There is a complex N-transforming guild composition in benthic habitats of mountain lakes, which is deeply embedded in the overall prokaryotic community. These N-transforming guilds differ in the dominant pathway depending on the habitat and productivity of the lake, with the DNRA-denitrification dichotomy as the greatest differentia- tion (article I). The denitrification temperature dependence increases with nitrate limitation (article III). There is an average current denitrification rate of 1.5 μmol N O m-2 h-1 in the sediments of the Pyrenean lakes with higher activity in littoral than in the deep zones. The factors controlling current and potential denitrification rates differ. Current denitrification is controlled by the NO -/ NO - availability and secondarily by temperature; whereas potential denitrification is controlled by landscape productivity, the sulphate content, and the DNRA-denitrification (nrfA-nirS) competition (article IV). The best candidate drivers, i.e. temperature, organic matter quantity and quality, and spatio- temporal redox conditions affecting the overall prokaryotic community and the N-transforming guilds are discussed. The estimated denitrification rates are compared to other mountain lake sediments, discussing the spatial variations, the controls, and the methods used. Finally, some unsolved questions of the N cycle in mountain lakes are discussed. Overall this thesis contributes to increasing the knowledge of denitrification and other processes of the N cycle. The findings are probably not restricted only to mountain lakes encompassing other oligotrophic and remote ecosystems.
El ciclo del nitrógeno (N) ha sido profundamente modificado por la fijación industrial de N, el aumento de emisiones, y el calentamiento global, doblando el nitrógeno reactivo (Nr) disponible en la biosfera. La desnitrificación transforma el nitrato en gases nitrogenados eliminando Nr del sistema. Hay un conocimiento muy limitado de las tasas de desnitrificación, así como de los gremios microbianos implicados en la transformación de N, especialmente en sistemas oligotróficos de aguadulce. El principal objetivo de esta tesis es ampliar el conocimiento actual de la desnitrificación en lagos de montaña. Para ello, se estudiaron once lagos oligotróficos afectados por una alta deposición de N, que muestran un gradiente de nitrógeno inorgánico disuelto debido a una diferente productividad dentro de la oligotrofia. Concretamente se estudiaron los sedimentos del punto más profundo del lago, las biopelículas de las piedras litorales, y los sedimentos litorales de las áreas rocosas, de los cinturones de Carex rostrata, y de los lechos de macrófitas isoétidas y elodéidas. Se ha encontrado una compleja composición de gremios transformadores de N profundamente arraigada en la comunidad procariota general de los hábitats bentónicos. La ruta dominante de transformación de N cambia dependiendo del hábitat y la productividad del lago con la dicotomía DNRA-desnitrificación como mayor diferencia, con una dominancia de los desnitrificantes reductores de nitritos (nirS) en las capas superficiales de los sedimentos de los lagos someros, más cálidos y productivos. También una creciente dependencia de la temperatura en la desnitrificación al aumen- tar la limitación de nitratos. Se ha estimado una tasa promedio de desnitrificación de 1.5 μmol N2O m-2 h-1 en los sedimentos de los lagos pirenaicos, con mayor actividad en la zona litoral que en la profunda. Diferentes variables controlan las tasas de desnitrificación actuales y potenciales; las pri- meras están controladas por la disponibilidad de nitratos y secundariamente por la temperatura, las potenciales están controladas por la productividad del sistema, el contenido de sulfatos y la compe- tencia DNRA-desnitrificación (nrfA-nirS). Esta tesis contribuye a aumentar el conocimiento del ciclo del N, y probablemente, los resultados obtenidos son extrapolables a otros ecosistemas oligotróficos y de áreas remotas.

La contamination des aquifères de proche subsurface par les intrants d'origine agricole (nitrates) est un problème mondial.L'utilisation excessive d'engrais depuis plusieurs décennies a impacté la qualité des masses d'eau souterraines et soulève des enjeux pour la santé humaine comme pour celle des écosystèmes. Les nitrates dans les aquifères peuvent être réduits en diazote gazeux par l'activité microbienne hétérotrophique (la biomasse microbienne obtenant l'énergie nécessaire à ce processus via le carbone organique issu de la surface) et/ou par l'activité autotrophique (la biomasse microbienne obtenant cette fois ci son énergie depuis une source proche, lithologique). Les taux de dénitrification sont très variables spatialement, et sont régulés par l'interaction entre la structure des flux d'eau souterrains avec l'activité biogéochimique. Localiser l'activité biogéochimique dans les aquifères est difficilement réalisable à l'échelle des bassins versants, mais paraît crucial pour la gestion des masses d'eau souterraines. Bien que les processus de l'activité microbienne ne puissent pas être entièrement résolus à l'échelle locale, ce manuscrit de thèse propose une caractérisation des taux de dénitrification à l'échelle du bassin versant, basée sur l'analyse de données et sur une approche de modélisation intégrée. Cette thèse propose d'utiliser de manière extensive des traceurs conservatifs et réactifs associés aux flux d'eau souterraine et des modèles de transport afin d'identifier les contrôles géologiques et biogéochimiques sur les capacités de dénitrification dans les aquifères. Cette méthodologie a été appliquée à un aquifère libre cristallin de 76 km² situé en Bretagne. A partir des concentrations en CFC-12, O2, NO3- et N2 dissous mesurées dans 16 puits, il a été possible de reconstituer les chroniques d'apports de nitrate dans la zone saturée et de définir les variations spatio-temporelles de la dénitrification. Il est prouvé ici que la dénitrification est en premier lieu contrôlée par la position des donneurs d'électron. Ce travail propose un cadre d'interprétation général sur la base de l'utilisation combinée et complémentaire des traceurs et sur la modélisation semi-explicite pour estimer à l'échelle régionale les capacités de dénitrification et les stocks de nitrates dans les aquifères
Unconfined shallow aquifers in agricultural areas are contaminated by nitrates worldwide. Excessive fertilization over the last decades has affected groundwater quality as well as human and ecosystem wellbeing. Nitrate in groundwater can be microbially reduced to dinitrogen gas by heterotrophic (microbes obtaining their energy from surface-derived organic carbon) and autotrophic (microbes obtaining their energy from a lithological source) processes. However, denitrification rates are highly spatially variable, following involved interactions between groundwater flow structures and biogeochemical activity. The location of biogeochemical activity in the aquifer is difficult to access at the catchment scale, but of vast importance to gain predictive capabilities for groundwater management. Even though microbial processes cannot be resolved at the local scale, this dissertation proposes a catchment scale characterization of denitrification rates based on an integrated model- and data-driven approach. The dissertation proposes an extensive use of conservative and reactive tracers combined with groundwater flow and transport models to identify the geological and biogeochemical controls on aquifer denitrification capacities. The methodology is applied to a crystalline unconfined aquifer of 76 km2 size in Brittany, France. Based on CFC-12, O2, NO3-, and dissolved N2 concentrations measured in 16 wells, it is possible to reconstruct historical nitrate inputs to the saturated zone and to define spatiotemporal denitrification activity. It is shown that denitrification is primarily controlled by the location of electron donors. The dissertation proposes a general interpretation framework based on tracer information combined with complementary semi-explicit lumped parameter models to assess regional denitrification capacities and nitrate legacy