Microbial Nitrate Reduction Research Articles

Nitrate removal rate, efficiency and pollution swapping potential of different organic carbon media in laboratory denitrification bioreactors

Laboratory denitrifying bioreactors, which use an organic carbon (C) rich media to enhance microbial reduction of nitrate (NO3−) to nitrogen (N) gases, are used worldwide to protect surface and groundwater. To highlight potential adverse effects of denitrifying bioreactors, NO3− removal rates (gNO3-Nm−3d−1 removed), NO3− removal efficiencies (% removed minus production of other N species) and release of greenhouse gases and solutes (ammonium (NH4+), phosphorus (P) and organic carbon (C)) were compared in this study using different media: lodgepole pine woodchips (LPW), cardboard, lodgepole pine needles (LPN), barley straw (BBS) and a soil control. Results showed that NO3− removals were consistently >99% for all media for initial leaching and steady-state periods. When pollution swapping was considered, this ranged from 67% for LPW to 95% for cardboard. Sustained P releases over the threshold for the occurrence of eutrophication were measured in all media. Greenhouse gas emissions were dominated by carbon dioxide (CO2) and methane (CH4) fluxes with little nitrous oxide (N2O) release due to the anaerobic conditions prevalent within the bioreactors. Comparisons of different media, under steady-state conditions, showed that C fluxes were highest for cardboard and BBS bioreactors. Carbon fluxes from cardboard bioreactors ranged from 11.6gCm−2d−1 to 13.9gCm−2d−1, whilst BBS emissions ranged from 3.9gCm−2d−1 to 4.4gCm−2d−1. These C emissions were correlated with the total surface area exposed within the media. Such investigations highlight the need to consider pollution swapping during the initial leaching period and should improve design criteria for field-scale bioreactors used to mitigate shallow groundwater NO3−.

Sulfide‐ and nitrite‐dependent nitric oxide production in the intestinal tract

SummaryIn the gut ecosystem, nitric oxide (NO) has been described to have damaging effects on the energy metabolism of colonocytes. Described mechanisms of NO production are microbial reduction of nitrate via nitrite to NO and conversion of l‐arginine by NO synthase. The aim of this study was to investigate whether dietary compounds can stimulate the production of NO by representative cultures of the human intestinal microbiota and whether this correlates to other processes in the intestinal tract. We have found that the addition of a reduced sulfur compound, i.e. cysteine, contributed to NO formation. This increase was ascribed to higher sulfide concentrations generated from cysteine that in turn promoted the chemical conversion of nitrite to NO. The NO release from nitrite was of the order of 4‰ at most. Overall, it was shown that two independent biological processes contribute to the chemical formation of NO in the intestinal tract: (i) the production of sulfide by fermentation of sulfur containing amino acids or reduction of sulfate by sulfate reducing bacteria, and (ii) the reduction of nitrate to nitrite. Our results indicate that dietary thiol compounds in combination with nitrate may contribute to colonocytes damaging processes by promoting NO formation.

Impact of elevated nitrate on sulfate-reducing bacteria: a comparative Study of Desulfovibrio vulgaris

Sulfate-reducing bacteria have been extensively studied for their potential in heavy-metal bioremediation. However, the occurrence of elevated nitrate in contaminated environments has been shown to inhibit sulfate reduction activity. Although the inhibition has been suggested to result from the competition with nitrate-reducing bacteria, the possibility of direct inhibition of sulfate reducers by elevated nitrate needs to be explored. Using Desulfovibrio vulgaris as a model sulfate-reducing bacterium, functional genomics analysis reveals that osmotic stress contributed to growth inhibition by nitrate as shown by the upregulation of the glycine/betaine transporter genes and the relief of nitrate inhibition by osmoprotectants. The observation that significant growth inhibition was effected by 70 mM NaNO(3) but not by 70 mM NaCl suggests the presence of inhibitory mechanisms in addition to osmotic stress. The differential expression of genes characteristic of nitrite stress responses, such as the hybrid cluster protein gene, under nitrate stress condition further indicates that nitrate stress response by D. vulgaris was linked to components of both osmotic and nitrite stress responses. The involvement of the oxidative stress response pathway, however, might be the result of a more general stress response. Given the low similarities between the response profiles to nitrate and other stresses, less-defined stress response pathways could also be important in nitrate stress, which might involve the shift in energy metabolism. The involvement of nitrite stress response upon exposure to nitrate may provide detoxification mechanisms for nitrite, which is inhibitory to sulfate-reducing bacteria, produced by microbial nitrate reduction as a metabolic intermediate and may enhance the survival of sulfate-reducing bacteria in environments with elevated nitrate level.

Future shock from the microbe electric

How can the future not look bright when you are dealing with a microbial process that can power a light bulb? The study of microbial fuel cells and, more generally, microbe–electrode interactions is rapidly amping up, not only in power production, but also in the number of investigators and areas of study.

Spatial sequencing of microbial reduction of chromate and nitrate in membrane bioreactor

Sequential reduction of chromate and nitrate, two competitive electron acceptors, has been demonstrated for strains of Pseudomonas genus for both planktonic cells and cells immobilised in agar layers on the surface of synthetic membrane. Denitrification occurs practically after chromate depletion. This order of reduction process is consistent with redox potentials of the respective reactions. In a membrane bioreactor, competitive inhibition results in nitrate transfer through the membrane without transformation. Thus the receiving phase is contaminated with nitrate. To address this problem, a membrane has been used for spatial sequencing of chromate and nitrate reduction. Bacterial cells were immobilised in two layers with each layer placed on opposing sides of the membrane. By this means, chromate reduction is localised into the layer contacting the feed phase while nitrate reduction occurs in the layer facing the receiving phase. As a result, only traces of the pollutants are detected in the receiving phase.

Functional Diversity and Electron Donor Dependence of Microbial Populations Capable of U(VI) Reduction in Radionuclide-Contaminated Subsurface Sediments

In order to elucidate the potential mechanisms of U(VI) reduction for the optimization of bioremediation strategies, the structure-function relationships of microbial communities were investigated in microcosms of subsurface materials cocontaminated with radionuclides and nitrate. A polyphasic approach was used to assess the functional diversity of microbial populations likely to catalyze electron flow under conditions proposed for in situ uranium bioremediation. The addition of ethanol and glucose as supplemental electron donors stimulated microbial nitrate and Fe(III) reduction as the predominant terminal electron-accepting processes (TEAPs). U(VI), Fe(III), and sulfate reduction overlapped in the glucose treatment, whereas U(VI) reduction was concurrent with sulfate reduction but preceded Fe(III) reduction in the ethanol treatments. Phyllosilicate clays were shown to be the major source of Fe(III) for microbial respiration by using variable-temperature Mössbauer spectroscopy. Nitrate- and Fe(III)-reducing bacteria (FeRB) were abundant throughout the shifts in TEAPs observed in biostimulated microcosms and were affiliated with the genera Geobacter, Tolumonas, Clostridium, Arthrobacter, Dechloromonas, and Pseudomonas. Up to two orders of magnitude higher counts of FeRB and enhanced U(VI) removal were observed in ethanol-amended treatments compared to the results in glucose-amended treatments. Quantification of citrate synthase (gltA) levels demonstrated a stimulation of Geobacteraceae activity during metal reduction in carbon-amended microcosms, with the highest expression observed in the glucose treatment. Phylogenetic analysis indicated that the active FeRB share high sequence identity with Geobacteraceae members cultivated from contaminated subsurface environments. Our results show that the functional diversity of populations capable of U(VI) reduction is dependent upon the choice of electron donor.

Microbial reduction of nitrate in the presence of nanoscale zero-valent iron

Microbial reduction of nitrate in the presence of nanoscale zero-valent iron (NZVI) was evaluated to assess the feasibility of employing NZVI in the biological nitrate treatment. Nitrate was completely reduced within 3 d in a nanoscale Fe(0)-cell reactor, while only 50% of the nitrate was abiotically reduced over 7 d at 25 °C. The removal rate of nitrate in the integrated NZVI-cell system was unaffected by the presence of high amounts of sulfate. Efficient removal of nitrate by Fe(II)-supported anaerobic culture in 14 d indicated that Fe(II), which is produced during anaerobic iron corrosion in the Fe(0)-cell system, might act as an electron donor for nitrate. Unlike abiotic reduction, microbial reduction of nitrate was not significantly affected by low temperature conditions. This study demonstrated the potential applicability of employing NZVI iron as a source of electrons for biological nitrate reduction. Use of NZVI for microbial nitrate reduction can obviate the disadvantages associated with traditional biological denitrification, that relies on the use of organic substrates or explosive hydrogen gas, and maintain the advantages offered by nano-particle technology such as higher surface reactivity and functionality in suspensions.

The distribution of nitrate 15N/ 14N in marine sediments and the impact of benthic nitrogen loss on the isotopic composition of oceanic nitrate

We report 15N/ 14N ratios of porewater nitrate in sediments from the Bering Sea basin, where microbial nitrate reduction has been identified as a significant sink for fixed nitrogen (N). Strong 15N enrichment in porewater nitrate is observed as one goes deeper in the sediments and nitrate concentration [ NO 3 - ] decreases ( δ 15N generally reaches 25–35‰). Analysis of profiles with a one-dimensional diffusion-reaction model yields organism-scale isotope effects for dissimilatory nitrate reduction ( ε cell) of 11‰ to 30‰, in the same range as measured in previous studies of cultures and the marine and lacustrine water column. Estimates of ε cell, while uncertain, show a negative correlation with bottom water [O 2]; we propose that this relates to the [ NO 3 - ] at the depth of denitrification. The N isotope effect at the scale of nitrate sediment–water exchange ( ε app) is ∼0‰ in two unreactive deep sites and is typically <3‰ at more reactive sites at various depths. ε app is much lower than ε cell because nitrate consumption is nearly complete at the sediment depth of denitrification, minimizing the escape of 15N-enriched nitrate from the sediments. In reactive sediments, this is due to rapid denitrification, while in less reactive sediments, it is due to greater diffusive distances for nitrate to the depth of denitrification. The data suggest that low bottom water [O 2] tends to yield more complete expression of ε cell at the sediment–water scale, due to higher [ NO 3 - ] at the depth of denitrification. While porewater ammonium-N isotopes were not measured, our porewater model suggests that, in sediments with high organic matter supply and/or low-[O 2] bottom waters, the efflux and subsequent oxidation of ammonium enriched in 15N by incomplete nitrification can significantly enhance the total net isotope effect of sedimentary N loss ( ε sed, equivalent to ε app but including ammonium fluxes). Model analysis of representative sedimentary environments suggests a global mean ε sed of ∼4‰ (∼2‰ if restricted to seafloor below 1 km depth).

Iron Sulfide and Pyrite as Potential Electron Donors for Microbial Nitrate Reduction in Freshwater Wetlands

The potential of iron sulfide minerals to function as electron donors for microbial nitrate reduction has important consequences for freshwater wetland ecosystems. Nitrate influxes in iron sulfide-rich soils, could ultimately lead to adverse processes, such as internal eutrophication or metal toxicity. In the present study, the potential of two iron sulfide minerals (pyrite and iron sulfide) to function as electron donors for microbial nitrate reduction was investigated in a bioreactor under anoxic, freshwater conditions. Crystalline pyrite did not function as an electron donor. In contrast, an active denitrifying microbial community developed upon addition of iron sulfide. The dominant process in the iron sulfide amended bioreactor was identified, based upon stoichiometric calculations, as the oxidation of sulfide to sulfate, at the expense of nitrate or nitrite. A clone library (16S rRNA gene) generated from the reactors biomass contained sequences of Thiobacillus-like, Geothrix-like, OP10-like, and Acidovorax-like bacteria, which covered 90% of all clones. Together these results suggest that the type of iron sulfide mineral is an important determinant for the occurrence of anoxic iron sulfide mineral oxidation, and Thiobacillus-species may be key players in nitrate-dependent iron sulfide dissolution in freshwater ecosystems.

Nitrification and denitrification in subalpine coniferous forests of different restoration stages in western Sichuan, China

Nitrification is the biological conversion of organic or inorganic nitrogen compounds from a reduced to a more oxidized state. Denitrification is generally referred to as the microbial reduction of nitrate to nitrite and further gaseous forms of nitric oxide, nitrous oxide and molecular nitrogen. They are functionally interconnected processes in the soil nitrogen cycle that are involved in the control of long-term nitrogen losses in ecosystems through nitrate leaching and gaseous N losses. In order to better understand how nitrification and denitrification change during the process of ecosystem restoration and how they are affected by various controlling factors, gross nitrification rates and denitrification rates were determined using the barometric process separation (BaPS) technique in subalpine coniferous forests of different restoration stages. The results showed that forest restoration stage had no significant effects on gross nitrification rates or denitrification rates (One-way ANOVA (analysis of variance), p < 0.05). There was no significant difference in the temperature coefficient (Q 10) for gross nitrification rate among all the forest sites (One-way ANOVA, p < 0.05). Gross nitrification rates were positively correlated with water content (p < 0.05), but not with soil pH, organic matter, total nitrogen, or C/N ratios. Denitrification rates in all the forest soils were low and not closely correlated with water content, soil pH, organic matter, or total nitrogen. Nevertheless, we found that C/N ratios obviously affected denitrification rates (p < 0.05). Results from this research suggest that gross nitrification is more responsible for the nitrogen loss from soils compared with denitrification.

Nitrogen removal capacity of wetlands: sediment versus epiphytic biofilms

Wetlands are important sinks for nutrients and constructed wetlands are current practice for stormwater treatment. For nitrogen, the main removal process is denitrification (microbial reduction of nitrate to nitrogen gas). The bacteria responsible for this process are mostly found in the sediments and in epiphytic biofilms growing on wetland macrophytes. This paper reports on a project which aimed at measuring denitrification potential in sediments and epiphyton in urban wetlands. This study showed that wetland sediments could support high rates of denitrification. Interestingly, the most polluted of the wetlands studied had the highest denitrification potential. The management implication from this result is that indicators of pollution, such as hydrocarbon levels, will not necessarily reflect the ability of a wetland to denitrify. Two of the wetlands were studied in more detail. Here the denitrification potential of the epiphyton on dominant macrophytes and sediments were measured. The results indicated that the potential denitrification activity of the epiphyton was comparable to those measured in the sediments. Hence, biofilms could play a significant part in removing nitrogen loads. This work contributes to a better knowledge of the functioning of wetlands. This will lead to improved design and management of wetlands used for treating stormwater.

Modeling in-situ uranium(VI) bioreduction by sulfate-reducing bacteria

We present a travel-time based reactive transport model to simulate an in-situ bioremediation experiment for demonstrating enhanced bioreduction of uranium(VI). The model considers aquatic equilibrium chemistry of uranium and other groundwater constituents, uranium sorption and precipitation, and the microbial reduction of nitrate, sulfate and U(VI). Kinetic sorption/desorption of U(VI) is characterized by mass transfer between stagnant micro-pores and mobile flow zones. The model describes the succession of terminal electron accepting processes and the growth and decay of sulfate-reducing bacteria, concurrent with the enzymatic reduction of aqueous U(VI) species. The effective U(VI) reduction rate and sorption site distributions are determined by fitting the model simulation to an in-situ experiment at Oak Ridge, TN. Results show that (1) the presence of nitrate inhibits U(VI) reduction at the site; (2) the fitted effective rate of in-situ U(VI) reduction is much smaller than the values reported for laboratory experiments; (3) U(VI) sorption/desorption, which affects U(VI) bioavailability at the site, is strongly controlled by kinetics; (4) both pH and bicarbonate concentration significantly influence the sorption/desorption of U(VI), which therefore cannot be characterized by empirical isotherms; and (5) calcium–uranyl–carbonate complexes significantly influence the model performance of U(VI) reduction.

Salivary nitrate – an ecological factor in reducing oral acidity

Human oral cavities represent a novel environment with a constant supply of concentrated nitrate. For humans, over 80% of dietary nitrate originates from fruit and vegetables. With a healthy, balanced diet, rich in fruit and vegetables, the concentration of nitrate in saliva can reach up to more than three times the European drinking water standard. The physiological function of the active excretion of salivary nitrate is unknown. Furthermore, little is known of the ecological function of oral nitrate and the effect on the oral environment during its subsequent oral microbial conversions. The objectives of the research were to investigate the effect on salivary pH coupled with oral microbial nitrate and/or nitrite reduction. Human saliva samples were incubated anaerobically in the presence of 111.0 mmol glucose (2%), with and without 1.5 mmol nitrate/nitrite, and pH and nitrate/nitrite consumption were measured during the time-course of the incubations. We found that anaerobic incubation of saliva containing a mixture of oral bacteria in the presence of nitrate/nitrite substrates and glucose resulted in a higher pH than was found in controls in the absence of nitrate/nitrite. These results suggest that the presence of these electron acceptors repressed acid fermentation, or increased alkali production, or consumed acid produced, thus reducing salivary acidity. This finding identifies salivary nitrate as a possible ecological factor in reducing oral acidity. The possibility that a symbiotic relationship between host nitrate excretion and nitrate-reducing microorganisms might help to protect against tooth decay should be explored further.

Potential rates and pathways of microbial nitrate reduction in coastal sediments

Nitrate reduction plays a key role in the biogeochemical dynamics and microbial ecology of coastal sediments. Potential rates of nitrate reduction were measured on undisturbed sediment slices from two eutrophic coastal environments using flow-through reactors (FTR). Maximum potential nitrate reduction rates ranged over an order of magnitude, with values of up to 933 nmol cm(-3) h(-1), whereas affinity constants for NO(3) (-) fell mostly between 200 and 600 microM. Homogenized sediment slurries systematically yielded higher rates of nitrate reduction than the FTR experiments. Dentrification was the major nitrate removal pathway in the sediments, although excess ammonium production indicated a contribution of dissimilatory nitrate reduction to ammonium under nitrate-limiting conditions.

Microbial nitrate respiration of lactate at in situ conditions in ground water from a granitic aquifer situated 450 m underground

ABSTRACTThere is widespread interest in developing methods to investigate in situ microbial activity in subsurface environments. Novel experiments based on single borehole push–pull methods were conducted to measure in situ microbial activity at the Äspö Hard Rock Laboratory (HRL). Microbial nitrate reduction and lactate consumption were measured at in situ conditions at a depth of 450 m in the HRL tunnel. A circulation system was used to circulate ground water from the aquifer through pressure‐maintaining flow cells containing coupons for biofilm growth. The system allows microbial investigations at in situ pressure, temperature and chemistry. Four experiments were conducted in which a combination of a conservative tracer, nitrate and lactate was injected into the circulation system. Rate of nitrate utilization was 5 µm h−1 without lactate and 13 µm h−1 with lactate. Lactate consumption increased from 30 to 50 µm h−1 with the addition of an exogenous electron acceptor (nitrate). Attached and unattached cells were enumerated using epifluorescence microscopy to calculate cell‐specific rates of activity. The biofilm had an average cell density of 1 × 106 cells cm−2 and there was an average of 6 × 105 unattached cells mL−1 in circulation. Cell‐specific rates of lactate consumption were higher than previously reported using radiotracer methods in similar environments. The differences highlight the importance of conducting microbial investigations at in situ conditions. The results demonstrate that an indigenous community of microbes survives at a depth of 450 m in the Fennoscandian shield aquifer with the potential to oxidize simple organic molecules such as lactate.

Geomicrobiological factors that control uranium mobility in the environment: Update on recent advances in the bioremediation of uranium-contaminated sites

Understanding the behavior of uranium (U) in the environment is essential not only for the protection of aquifers from U contamination but also for predicting the fate of U and other actinides disposed of in deep geological settings. It has long been believed that the redox chemistry of U can be simply predicted by thermodynamics and that the development of a low redox potential is a sufficient condition for U reduction. However, recent studies have demonstrated that redox transformations of U are controlled by kinetic factors that are strongly influenced by microbial activity. Although abiological U oxidation proceeds efficiently under oxygenic conditions, abiological reduction of U is inhibited by the formation of negatively charged U(VI)-CO3 complexes that prevail in nature. Phylogenetically diverse microorganisms are capable of enzymatically reducing U(VI)-CO3 complexes to form U(IV)-bearing minerals such as uraninite (UO2+x). The only abiological pathway currently known for the reduction of U(VI)-CO3 complexes involves the Fe(II) monohydroxo surface complex ≡FeIIIOFeIIOH0. This complex is mainly produced by the microbial reduction of Fe(III) in natural systems. Thus, U(VI) reduction is controlled both directly and indirectly, at least in part, by microbial activity. Several mechanisms of U oxidation under anoxic conditions have been revealed recently by laboratory and field studies. U(IV) is abiologically oxidized by Fe(III) and Mn(IV) oxides. Microbial reduction of nitrate to molecular nitrogen, which occurs following the depletion of O2, produces nitrogen intermediates including nitrite (NO2−), nitrous oxide (NO), and nitric oxide (N2O). Although the nitrogen intermediates oxidize U(IV), poorly crystalline Fe(III)-oxide minerals resulting from the oxidation of aqueous Fe(II) species by the nitrogen intermediates oxidize U(IV) more efficiently than the nitrogen intermediates alone. Remarkably, the formation of Ca-U(VI)-CO3 complexes resulting from increased levels of Ca2+ and/or HCO3− leads to the reoxidation of bioreduced U(IV) under reducing conditions. These geomicrobiological factors pose challenges in manipulating and/or predicting the mobility and fate of U in complex and heterogeneous environmental settings.

Role for Fe(III) Minerals in Nitrate-Dependent Microbial U(IV) Oxidation

Microbiological reduction of soluble U(VI) to insoluble U(IV) is a means of preventing the migration of that element in groundwater, but the presence of nitrate in U(IV)-containing sediments leads to U(IV) oxidation and remobilizaton. Nitrite or iron(III) oxyhydroxides may oxidize U(IV) under nitrate-reducing conditions, and we determined the rate and extent of U(IV) oxidation by these compounds. Fe(III) oxidized U(IV) at a greater rate than nitrite (130 and 10 microM U(IV)/day, respectively). In aquifer sediments, Fe(III) may be produced during microbial nitrate reduction by oxidation of Fe(II) with nitrite, or by enzymatic Fe(II) oxidation coupled to nitrate reduction. To determine which of these mechanisms was dominant, we isolated a nitrate-dependent acetate- and Fe(ll)-oxidizing bacterium from a U(VI)- and nitrate-contaminated aquifer. This organism oxidized U(IV) at a greater rate and to a greater extent under acetate-oxidizing (where nitrite accumulated to 50 mM)than under Fe(II)-oxidizing conditions. We showthatthe observed differences in rate and extent of U(IV) oxidation are due to mineralogical differences between Fe(III) produced by reaction of Fe(II) with nitrite (amorphous) and Fe(III) produced enzymatically (goethite or lepidocrocite). Our results suggest the mineralogy and surface area of Fe(III) minerals produced under nitrate-reducing conditions affect the rate and extent of U(IV) oxidation. These results may be useful for predicting the stability of U(IV) in aquifers.

Microbial reduction of selenate and nitrate: common themes and variations

A number of biochemically distinct systems have been characterized for the microbial reduction of the oxyanions, selenate (SeO(4)(2-)) and nitrate (NO(3)(-)). Two classes of molybdenum-dependent nitrate reductase catalyse the respiratory-linked reduction of nitrate (NO(3)(-)) to nitrite (NO(2)(-)). The main respiratory nitrate reductase (NAR) is membrane-anchored, with its active site facing the cytoplasmic compartment. The other enzyme (NAP) is water-soluble and located in the periplasm. In recent years, our understanding of each of these enzyme systems has increased significantly. The crystal structures of both NAR and NAP have now been solved and they provide new insight into the structure, function and evolution of these respiratory complexes. In contrast, our understanding of microbial selenate (SeO(4)(2-)) reduction and respiration is at an early stage; however, similarities to the nitrate reductase systems are emerging. This review will consider some of the common themes and variations between the different classes of nitrate and selenate reductases.

Complete conversion of nitrate into dinitrogen gas in co-cultures of denitrifying bacteria

In the past 10 years many molecular aspects of microbial nitrate reduction have been elucidated, but the ecophysiology of this process is hardly understood. In this contribution, our efforts to study the complex microbial communities and interactions involved in the reduction of nitrate to dinitrogen gas are summarized. The initial work concentrated on emission of the greenhouse gas nitrous oxide during incomplete denitrification by Alcaligenes faecalis. As more research methods became available, the fitness of A. faecalis could be tested in mixed cultures with other denitrifying bacteria, most notably with the nitrate-reducing bacterium Pseudomonas G9. Finally, the advancement of molecular diagnostic tools made it possible to survey complex microbial communities using specific primer sets for/and antibodies raised against the various NO(x) reductases. Given the enormous complexity of substrates and environmental conditions, it is evident that mixed cultures rather than single species are responsible for denitrification in man-made and natural ecosystems. However, it is surprising that even for the breakdown of a single compound, such as acetate, mixed cultures are responsible, and that the consecutive denitrification steps are commonly performed by mutualistic co-operating species. Our observations also indicate that we seldom know the identity of the major key players in the nitrogen cycle of these ecosystems.

Retracing the development of raw water quality in water works applying reactive controlled material flux analyses

The Fuhrberger Feld aquifer in northern Germany provides the majority of water supply for the city of Hannover. Although parts of the recharge area have received a strong intake of nitrate from agricultural activities during the past 40 years, the extracted raw waters show little sign of nitrate contamination due to microbial nitrate reduction coupled with iron sulfide oxidation in the aquifer. Accordingly, iron and sulfate concentrations, in particular, are temporarily objectively high. To evaluate the contribution of quantitatively important processes to the development of raw water quality, all relevant natural and anthropogenic induced processes were identified, quantified and stepwise integrated in a reactive controlled material flux model using PhreeqC.