Heterotrophic nitrification and aerobic denitrification inAlcaligenes faecalis strain TUD

Heterotrophic nitrification and aerobic and anaerobic denitrification byAlcaligenes faecalis strain TUD were studied in continuous cultures under various environmental conditions. Both nitrification and denitrification activities increased with the dilution rate. At dissolved oxygen concentrations above 46% air saturation, hydroxylamine, nitrite and nitrate accumulated, indicating that both the nitrification and denitrification were less efficient. The overall nitrification activity was, however, essentially unaffected by the oxygen concentration. The nitrification rate increased with increasing ammonia concentration, but was lower in the presence of nitrate or nitrite. When present, hydroxylamine, was nitrified preferentially. Relatively low concentrations of acetate caused substrate inhibition (K_I=109 M acetate). Denitrifying or assimilatory nitrate reductases were not detected, and the copper nitrite reductase, rather than cytochrome cd, was present. Thiosulphate (a potential inhibitor of heterotrophic nitrification) was oxidized byA. faecalis strain TUD, with a maximum oxygen uptake rate of 140–170nmol O₂·min^-1·mg prot^-1. Comparison of the behaviour ofA. faecalis TUD with that of other bacteria capable of heterotrophic nitrification and aerobic denitrification established that the response of these organisms to environmental parameters is not uniform. Similarities were found in their responses to dissolved oxygen concentrations, growth rate and ammonia concentration. However, they differed in their responses to externally supplied nitrite and nitrate.     

Bioenergetic examination of the heterotrophic nitrifier-denitrifier Thiosphaera pantotropha

The heterotrophic nitrifying-denitrifying bacterium Thiosphaera pantotropha is remarkable as it nitrifies and denitrifies simultaneously. With respect to nitrogenous compounds, whether nitrification or denitrification results in energy conservation is of interest. Proton translocation studies were performed to determine if energy was conserved by the bacterium during heterotrophic nitrification and denitrification. Hydrazine (N₂H _inf5 ^sup+ ) was employed as the heterotrophic nitrification substrate while nitrate, nitrite and nitrous oxide were used as denitrification substrates. Analysis of the data indicate that the bacterium does not conserve energy when hydrazine was the substrate. Conversely, energy was conserved when either nitrate, nitrite or nitrous oxide functioned as the oxidants during denitrification-dependent proton translocation experiments. Thiosphaera pantotropha thus is similar to other heterotrophic nitrifiers-denitrifiers in that it conserves energy while denitrifying but has not been observed to do so when heterotrophically nitrifying.     

Role of heterotrophic nitrifiers and aerobic denitrifiers in simultaneous nitrification and denitrification process: a nonconventional nitrogen removal pathway in wastewater treatment

Bacterial species capable of performing both nitrification and denitrification in a single vessel under similar conditions have gained significance in the wastewater treatment scenario considering their unique character of performing the above reactions under heterotrophic and aerobic conditions respectively. Such a novel strategy often referred to as simultaneous nitrification and denitrification (SND) has a tremendous potential in dealing with various wastewaters having low C : N content, considering that the process needs very little or no external carbon source and oxygen supply thus adding to its cost-effective and environmentally friendly nature. Though like other micro-organisms, heterotrophic nitrifiers and aerobic denitrifiers convert inorganic or organic nitrogen-containing substances into harmless dinitrogen gas in the wastewater, their ecophysiological role in the global nitrogen cycle is still not fully understood. Attempts to highlight the role played by the heterotrophic nitrifiers and aerobic denitrifiers in dealing with nitrogen pollution under various environmental operating conditions will help in developing a mechanistic understanding of the SND process to address the issues faced by the traditional methods of aerobic autotrophic nitrification–anaerobic heterotrophic denitrification.     

Aerobic denitrification in various heterotrophic nitrifiers

Various heterotrophic nitrifiers have been tested and found to also be aerobic denitrifiers. The simultaneous use of two electron acceptors (oxygen and nitrate) permits these organisms to grow more rapidly than on either single electron acceptor, but generally results in a lower yield than is obtained on oxygen, alone. One strain, formerly known as Pseudomonas denitrificans, was grown in the chemostat and shown to achieve nitrification rates of up to 44 nmol NH₃ min^–1 mg protein^–1 and denitrification rates up to 69 nmol NO _inf3 ^sup–1 min^–1 mg protein^–1.Unlike Thiosphaera pantotropha, this strain needed to induce its nitrate reductase. However, the remainder of the denitrifying pathway was constitutive and, like T. pantotropha, Ps. denitrificans probably possesses the copper nitrite reductase.     

Variation in Heterotrophic and Autotrophic Nitrifier Populations in Relation to Nitrification in Organic Soils

The occurrence of heterotrophic and autotrophic nitrifiers in Pahokee muck and the role of these organisms in the ecosystem were assessed by surveying their population densities under different field conditions and by observing the relationship of these populations with aerobic bacteria and soil moisture. Heterotrophic nitrifier populations varied from 2.0 × 10⁵ to 3.8 × 10⁶ bacteria per cm³ of muck in surface fallow (bare) Pahokee muck during the annual cycle. This population decreased 40-fold between the surface and the 60- to 70-cm depths of soil. Similar variations were noted with autotrophic nitrifier populations. Significant correlations were found between heterotrophic nitrifiers and both soil moisture and aerobic bacteria. These relationships did not exist for the autotrophic nitrifiers. In soil that had been heated to kill the autotrophic nitrifiers, while preserving a population of the heterotrophs, and then amended with sodium acetate or ammonium sulfate or both, no nitrate or nitrite accumulated, although significant increases in heterotrophic nitrifiers were detected. In unheated control soil, nitrate plus nitrite-N increased from 14.3 to 181 μg/g of wet soil, and 48 μg of nitrite-N per g was produced. These data suggest that the autotrophic nitrifiers were the sole population responsible for nitrification in Pahokee muck.     

Nitrogen Redox Metabolism of a Heterotrophic, Nitrifying-Denitrifying Alcaligenes sp. from Soil

Metabolic characteristics of a heterotrophic, nitrifier-denitrifier Alcaligenes sp. isolated from soil were further characterized. Pyruvic oxime and hydroxylamine were oxidized to nitrite aerobically by nitrification-adapted cells with specific activities (V_max) of 0.066 and 0.003 μmol of N × min⁻¹ × mg of protein⁻¹, respectively, at 22°C. K_m values were 15 and 42 μM for pyruvic oxime and hydroxylamine, respectively. The greater pyruvic oxime oxidation activity relative to hydroxylamine oxidation activity indicates that pyruvic oxime was a specific substrate and was not oxidized appreciably via its hydrolysis product, hydroxylamine. When grown as a denitrifier on nitrate, the bacterium could not aerobically oxidize pyruvic oxime or hydroxylamine to nitrite. However, hydroxylamine was converted to nearly equimolar amounts of ammonium ion and nitrous oxide, and the nature of this reaction is discussed. Cells grown as heterotrophic nitrifiers on pyruvic oxime contained two enzymes of denitrification, nitrate reductase and nitric oxide reductase. The nitrate reductase was the dissimilatory type, as evidenced by its extreme sensitivity to inhibition by azide and by its ability to be reversibly inhibited by oxygen. Cells grown aerobically on organic carbon sources other than pyruvic oxime contained none of the denitrifying enzymes surveyed but were able to oxidize pyruvic oxime to nitrite and reduce hydroxylamine to ammonium ion.     

Combined heterotrophic nitrification and aerobic denitrification in Thiosphaera pantotropha and other bacteria

Reports of the simultaneous use of oxygen and denitrification by different species of bacteria have become more common over the past few years. Research with some strains (e.g. Thiosphaera pantotropha) has indicated that there might be a link between this aerobic denitrification and a form of nitrification which requires rather than generates energy and is therefore known as heterotrophic nitrification. This paper reviews recent research into heterotrophic nitrification and aerobic denitrification, and presents a preliminary model which, if verified, will provide at least a partial explanation for the simultaneous occurrence of nitrification and denitrification in some bacteria.     

Modelling of biological processes during aerobic treatment of piggery wastewater aiming at process optimisation

A dynamic mathematical model was developed for the simulation of the aerobic treatment of piggery wastewater. This model includes the carbon oxidation, the nitrification and the denitrification. According to the experimental results obtained during this study, a modified version of the activated sludge model No. 1 has been developed. The model includes (1) nitrite as intermediate of nitrification and denitrification, (2) the distinction between the anoxic heterotrophic yield and the aerobic heterotrophic yield, respectively equal to 0.53 and 0.6 and (3) the first-order hydrolysis of the slowly biodegradable fraction. The calibration and the validation of the model was performed using experimental data from three experiments with two piggery wastewaters. A set of kinetic and stoichiometric parameters emerged from these tests. Except the kinetic of hydrolysis of the slowly biodegradable organic matter varying from 6 to 25 gCOD(gCODday)(-1), all other parameters were similar for all experiments. The dissolved oxygen concentration was identified as the main variable influencing the nitrite accumulation during nitrification. In the calibrated model, the oxygen half-saturation coefficient of the ammonium oxidisers (0.3g O(2)m(-3)) was lower than for the nitrite oxidisers (1.1 gO(2)m(-3)), leading to nitrite accumulation when the dissolved oxygen concentration was low. Simulations with the proposed model could be very useful for improved design and management of biological treatment of piggery wastewaters, particularly in case of partial nitrification to nitrite directly followed by denitrification.     

Nitrite oxidation in the Namibian oxygen minimum zone

Nitrite oxidation is the second step of nitrification. It is the primary source of oceanic nitrate, the predominant form of bioavailable nitrogen in the ocean. Despite its obvious importance, nitrite oxidation has rarely been investigated in marine settings. We determined nitrite oxidation rates directly in ¹⁵N-incubation experiments and compared the rates with those of nitrate reduction to nitrite, ammonia oxidation, anammox, denitrification, as well as dissimilatory nitrate/nitrite reduction to ammonium in the Namibian oxygen minimum zone (OMZ). Nitrite oxidation (⩽372 nM NO₂⁻ d⁻¹) was detected throughout the OMZ even when in situ oxygen concentrations were low to non-detectable. Nitrite oxidation rates often exceeded ammonia oxidation rates, whereas nitrate reduction served as an alternative and significant source of nitrite. Nitrite oxidation and anammox co-occurred in these oxygen-deficient waters, suggesting that nitrite-oxidizing bacteria (NOB) likely compete with anammox bacteria for nitrite when substrate availability became low. Among all of the known NOB genera targeted via catalyzed reporter deposition fluorescence in situ hybridization, only Nitrospina and Nitrococcus were detectable in the Namibian OMZ samples investigated. These NOB were abundant throughout the OMZ and contributed up to ∼9% of total microbial community. Our combined results reveal that a considerable fraction of the recently recycled nitrogen or reduced NO₃⁻ was re-oxidized back to NO₃⁻ via nitrite oxidation, instead of being lost from the system through the anammox or denitrification pathways.     

Degradation of n-Hexadecane and Its Metabolites by Pseudomonas aeruginosa under Microaerobic and Anaerobic Denitrifying Conditions

A strategy for sequential hydrocarbon bioremediation is proposed. The initial O₂-requiring transformation is effected by aerobic resting cells, thus avoiding a high oxygen demand. The oxygenated metabolites can then be degraded even under anaerobic conditions when supplemented with a highly water-soluble alternative electron acceptor, such as nitrate. To develop the new strategy, some phenomena were studied by examining Pseudomonas aeruginosa fermentation. The effects of dissolved oxygen (DO) concentration on n-hexadecane biodegradation were investigated first. Under microaerobic conditions, the denitrification rate decreased as the DO concentration decreased, implying that the O₂-requiring reactions were rate limiting. The effects of different nitrate and nitrite concentrations were examined next. When cultivated aerobically in tryptic soy broth supplemented with 0 to 0.35 g of NO₂⁻-N per liter, cells grew in all systems, but the lag phase was longer in the presence of higher nitrite concentrations. However, under anaerobic denitrifying conditions, even 0.1 g of NO₂⁻-N per liter totally inhibited cell growth. Growth was also inhibited by high nitrate concentrations (>1 g of NO₃⁻-N per liter). Cells were found to be more sensitive to nitrate or nitrite inhibition under denitrifying conditions than under aerobic conditions. Sequential hexadecane biodegradation by P. aeruginosa was then investigated. The initial fermentation was aerobic for cell growth and hydrocarbon oxidation to oxygenated metabolites, as confirmed by increasing dissolved total organic carbon (TOC) concentrations. The culture was then supplemented with nitrate and purged with nitrogen (N₂). Nitrate was consumed rapidly initially. The live cell concentration, however, also decreased. The aqueous-phase TOC level decreased by about 40% during the initial active period but remained high after this period. Additional experiments confirmed that only about one-half of the derived TOC was readily consumable under anaerobic denitrifying conditions.     

Aerobic denitrification: a controversy revived

During studies on the denitrifying mixotroph, Thiosphaera pantotropha, it has been found that this organism is capable of simultaneously utilizing nitrate and oxygen as terminal electron acceptors in respiration. This phenomenon, termed aerobic denitrification, has been found in cultures maintained at dissolved oxygen concentrations up to 90% of air saturation.The evidence for aerobic denitrification was obtained from a number of independant experiments. Denitrifying enzymes were present even in organisms growing aerobically without nitrate. Aerobic yields on acetate were higher (8.1 g protein/mol) without than with (6.0 g protein/mol) nitrate, while the anaerobic yield with nitrate was even lower (4 g protein/mol). The maximum specific growth rate of Tsa. pantotropha was higher (0.34 h^-1) in the presence of both oxygen (>80% air saturation) and nitrate than in similar cultures not supplied with nitrate (0.27 h^-1), indicating that the rate of electron transport to oxygen was limiting. This was confirmed by oxygen uptake experiments which showed that although the rate of respiration on acetate was not affected by nitrate, the total oxygen uptake was reduced in its presence. The original oxygen uptake could be restored by the addition of denitrification inhibitors.Dedicated to Professor Dr. H.-G. Schlegel on the occasion of his 60th birthday     

Aerobic denitrification by novel isolated strain using NO-₂-N as nitrogen source

Biological denitrification reaction can be achieved under aerobic environment. Few aerobic denitrifiers using nitrite as sole nitrogen source were identified. Using nitrite as the sole nitrogen source, this work assessed the denitrification activity of yy7, an aerobic heterotrophic denitrifier identified as Pseudomonas sp. (94% similarity) by 16S rRNA sequencing analysis. The logistic equation describes the cell growth curve, yielding a generation time of 2.9h at an initial 18 mg l(-1)NO(-)?-N. Reduction of NO(-)?-N was primarily achieved during its logarithmic growth phase, and was accompanied by an increase in suspension pH and near complete consumption of dissolved oxygen. Three genes relating to nirK, norB, and nosZ were noted to involve in isolate strain. Isolate yy7 can survive and remove up to 40 mg l(-1)NO(-)?-N and, hence, can be applied as an effective aerobic denitrifier during simultaneous nitrification and denitrification via nitrite processes.     

Nitrate reduction by Citrobacter diversus under aerobic environment

A new aerobic denitrifier, Citrobacter diversus, was isolated from both nitrification and denitrification sludge. To monitor the variation in the concentration of nitrogen oxides, aerobic denitrification by C. diversus was carried out in a batch reactor. When the nitrate concentration was greater than 180 mg N l⁻¹, the nitrate reduction rate became stable. The effect of the C/N ratio on the denitrification activity was also investigated. The results showed that the optimum denitrification activity was obtained when the C/N ratio was 4–5. The range of the C/N ratio was higher than that for traditional anoxic denitrification. The effect of the dissolved oxygen concentration was further studied; and it was found that the range of dissolved oxygen concentrations, both for specific growth rates and for specific denitrification rates, was 2–6 mg⁻¹. From these results, it can be concluded that both the concentration of dissolved oxygen and the C/N ratio are key factors in the aerobic denitrification by C. diversus. Received: 23 November 1999 / Received revision: 4 February 2000 / Accepted: 13 February 2000     

New pathways for ammonia conversion in soil and aquatic systems

Ammonia conversion processes are essential for most soil and aquatic systems. Under natural conditions, the many possible reactions are difficult to analyze. For example, nitrification and denitrification have long been regarded as separate phenomena performed by different groups of bacteria in segregated areas of soils, sediments or aquatic systems sequentially in time. It has now been established that strict segregation in place and time of the two processes is not necessary and that both denitrifiers and nitrifiers have versatile metabolisms. However, the rates described for aerobic denitrifiers are very low compared to the rates observed under anoxic conditions. Also the rates of nitrifier denitrification are quite low, indicating that these conversions may not play an important role under natural conditions. In addition, these processes often result in the emission of quite large amounts of undesirable products, NO and N₂O. Heterotrophic nitrification might be of relevance for systems, that contain a high carbon to nitrogen ratio. Recently, a novel process (Anammox) has been discovered in which ammonium serves as the electron donor for denitrification of nitrite into dinitrogen gas. ¹⁵N labeling studies showed that hydrazine and hydroxylamine were important intermediates in this process. Enrichment cultures on ammonium, nitrite and bicarbonate resulted in the dominance of one morphotypical microorganism. The growth rate of the cultures is extremely low (doubling time 11 days), but the affinity for ammonium and nitrite and the conversion rates (9.2 10^–4 mol kg^–1 s^–1) are quite high. Some of the reported high nitrogen losses in soil and aquatic systems might be attributed to anaerobic ammonium oxidation. In addition, this conversion offers new opportunities for nitrogen removal, when it is combined with recently developed processes for partial nitrification.     

Nitrogen transformation under different dissolved oxygen levels by the anoxygenic phototrophic bacterium <Emphasis Type="Italic">Marichromatium gracile</Emphasis>

Marichromatium gracile: YL28 (M. gracile YL28) is an anoxygenic phototrophic bacterial strain that utilizes ammonia, nitrate, or nitrite as its sole nitrogen source during growth. In this study, we investigated the removal and transformation of ammonium, nitrate, and nitrite by M. gracile YL28 grown in a combinatorial culture system of sodium acetate-ammonium, sodium acetate-nitrate and sodium acetate-nitrite in response to different initial dissolved oxygen (DO) levels. In the sodium acetate-ammonium system under aerobic conditions (initial DO?=?7.20–7.25 mg/L), we detected a continuous accumulation of nitrate and nitrite. However, under semi-anaerobic conditions (initial DO?=?4.08–4.26 mg/L), we observed a temporary accumulation of nitrate and nitrite. Interestingly, under anaerobic conditions (initial DO?=?0.36–0.67 mg/L), there was little accumulation of nitrate and nitrite, but an increase in nitrous oxide production. In the sodium acetate-nitrite system, nitrite levels declined slightly under aerobic conditions, and nitrite was completely removed under semi-anaerobic and anaerobic conditions. In addition, M. gracile YL28 was able to grow using nitrite as the sole nitrogen source in situations when nitrogen gas produced by denitrification was eliminated. Taken together, the data indicate that M. gracile YL28 performs simultaneous heterotrophic nitrification and denitrification at low-DO levels and uses nitrite as the sole nitrogen source for growth. Our study is the first to demonstrate that anoxygenic phototrophic bacteria perform heterotrophic ammonia-oxidization and denitrification under anaerobic conditions.     

15N Kinetic Analysis of N2O Production by Nitrosomonas europaea: an Examination of Nitrifier Denitrification

A series of ¹⁵N isotope tracer experiments showed that Nitrosomonas europaea produces nitrous oxide only under oxygen-limiting conditions and that the labeled N from nitrite, but not nitrate, is incorporated into nitrous oxide, indicating the presence of the “denitrifying enzyme” nitrite reductase. A kinetic analysis of the m/z 44, 45, and 46 nitrous oxide produced by washed cell suspensions of N. europaea when incubated with 4 mM ammonium (99% ¹⁴N) and 0.4 mM nitrite (99% ¹⁵N) was performed. No labeled nitrite was reduced to ammonium. All labeled material added was accounted for as either nitrite or nitrous oxide. The hypothesis that nitrous oxide is produced directly from nitrification was rejected since (i) it does not allow for the large amounts of double-labeled (m/z 46) nitrous oxide observed; (ii) the observed patterns of m/z 44, 45, and 46 nitrous oxide were completely consistent with a kinetic analysis based on denitrification as the sole mechanism of nitrous oxide production but not with a kinetic analysis based on both mechanisms; (iii) the asymptotic ratio of m/z 45 to m/z 46 nitrous oxide was consistent with denitrification kinetics but inconsistent with nitrification kinetics, which predicted no limit to m/z 45 production. It is concluded that N. europaea is a denitrifier which, under conditions of oxygen stress, uses nitrite as a terminal electron acceptor and produces nitrous oxide.     

Potential of Aerobic Denitrification by Pseudomonas stutzeri TR2 To Reduce Nitrous Oxide Emissions from Wastewater Treatment Plants

In contrast to most denitrifiers studied so far, Pseudomonas stutzeri TR2 produces low levels of nitrous oxide (N₂O) even under aerobic conditions. We compared the denitrification activity of strain TR2 with those of various denitrifiers in an artificial medium that was derived from piggery wastewater. Strain TR2 exhibited strong denitrification activity and produced little N₂O under all conditions tested. Its growth rate under denitrifying conditions was near comparable to that under aerobic conditions, showing a sharp contrast to the lower growth rates of other denitrifiers under denitrifying conditions. Strain TR2 was tolerant to toxic nitrite, even utilizing it as a good denitrification substrate. When both nitrite and N₂O were present, strain TR2 reduced N₂O in preference to nitrite as the denitrification substrate. This bacterial strain was readily able to adapt to denitrifying conditions by expressing the denitrification genes for cytochrome cd₁ nitrite reductase (NiR) (nirS) and nitrous oxide reductase (NoS) (nosZ). Interestingly, nosZ was constitutively expressed even under nondenitrifying, aerobic conditions, consistent with our finding that strain TR2 preferred N₂O to nitrite. These properties of strain TR2 concerning denitrification are in sharp contrast to those of well-characterized denitrifiers. These results demonstrate that some bacterial species, such as strain TR2, have adopted a strategy for survival by preferring denitrification to oxygen respiration. The bacterium was also shown to contain the potential to reduce N₂O emissions when applied to sewage disposal fields.Wastewater treatment processes produce one of the major greenhouse effect gases, nitrous oxide (N₂O) (7, 25, 30). The global warming potential of N₂O relative to that of carbon dioxide (CO₂) is 298 for a 100-year time horizon, and its concentration in the atmosphere continues to increase by about 0.26% per year (9). Nitrogen removal in wastewater treatment plants is essentially based on the activities of nitrifying and denitrifying microorganisms, both of which are inhabitants of activated sludge. Nitrifying bacteria aerobically oxidize ammonium to nitrite (NO₂⁻) and nitrate (NO₃⁻), which are then reduced anaerobically by denitrifying bacteria to gaseous nitrogen forms, such as N₂O and dinitrogen (N₂). It has long been known that N₂O can be produced during both nitrification and denitrification processes of wastewater treatment (3, 19, 23), but the cause of N₂O emission during the nitrification process was not clear. We recently showed, however, using activated sludge grown under conditions that mimicked a piggery wastewater disposal, that N₂O emission during the nitrification process depends on denitrification by ammonia-oxidizing bacteria (Nitrosomonas) (18). On the other hand, it is believed that denitrifying bacteria produce N₂O as a by-product when anaerobiosis is insufficient during the denitrification process, because N₂O reductase is the enzyme that is most sensitive to oxygen (6). Piggery wastewater, in particular, contains a high concentration of ammonia, and N₂O emission tends to take place during the nitrogen removal process (5, 10). Experiments on the removal of ammonia and organic carbon by the aerobic denitrifier Pseudomonas stutzeri SU2 (24) and the heterotrophic nitrifier-aerobic denitrifier Alcaligenes faecalis no. 4 (16, 17) have been reported as examples of bioaugmentation in piggery wastewater treatment. Reduction of N₂O emissions from pig manure compost by addition of nitrite-oxidizing bacteria has also been reported (11). However, there have been no reports of methods for reducing N₂O emissions by bioaugmentation using aerobic denitrifying bacteria.Takaya et al. isolated the aerobic denitrifying bacterium Pseudomonas stutzeri TR2 (26). The denitrification activity of strain TR2 was monitored in batch and continuous cultures, using denitrification and artificial wastewater media, and the strain was found to keep a distinct activity (producing N₂ from NO₃⁻) and to produce a very low level of N₂O at a dissolved oxygen (O₂) concentration of 1.25 mg liter⁻¹. Therefore, strain TR2 should be useful in the future for reducing N₂O emissions from wastewater treatment plants by bioaugmentation. To investigate the feasibility of using strain TR2 for future application to wastewater treatment processes, we examined its denitrification activity, N₂O production, growth rate, and expression of denitrifying genes in batch cultures, using a medium that mimics the composition found in nitrogen removal wastewater plants. Comparison of the properties of strain TR2 with those of well-characterized denitrifying bacteria revealed characteristics of the strain that favor denitrification, although it can also respire oxygen.     

Quantification of Ammonia-Oxidizing Bacteria and Factors Controlling Nitrification in Salt Marsh Sediments

To elucidate the geomicrobiological factors controlling nitrification in salt marsh sediments, a comprehensive approach involving sediment geochemistry, process rate measurements, and quantification of the genetic potential for nitrification was applied to three contrasting salt marsh habitats: areas colonized by the tall (TS) or short (SS) form of Spartina alterniflora and unvegetated creek banks (CBs). Nitrification and denitrification potential rates were strongly correlated with one another and with macrofaunal burrow abundance, indicating that coupled nitrification-denitrification was enhanced by macrofaunal burrowing activity. Ammonia monooxygenase (amoA) gene copy numbers were used to estimate the ammonia-oxidizing bacterial population size (5.6 × 10⁴ to 1.3 × 10⁶ g of wet sediment⁻¹), which correlated with nitrification potentials and was 1 order of magnitude higher for TS and CB than for SS. TS and CB sediments also had higher Fe(III) content, higher Fe(III)-to-total reduced sulfur ratios, higher Fe(III) reduction rates, and lower dissolved sulfides than SS sediments. Iron(III) content and reduction rates were positively correlated with nitrification and denitrification potential and amoA gene copy number. Laboratory slurry incubations supported field data, confirming that increased amounts of Fe(III) relieved sulfide inhibition of nitrification. We propose that macrofaunal burrowing and high concentrations of Fe(III) stimulate nitrifying bacterial populations, and thus may increase nitrogen removal through coupled nitrification-denitrification in salt marsh sediments.     

Evidence for the Cooccurrence of Nitrite-Dependent Anaerobic Ammonium and Methane Oxidation Processes in a Flooded Paddy Field

Anaerobic ammonium oxidation (anammox) and nitrite-dependent anaerobic methane oxidation (n-damo) are two of the most recent discoveries in the microbial nitrogen cycle. In the present study, we provide direct evidence for the cooccurrence of the anammox and n-damo processes in a flooded paddy field in southeastern China. Stable isotope experiments showed that the potential anammox rates ranged from 5.6 to 22.7 nmol N₂ g⁻¹ (dry weight) day⁻¹ and the potential n-damo rates varied from 0.2 to 2.1 nmol CO₂ g⁻¹ (dry weight) day⁻¹ in different layers of soil cores. Quantitative PCR showed that the abundance of anammox bacteria ranged from 1.0 × 10⁵ to 2.0 × 10⁶ copies g⁻¹ (dry weight) in different layers of soil cores and the abundance of n-damo bacteria varied from 3.8 × 10⁵ to 6.1 × 10⁶ copies g⁻¹ (dry weight). Phylogenetic analyses of the recovered 16S rRNA gene sequences showed that anammox bacteria affiliated with “Candidatus Brocadia” and “Candidatus Kuenenia” and n-damo bacteria related to “Candidatus Methylomirabilis oxyfera” were present in the soil cores. It is estimated that a total loss of 50.7 g N m⁻² per year could be linked to the anammox process, which is at intermediate levels for the nitrogen flux ranges of aerobic ammonium oxidation and denitrification reported in wetland soils. In addition, it is estimated that a total of 0.14 g CH₄ m⁻² per year could be oxidized via the n-damo process, while this rate is at the lower end of the aerobic methane oxidation rates reported in wetland soils.     

Simultaneous nitrification and denitrification in a mixed methanotrophic culture

Simultaneous nitrification and denitrification using a mixed methanotrophic culture was investigated. When both NO₃ ^–-N (108 mg l^–1) and NH₃-N (59 mg l^–1) were added into batch reactors, nitrate removal was complete within 10 h at the rate of 47 mg NO₃ ^–-N g VSS^–1 day^–1 when dissolved oxygen (DO) concentration was maintained at 2 mg DO l^–1. Ammonia removal started simultaneously with nitrate removal at a slower rate of 14 NH₃-N g VSS^–1 day^–1. No significant accumulation of nitrite or nitrate during ammonia utilization suggested the occurrence of simultaneous nitrification and denitrification.