Ammonia oxidation coupled nitrate reduction driven by bio-iron sludge in low-carbon mode: Performance and mechanisms

The effects of temperature, water potential and ammonium concentrations were studied in field and laboratory experiments on arable soil. The two field experiments used different sampling intervals, one at daily (short-term) and the other at monthly (long-term) intervals. In the short-term field experiment, the numbers and activities of nitrifiers were assessed before and after natural rain or irrigation. The nitrifiers were apparently outcompeted by heterotrophs during the first days after wetting the soil. Potential nitrification was affected only slightly by changes in water potential, whereas the numbers of ammonium and nitrite oxidizers appeared more sensitive to these changes. The numbers of ammonium and nitrite oxidizers correlated strongly during the daily samplings. The potential nitrite-oxidation rates correlated with water potentials whereas the potential ammonium oxidation rates did not. Extractable ammonium decreased in proportion to increasing nitrate concentrations in both the rain-fed and the irrigated plots. In the long-term field experiments, the numbers of ammonium oxidizers correlated with water potentials but not with in situ temperature or with ammonium concentrations. The potential ammonium-oxidation rates correlated with water potentials and with ammonium-oxidizer numbers. The potential nitrite-oxidation rates correlated strongly with the potential ammonium-oxidation rates. The field experiments implied that nitrite oxidizers obtained substrate from ammonium oxidizers but also from nitrate reduction. In laboratory experiments nitrate accumulated at a Q 10 of about 2 and the V max for nitrification was observed at a water potential of −0.11 MPa (65% of water-holding capacity). The K m for ammonium oxidation at pH 8.2 was 1.72 mg l−1 soil water.

Nitrous oxide (N2O) is a strong greenhouse gas and an ozone depleting agent. In marine environments, N2O is produced biologically via ammonium oxidation, nitrite, and nitrate reduction. The relative importance of these principle production pathways is strongly influenced by oxygen availability. We conducted 15N tracer experiments of N2O production in parallel with measurements of N2O concentration and natural abundance isotopes/isotopomers in Saanich Inlet, a seasonally anoxic fjord, to investigate how temporal and vertical oxygen gradients regulate N2O production pathways and rates. In April, June, and August 2018, the depth of the oxic‐anoxic interface (dissolved oxygen = 2.5 μmol L−1 isoline) progressively deepened from 110 to 160 m. Within the oxygenated and suboxic water column, N2O supersaturation coincided with peak ammonium oxidation activity. Conditions in the anoxic deep water were potentially favorable to N2O production from nitrate and nitrite reduction, but N2O undersaturation was observed indicating that N2O consumption exceeded rates of production. In October, tidal mixing introduced oxygenated water from outside the inlet, displacing the suboxic and anoxic deep water. This oxygenation event stimulated N2O production from ammonium oxidation and increased water column N2O supersaturation while inhibiting nitrate and nitrite reduction to N2O. Results from 15N tracer incubation experiments and natural abundance isotopomer measurements both implicated ammonium oxidation as the dominant N2O production pathway in Saanich Inlet, fueled by high ammonium fluxes (0.6–3.5 nmol m−2 s−1) from the anoxic depths. Partial denitrification contributed little to water column N2O production because of low availability of nitrate and nitrite.

The intramolecular distribution of nitrogen isotopes in N2O is an emerging tool for defining the relative importance of microbial sources of this greenhouse gas. The application of intramolecular isotopic distributions to evaluate the origins of N2O, however, requires a foundation in laboratory experiments in which individual production pathways can be isolated. Here we evaluate the site preferences of N2O produced during hydroxylamine oxidation by ammonia oxidizers and by a methanotroph, ammonia oxidation by a nitrifier, nitrite reduction during nitrifier denitrification, and nitrate and nitrite reduction by denitrifiers. The site preferences produced during hydroxylamine oxidation were 33.5 +/- 1.2 per thousand, 32.5 +/- 0.6 per thousand, and 35.6 +/- 1.4 per thousand for Nitrosomonas europaea, Nitrosospira multiformis, and Methylosinus trichosporium, respectively, indicating similar site preferences for methane and ammonia oxidizers. The site preference of N2O from ammonia oxidation by N. europaea (31.4 +/- 4.2 per thousand) was similar to that produced during hydroxylamine oxidation (33.5 +/- 1.2 per thousand) and distinct from that produced during nitrifier denitrification by N. multiformis (0.1 +/- 1.7 per thousand), indicating that isotopomers differentiate between nitrification and nitrifier denitrification. The site preferences of N2O produced during nitrite reduction by the denitrifiers Pseudomonas chlororaphis and Pseudomonas aureofaciens (-0.6 +/- 1.9 per thousand and -0.5 +/- 1.9 per thousand, respectively) were similar to those during nitrate reduction (-0.5 +/- 1.9 per thousand and -0.5 +/- 0.6 per thousand, respectively), indicating no influence of either substrate on site preference. Site preferences of approximately 33 per thousand and approximately 0 per thousand are characteristic of nitrification and denitrification, respectively, and provide a basis to quantitatively apportion N2O.

Nitrification and denitrification are two critical processes in the global nitrogen cycle. Many microorganisms have been shown to contribute to these biological nitrogen transformation processes. However, our understanding of the nitrogen cycle is still evolving. In the recent years, novel microorganisms and metabolic pathways involved in the nitrogen cycle have been discovered.n Therefore, the overall aim of this thesis is to enrich and characterise novel microorganisms involved in the nitrogen cycle, in order to improve our understanding of the microbial nitrogen conversion processes and their interactions with other important nutrients cycles including carbon and metals.In terms of ammonium oxidation, ammonia-oxidising archaea (AOA) are critical ammonium oxidizers that regulate the nitrogen transformation in ubiquitous environments. AOA may have advantages when competing with ammonia-oxidising bacteria (AOB) in certain extreme environments such as acidic soils. Although 30% of the soils in the world are acidic soils, our understanding of the AOA living under low pH is insufficient. Therefore, the first research objective of this thesis is to enrich and identify a novel AOA under low pH. Using a fresh water reservoir sediments as inoculum, two novel microorganisms (an AOA and a NOB) were enriched in a bioreactor operated at pH 4.5, and proved to play a critical role in ammonium oxidation to nitrate. The AOA strain was assigned to the genus of Nitrosotalea and identified as a novel species Candidatus Nitrosotalea sp. GC1, while the NOB was clustered into a novel lineage within genus Nitrospira. According to the metagenomic analysis on Candidatus Nitrosotalea sp. GC1, the genes encoding enzymes for ammonium oxidation to nitrite were all identified and the Thaumarcheal HP/HB pathway was used for carbon fixation.As a member affiliating in the genus of Nitrosotalea, Candidatus Nitrosotalea sp. GC1 might have similar features to the other members, such as its acidophily, substrate affinity and nitrous oxide (N2O) production. As a critical step to help understand this novel AOA culture, the second research objective of this thesis is to characterise the effect of environmental conditions on Candidatus Nitrosotalea sp. GC1 and its N2O emission potential. With a series batch tests, the acidophily of strain GC was verified, with an optimal pH of 4.5. The Km of ammonium and O2 for Candidatus Nitrosotalea sp. GC1 are 39.7 p 3.2 and 35.2 p 3.0 mM, respectively, which were much higher than other AOA isolates, although still lower than most AOB. In addition, N2O emission from Candidatus Nitrosotalea sp. GC1 has a remarkable yield of 6.7%, which suggested the critical role of acidophilic AOA in greenhouse gas emission.In terms of nitrate reduction, dissimilatory nitrate reduction to ammonium (DNRA) and denitrification are the main processes that convert nitrate to ammonium or inert dinitrogen gas respectively. Although organic carbon compounds are the main energy and electron sources for the most of known DNRA microorganisms, the mechanisms of methane driven DNRA process have not been investigated. Therefore, the third research objective of this thesis is to characterise the microorganisms and pathways involved in a culture performing methane driven DNRA process. In a bioreactor fed with methane and limited nitrate supply, continuous ammonium production from nitrate coupled with anaerobic oxidation of methane was observed, while an anaerobic methanotrophic archaea, Candidatus Methanoperedens nitroreducens (M. nitroreducens), dominated the microbial community. During batch tests, metagenomic and metatranscriptomic analyses showed that the DNRA related genes in Candidatus M. nitroreducens have significantly higher expression levels when the culture was producing ammonium, compared to their levels when the culture was fed with large quantity of nitrate to inhibit DNRA activity. Overall, the results confirmed that Candidatus M. nitroreducens can facilitate the methane-driven DNRA process.It has been shown that denitrification can couple to the oxidation of methane or metals (e.g. Fe or Mn), thus linking the nitrogen cycle to the methane and metal cycles, respectively. However, research works focusing on the interactions between nitrogen, methane and metal cycles in one system are rarely done. Therefore, the last research objective is to investigate interactions between anaerobic oxidation of methane and nitrate and metals reduction processes in one reactor. By setting up and incubating the bioreactor with methane, nitrate and ferrihydrite for more than 900 days, simultaneous methane oxidation and nitrate reduction were observed. Mass and electron balances suggested that in this system, nitrite/iron-dependent anaerobic oxidation of methane processes are coupled with nitrate-dependent iron (Fe) oxidation (NDFO) process. The community analysis suggested that Azospira sp. and Methylomirabiliaceae sp. may be responsible for these processes, while an unknown microorganism facilitated metal-dependent anaerobic oxidation of methane process.Overall, the discovery and characterisation of these novel microorganisms and metabolic pathways mediating ammonium oxidation and nitrate reduction will provide important insights into the understanding of the nitrogen cycle.n

Sodium nitrate and nitrite are major components of alkaline nuclear waste streams and contribute to environmental release hazards. The electrochemical reduction of these materials to gaseous products has been studied in a synthetic waste mixture. The effects of electrode materials, cell design, and other experimental parameters have been investigated. Lead was found to be the best cathode material in terms of current efficiency for the reduction of nitrate and nitrite in the synthetic mix. The current efficiency for nitrite and nitrate removal is improved in divided cells due to the elimination of anodic oxidation of nitrite. Operation of the divided cells at high current densities (300–600 mA cm−2) and high temperatures (80°C) provides more efficient reduction of nitrite and nitrate. Nearly complete reduction of nitrite and nitrate to nitrogen, ammonia, or nitrous oxide was demonstrated in 1000 h tests in a divided laboratory electrochemical flow cell using a lead cathode, Nafion® 417 cation exchange membrane, and oxygen evolving DSA® or platinum clad niobium anode at a current density of 500 mA cm−2 and a temperature of 70° C. Greater than 99% of the nitrite and nitrate was removed from the synthetic waste mix batch in the 1000 h tests at an overall destruction efficiency of 55%. The process developed shows promise for treating large volumes of waste.

Abstract. Light is considered a strong controlling factor of nitrification rates in the surface ocean. Previous work has shown that ammonia oxidation and nitrite oxidation may be inhibited by high light levels, yet active nitrification has been measured in the sunlit surface ocean. While it is known that photosynthetically active radiation (PAR) influences microbial nitrite production and consumption, the level of inhibition of nitrification is variable across datasets. Additionally, phytoplankton have light-dependent mechanisms for nitrite production and consumption that co-occur with nitrification around the depths of the primary nitrite maximum (PNM). In this work, we experimentally determined the direct influence of light level on net nitrite production, including all major nitrite cycling processes (ammonia oxidation, nitrite oxidation, nitrate reduction and nitrite uptake) in microbial communities collected from the base of the euphotic zone. We found that although ammonia oxidation was inhibited at the depth of the PNM and was further inhibited by increasing light at all stations, it remained the dominant nitrite production process at most stations and treatments, even up to 25 % surface PAR. Nitrate addition did not enhance ammonia oxidation in our experiments but may have increased nitrate and nitrite uptake at a coastal station. In contrast to ammonia oxidation, nitrite oxidation was not clearly inhibited by light and sometimes even increased at higher light levels. Thus, accumulation of nitrite at the PNM may be modulated by changes in light, but light perturbations did not exclude nitrification from the surface ocean. Nitrite uptake and nitrate reduction were both enhanced in high-light treatments relative to low light and in some cases showed high rates in the dark. Overall, net nitrite production rates of PNM communities were highest in the dark treatments.

<strong class="journal-contentHeaderColor">Abstract.</strong> Light is considered a strong controlling factor on nitrification rates in the surface ocean. Previous work has shown that ammonia oxidation and nitrite oxidation may be inhibited by high light levels, yet active nitrification has been measured in the sunlit surface ocean. While it is known that photosynthetically active radiation (PAR) influences microbial nitrite production and consumption, the level of inhibition of nitrification is variable across datasets. Additionally, phytoplankton have light-dependent mechanisms for nitrite production and consumption that co-occur with nitrification around the depths of the primary nitrite maximum (PNM). In this work, we experimentally determined the direct influence of light level on net nitrite production, including all major nitrite cycling processes (ammonia oxidation, nitrite oxidation, nitrate reduction, nitrite uptake) in microbial communities collected from the base of the euphotic zone. We found that although ammonia oxidation was inhibited at the depth of the PNM and was further inhibited by increasing light at all stations, it remained the dominant nitrite production process at most stations and treatments, even up to 25 % surface PAR. Nitrate addition did not enhance ammonia oxidation in our experiments, but may have increased nitrate and nitrite uptake at a coastal station. In contrast to ammonia oxidation, nitrite oxidation was not clearly inhibited by light, and sometimes even increased at higher light levels. Thus, accumulation of nitrite at the PNM may be modulated by changes in light, but light perturbations did not exclude nitrification from the surface ocean. Nitrite uptake and nitrate reduction were both enhanced in high light treatments relative to low light, and in some cases showed high rates in the dark. Overall, net nitrite production rates of PNM communities were highest in the dark treatments.

Simulated concentrated nitrate brine waste was treated in a two-stage electrolysis process, where reduction of nitrate in a cathodic chamber was coupled with oxidation of ammonium (produced in the previous stage) in an anodic chamber. The influence of operating conditions such as applied potential, electrolyte composition, and initial concentration on conversion of nitrate to ammonium in the cathodic chamber was investigated. The effects of chloride and current density on the two-stage treatment process were also examined. More negative potential at the cathode (−2.0 vs. −1.7 and −1.5 V) and higher current density (33 vs. 22 and 13.9 mA/cm2) favored ammonium as the product of nitrate reduction at a copper cathode and increased the overall amount of nitrate conversion. Faster reduction was seen when nitrate concentration was lower (0.01 vs. 0.02 N), but total mass conversion of nitrate in the same time period was higher for 0.02 N. The reduction of nitrate was not sensitive to sulfate or chloride concentration in the ranges studied (0–2 and 1–1.5 mg/L, respectively), but the oxidation of ammonium was noticeably higher when chloride was present. Only small amounts of ammonium and nitrate remained in the second-stage effluent after coupled treatment.

Potential benthic habitats of early Mars lakes, probably oligotrophic, could range from hydrothermal to cold sediments. Dynamic processes in the water column (such as turbidity or UV penetration) as well as in the benthic bed (temperature gradients, turbation, or sedimentation rate) contribute to supply nutrients to a potential microbial ecosystem. High altitude, oligotrophic, and deep Andean lakes with active deglaciation processes and recent or past volcanic activity are natural models to assess the feasibility of life in other planetary lake/ocean environments and to develop technology for their exploration. We sampled the benthic sediments (down to 269 m depth) of the oligotrophic lake Laguna Negra (Central Andes, Chile) to investigate its ecosystem through geochemical, biomarker profiling, and molecular ecology studies. The chemistry of the benthic water was similar to the rest of the water column, except for variable amounts of ammonium (up to 2.8 ppm) and nitrate (up to 0.13 ppm). A life detector chip with a 300-antibody microarray revealed the presence of biomass in the form of exopolysaccharides and other microbial markers associated to several phylogenetic groups and potential microaerobic and anaerobic metabolisms such as nitrate reduction. DNA analyses showed that 27% of the Archaea sequences corresponded to a group of ammonia-oxidizing archaea (AOA) similar (97%) to Nitrosopumilus spp. and Nitrosoarchaeum spp. (Thaumarchaeota), and 4% of Bacteria sequences to nitrite-oxidizing bacteria from the Nitrospira genus, suggesting a coupling between ammonia and nitrite oxidation. Mesocosm experiments with the specific AOA inhibitor 2-Phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO) demonstrated an AOA-associated ammonia oxidation activity with the simultaneous accumulation of nitrate and sulfate. The results showed a rich benthic microbial community dominated by microaerobic and anaerobic metabolisms thriving under aphotic, low temperature (4°C), and relatively high pressure, that might be a suitable terrestrial analog of other planetary settings.

Micro-oxygen supply combined with applied voltage enhance ammonium oxidation and denitrification in a single-chamber microbial electrolysis cell: Performance and mechanism

Paired electrolysis in a solid polymer electrolyte reactor—Simultaneously reduction of nitrate and oxidation of ammonia

Water samples were collected in May 1992 from turbid plume water along several transects of increasing salinity from the RhBne River mouth to the sea. Nitrogen salt concentrations (NH,', NO?. NO,), nitrification, denitrification and nitrate reduction were determined. NH,' values, measured in the plume water, were lower than those corresponding to a conservative dilution, demonstrating a loss of 2 pm01 1 ' of NH,' In situ concentrations of N O 1 differed by 10 to 30 pm01 I-' from theoretical values (conservat~ve dilution), showing a net consumption of this compound. NO2 concentrations stayed closed to the conservative dilution curve plots. Along the salinity gradient, ammonium and nitrite oxidation rates decreased from 2 to 0.2 and 1 to 0.1 pm01 I-' d' respectively. These 2 rates correlated well with in situ NH,' concentrations. 15 ':c, of the allochtonous NH,' was nitrified. Dissimilative nitrate and nitrite reduction rates displayed similar values, decreasing from 380 to 7 umol I' d' Denitrification ranged from 0 to 7 pm01 I-' d . ' , independently of the salinity value. 3.5:1 of the allochthonous NO; was denitrified. In the plume, denitrification rates were 30 to 100 times lower than nltrite reduction, whlle at a salinity > 20 psu, these 2 processes occurred at similar rates. A significant correlation was demonstrated between the nitrate reduction rate and the difference between theoretlcal and In sltu N O 3 concentrations. The deficit in nitrate in the plume could a r ~ s e from the nitrate reduction process.

Chronic nitrogen inputs can alleviate N limitation and potentially impose N losses in forests, indicated by soil enrichment in 15 N over 14 N. However, the complexity of the nitrogen cycle hinders accurate quantification of N fluxes. Simultaneously, soil ecologists are striving to find meaningful indicators to characterise the "openness" of the nitrogen cycle. We integrate soil δ15 N with constrained ecosystem N losses and the functional gene potential of the soil microbiome in 14 temperate forest catchments. We show that N losses are associated with soil δ15 N and that δ15 N scales with the abundance of soil bacteria. The abundance of the archaeal amoA gene, representing the first step in nitrification (ammonia oxidation to nitrite), followed by the abundance of narG and napA genes, associated with the first step in denitrification (nitrate reduction to nitrite), explains most of the variability in soil δ15 N. These genes are more informative than the denitrification genes nirS and nirK, which are directly linked to N2 O production. Nitrite formation thus appears to be the critical step associated with N losses. Furthermore, we show that the genetic potential for ammonia oxidation and nitrate reduction is representative of forest soil 15 N enrichment and thus indicative of ecosystem N losses.

Hydrogen water chemistry reduces the stress corrosion cracking (SCC) of boiling water reactor components by lowering the corrosion potential. However, the amount of N16 carried into the main steam line increases as the corrosion potential decreases. This is attributed to the reduction of nitrogen-containing species to ammonia, which is more volatile than nitrate or nitrite and readily partitions to the steam phase. An alternative method to lower the corrosion potential and mitigate SCC without large hydrogen additions is in situ noble metal deposition. With this technique, noble metals catalyze the recombination of hydrogen and oxygen and produce a low corrosion potential with stoichiometric amounts of H2 to O2. However, there is some concern that noble metals will also increase the amount of N16 in the main steam line as a result of the low corrosion potentials and the catalytic nature of these metals. The stability of nitrate and ammonia was studied as a function of corrosion potential, surface area, flow rate and catalytic nature of the surface. Undoped surfaces of 304 stainless steel with surface to volume ratios from 0.65 to 15.7cm−1 and flow rates up to 4.8cm/s produced no reduction of nitrate to ammonia at corrosion potentials down to -475mVSHE at 288°C. This indicates that the surface area of undoped 304 SS has little effect on the reduction of nitrate at corrosion potentials above -475mVSHE at 288°C. In situ Pt deposition of 304 SS did promote the reduction of nitrate to ammonia, although this effect was enhanced at very low flow rates/high residence times. This suggests that a surface plays a critical role in ammonia formation and consequently the level of N16 partitioned to the steam phase. At low flow rates, the Pt doped material promoted the reduction of nitrate to ammonia when the potential was -400mVSHE, whereas the undoped material produced only a small amount of nitrite. At higher flow rates, Pt doping promoted the formation of nitrate to a lesser extent than at low flow rates, with the amount of reduction being a function of the surface to volume ratio. The reduction of nitrate in the presence of Pt is most likely controlled by some combination of the low corrosion potential and the catalytic nature of noble metals. Under low flow conditions, Pt doping promoted the oxidation of ammonia to nitrate at 42 ppm O2 (258mVSHE), which was not observed on the undoped surfaces. At high flow, no oxidation of ammonia was detected up to 2 wppm O2.

The long-term effects of using nitrite and urea on the enrichment of comammox bacteria