Nitrification Denitrification Research Articles

Fate of dissolved inorganic nitrogen in turbulent rivers: The critical role of dissolved oxygen levels

Dissolved inorganic nitrogen (DIN) is considered the main factor that induces eutrophication in water, and is readily influenced by hydrodynamic activities. In this study, a 4-year field investigation of nitrogen dynamics in a turbulent river was conducted, and a laboratory study was performed in the approximately homogeneous turbulence simulation system to investigate potential mechanisms involved in DIN transformation under turbulence. The field investigation revealed that, contrary to NO-3 dynamics, the NH+4 concentrations in water were lower in flood seasons than in drought seasons. Further laboratory results demonstrated that limitation of dissolved oxygen (DO) caused inactive nitrification and active denitrification in static river sediment. In contrast, the increased DO levels in turbulent river intensified the mineralization of organic nitrogen in sediment; moreover, ammonification and nitrification were activated, while denitrification was first activated and then depressed. Turbulence therefore decreased NH+4 and NO-2 concentrations, but increased NO-3 and total DIN concentrations in the overlying water, causing the total DIN to increase from 0.4 mg/L to maximum of 1.0 and 1.7 mg/L at low and high turbulence, respectively. The DIN was maintained at 0.7 and 1.0 mg/L after the 30-day incubation under low and high turbulence intensities (ε) of 3.4 × 10−4 and 7.4 × 10−2 m2/s3, respectively. These results highlight the critical role of DO in DIN budgets under hydrodynamic turbulence, and provide new insights into the DIN transport and transformation mechanisms in turbulent rivers.

The successful integration of anammox to enhance the operational stability and nitrogen removal efficiency during municipal wastewater treatment

The anaerobic ammonium oxidation (anammox) process is a promising method for carbon reduction and energy conservation. However, the advantages of its application in municipal wastewater treatment have not been completely revealed. Stable nitrogen removal was successfully achieved in the anaerobic/aerobic/anoxic (AOA) process by directly integrating stable anammox. Over a 200 + day operating period, the abundance of anammox bacteria (3.1 ± 0.9 × 108 copies/gSS) and anammox activity (0.32 ± 0.07 mgN/(gSS·h)) remained high, despite a temperature decrease (27.8–17.2 °C) and fluctuating carbon-nitrogen (C/N) ratio (1.6–5.0). A stable nitrogen removal efficiency of 87.1 ± 2.5 % was successfully maintained, because the anammox activity compensated for the effect of low temperatures and low C/N ratios on the nitrification and denitrification. Further analysis found that anammox accounted for 52.6 %-57.5 % of N2 production, and the rest was through nitrification–denitrification. The stable anammox reaction was mainly due to the limitation of oxygen and carbon sources and the stable supply of nitrite by endogenous denitrification. Overall, these findings confirmed that the stability and efficiency of nitrogen removal from municipal wastewater could be increased by integrating anammox under appropriate conditions.

Biogas purification and ammonia load reduction in sewage treatment by two-stage down-flow hanging sponge reactor

In this study, a two-stage down-flow hanging sponge (TSDHS) reactor was used as biotrickling filter for biogas desulfurization by utilizing the anaerobic digester supernatant (ADS) of sewage sludge of an activated sludge process (ASP). The reactor comprises a closed-type first-stage down-flow hanging sponge (1st DHS) and an open-type second-stage down-flow hanging sponge (2nd DHS) reactors. In the 1st DHS, hydrogen sulfide in biogas was dissolved into the ADS, and then it was oxidized into elemental sulfur and sulfate by microbe using dissolved oxygen and nitrite in the ADS. More than 99.9 % of hydrogen sulfide was removed within 400 s of empty bed residence time, and >50 % of removed hydrogen sulfide was oxidized into elemental sulfur and accumulated at the surface of the sponge carrier in the 1st DHS. The 1st DHS effluent was fed into the 2nd DHS for nitrogen removal via nitrification and sulfur-based denitrification with the recirculation of the 2nd DHS effluent under nonaeration condition. In the 2nd DHS, 36.8 % of ammonia and 5.3 % of total inorganic nitrogen were removed. Sulfurimonas and Halothiobacillus were increased and contributed to the sulfur-based denitrification as well as the accumulation of elemental sulfur in the 1st DHS, respectively. In the 2nd DHS, Nitrosococcus, Nitrobacter, and Sulfuritalea were considered as the contributors of nitrogen removal via nitrification and sulfur-based denitrification. Further, this study shows that a TSDHS reactor can achieve not only desulfurization of biogas in the 1st DHS but also a 3.5 %–15 % reduction of the ammonia load in the 2nd DHS by effective utilization of the ADS during sewage treatment, assuming that the ADS is returned to the ASP.

Nitrogen removal from low COD/N interflow using a hybrid activated sludge membrane-aerated biofilm reactor (H-MABR)

A hybrid activated sludge membrane-aerated biofilm reactor based on a two-stage simultaneous nitrification–denitrification (SND) process was built, and its utility for treating interflow with low chemical oxygen demand (COD)/total nitrogen (TN) (COD/N) was explored. The operating performance, functional microbial communities, and functional genes for nitrogen metabolism were evaluated at low COD/N (4–1.3). The reactor could achieve stable operation at COD/N = 4–1.5, and the removal efficiency of COD, TN, and ammonia nitrogen was stable at 90.30 ± 2.36 %, 85.69 ± 2.22 %, and 89.52 ± 6.06 %, respectively. The SND rates were 70.89 % and 50.75 % when influent COD/N was 2.0 and 1.7, respectively, indicating that SND makes an important contribution to nitrogen removal under these two COD/N conditions. Microbial analysis revealed that the sampling sites with a high abundance of denitrification genes in the outer ring experienced aerobic conditions, inferring that aerobic denitrification also plays an important role in denitrification.

Performance of simultaneous nitrification–denitrification and denitrifying phosphorus and manganese removal by driving a single-stage moving bed biofilm reactor based on manganese redox cycling

Simultaneous removal of NH4+-N, NO3–-N, COD, and P by manganese redox cycling in nutrient wastewater was established with a single-stage moving bed biofilm reactor (MBBR) under low C/N ratio. When sodium succinate replaced the conventional denitrifying carbon source, removal efficiencies of TN, NO3–-N, NH4+-N, TP, and Mn2+ were 65.13 %, 79.63 %, 92.79 %, 51.57 %, and 68.10 %, respectively. Based on modified Stover-Kincannon model, 11.03 and 10.05 mg TN·L−1·h−1 of Umax values were obtained with sodium acetate and sodium succinate as substrates. Extracellular polymeric substances were used to evaluate the characteristics of biofilm, and microbial community of biofilm was identified. Transformation processes of NO3–-N, NH4+-N, Mn2+, and P were investigated, suggesting that the main functional groups (e.g., CO, Mn-O, and CN bonds) participated in N, P, and Mn2+ removal, and MnO2 was the main component of biogenic manganese oxides. This study provides a new strategy for nutrients removal by Mn2+ driven MBBR.

Significant cost savings using a newly developed single‐process technology compared to conventional processes for treating high‐strength effluents

AbstractThis study investigates the performance of a newly developed technology, known as simultaneous anaerobic oxidation/partial nitrification–denitrification (SAO/PND), for treating strong wastewater under laboratory conditions. Excellent results are achieved, with maximum chemical oxygen demand (COD) and nitrogen removal of 98% and 96%, respectively, when treating synthetic wastewater with influent 4120 mg/L COD and 210 mg/L NH4+–N. Appropriate living environment in this single‐process technology favoured the co‐existence of various bacterial communities, and removal mechanisms by these organisms are identified and confirmed. The results showed that the single‐process SAO/PND technology required 95% and 90% less oxygen compared to conventional aerobic processes and combined systems, respectively. Furthermore, this newly developed technology produced 60% and 44% less sludge, respectively, in comparison with the mentioned conventional systems. This study shows that the single‐process SAO/PND technology is a promising sustainable alternative to conventional systems, as it combines high efficiency and cost‐effectiveness with simple operation and maintenance requirements.

Organic and inorganic nitrogen removals by an ureolytic heterotrophic nitrification and aerobic denitrification strain Acinetobacter sp. Z1: Elucidating its physiological characteristics and metabolic mechanisms

Although heterotrophic nitrification-aerobic denitrification (HN-AD) is promising in nitrogen removal, it remains unclear for most HN-AD strains in physiological characteristics and metabolic mechanisms. In this study, a newly isolated strain Acinetobacter sp. Z1 converted not only inorganic nitrogen, but also organic nitrogen to N2. Among them, urea was the preferential nitrogen substrate. Single-factor experiments showed that efficient HN-AD process occurred with acetate as carbon source, C/N ratios of 12 for NH4+-N and 15 for NO3–-N, pH 8, 30 °C, DO of ∼5.8 mg/L and salinity less than 1.5 %. Subsequently, response surface analysis was applied to predict the optimal growth conditions. Its complete genome annotation in combination with enzymatic activity assay and nitrogen balance calculation showed that at least four pathways involved in nitrogen metabolism. This work indicates that ureolytic strain Z1 could be prepared as bacterial agents with other HN-AD strains to treat urea-containing wastewater like urine from urban community.

Nitrogen management for furrow‐irrigated rice produced on silt‐loam soils

AbstractRice is typically produced under flooded conditions, but upward of 120,000 ha of furrow‐irrigated rice (FIR) were produced in Arkansas and Missouri in 2020. Nitrogen management will inherently vary under FIR production due to greater N loss potential, especially via nitrification–denitrification stimulated by wetting and drying cycles of the soil. A series of trials were conducted from 2018 to 2020 to determine the optimum N management program for FIR grown on a silt‐loam soil. Eight sites were used in a split‐plot design, with the whole‐plot factor being location within the field (top vs. bottom) and the split‐plot factor being N management regime. The N management regimes ranged from a single application to a four‐way split. Trial results suggest that multiple N management programs can maximize rice grain yield, including those with a greater number of N applications or season total N rate. The recovery efficiency of N ranged from 53.6 to 76.2% and the agronomic N efficiency ranged from 8.3 to 29.3 kg kg–1. The findings suggest that three weekly applications of 52 kg N ha–1 will allow FIR producers across the Mid‐South to optimize their N management program, potentially increasing yield and decreasing environmental effects resulting from the transition to FIR.

Soil moisture-atmosphere feedback dominates land N2 O nitrification emissions and denitrification reduction.

Soil moisture (SM) is essential to microbial nitrogen (N)-cycling networks in terrestrial ecosystems. Studies have found that SM-atmosphere feedbacks dominate the changes in land carbon fluxes. However, the influence of SM-atmosphere feedbacks on the N fluxes changes, and the underlying mechanisms remain highly unsure, leading to uncertainties in climate projections. To fill this gap, we used in situ observation coupled with gridded and remote sensing data to analyze N2 O fluxes emissions globally. Here, we investigated the synergistic effects of temperature, hydroclimate on global N2 O fluxes, as the result of SM-atmosphere feedback impact on N fluxes. We found that SM-temperature feedback dominates land N2 O emissions by controlling the balance between nitrifier and denitrifier genes. The mechanism is that atmospheric water demand increases with temperature and thereby reduces SM, which increases the dominant N2 O production nitrifier (containing amoA AOB gene) and decreases the N2 O consumption denitrifier (containing the nosZ gene), consequently will potential increasing N2 O emissions. However, we find that the spatial variations of soil-water availability as a result of the nonlinear response of SM to vapor pressure deficit caused by temperature are some of the greatest challenges in predicting future N2 O emissions. Our data-driven assessment deepens the understanding of the impact of SM-atmosphere interactions on the soil N cycle, which remains uncertain in earth system models. We suggest that the model needs to account for feedback between SM and atmospheric temperature when estimating the response of the N2 O emissions to climatic change globally, as well as when conducting field-scale investigations of the response of the ecosystem to warming.

Aerobic ammonia removal with heterotrophic nitrification and denitrification of Alcaligenes faecalis strain No.4 to mitigate nitrogenous pollution caused by piggery wastewater: a feasibility study

AbstractThe ammonia removal ability of heterotrophic bacteria Alcaligenes faecalis strain No.4 isolated from sewage sludge was examined in a batch operation to mitigate ammonia from piggery wastewater, consequently preventing pollution by the inflow of wastewater from piggeries adjacent to rivers. If this process works functionally, it can be effective in controlling nitrous oxide (N2O) and nitrate (NO3−) emissions derived from animal agriculture, the heterotrophic nitrifying and the aerobic denitrifying effect of A. faecalis strain No.4 on high-strength ammonium (NH4+-N) were evaluated in wastewater collected from a piggery. The removal rate by A. faecalis strain No.4 on high-strength ammonium (NH4+-N) was 0.97 kg N/m3/day which was more than 100 fold greater than that achieved using conventional aerobic nitrification and anaerobic denitrification processes. An aerobic one-step denitrification system using A. faecalis strain No.4 can be proposed to remove ammonia and phytopathogens from piggery wastewater with high efficiency and prevent water pollution in adjacent rivers.

Biological nitrogen removal and metabolic characteristics of a novel cold-resistant heterotrophic nitrification and aerobic denitrification Rhizobium sp. WS7

For improving the poor de-nitrogen efficiency and effluent quality faced by wastewater treatment plants in winter, a novel cold-resistant strain, Rhizobium sp. WS7 was isolated. Strain WS7 presented dramatic de-nitrogen efficiencies including 98.73 % of NH4+-N, 99.98 % of NO3–-N, 100 % of NO2–-N and approximately 100 % of mixed nitrogen (NH4+-N and NO3–-N) at 15 °C. Optimum parameters of WS7 for aerobic denitrification were determined. Additionally, functional genes (amoA, napA, nirK, norB, and nosZ) and key enzymes (nitrate reductase and nitrite reductase) activities were determined. Nitrogen balance analysis suggested that assimilation played a dominant role in de-nitrogen by WS7, the NH4+-N metabolic pathway was deduced as NH4+-N → NH2OH → NO → N2O → N2, and the NO3–-N/NO2–-N metabolic pathway was deduced as NO3–-N → NO2–-N → NO → N2O → N2. The cold-resistant Rhizobium sp. WS7 has great application feasibility in cold sewage treatment.

Modified integrated fixed-film activated sludge process: Advanced nitrogen removal for low-C/N domestic wastewater

Actual low-C/N domestic wastewater was treated using the high-concentration powder carrier bio-fluidized bed (HPB) process comparing diatomite and Fe–C as the carriers. The total nitrogen removal efficiencies were increased from 50.08% to 65.40% and 78.58%, respectively. The diatomite HPB process increased the relative abundance of autotrophic N-cycle bacteria to more than twofold and the sludge size. Therefore, the contributions for nitrogen removal by anammox and simultaneous nitrification–denitrification were increased. The Fe–C HPB process improved the nitrogen removal efficiency mainly by increasing the biodegradability and activities of electron transfer system and key enzymes. The key device (hydrocyclone separator) of the HPB process significantly improved the recovery efficiency of the carriers. It also improved the capacity of microbial aggregations for adsorbing pollutants. Furthermore, it reduced the relative abundance of filamentous bacteria. This study demonstrated the feasibility and mechanism of the HPB process for improving the nitrogen removal efficiency for low-C/N wastewater.

Occurrence Characteristics of Inorganic Nitrogen in Groundwater in Silty-Clay Riparian Hyporheic Zones under Tidal Action: A Case Study of the Jingzi River in Shanghai, China

For comprehending the effect of tidal action on nitrogen cycle in silty-clay riparian hyporheic zones, the synchronous monitoring of water level and water quality was carried out along a test transect during a spring tidal period from 21 to 23 October 2021. Moreover, the permeability and chemical composition of soil samples from drilled holes were measured. Subsequently, the spatiotemporal variation of inorganic nitrogen concentrations in the groundwater in the riparian hyporheic zone was investigated during the study period, and the potential reason was discussed. It is shown that the delayed response time of groundwater level in the silty-clay riparian zone to the tide-driven fluctuation of the river stage increased with distance from the shore and reached 3.0 h at the position 3.83 m away from the shore. The continuous infiltration of the river water under tide action contributed to the aerobic and neutral riparian hyporheic zone conductive to nitrification. Within 4 m away from the bank, the dominant inorganic nitrogen form changed from NO3−-N to NH4+-N, upon increasing the distance from the bank. Additionally, the removal of nitrogen could occur in the riparian hyporheic zone with aerobic and neutral environment under the conjoint control of nitrification, microbial assimilation, and aerobic denitrification.

Effect of intermittent operation and shunt wastewater on pollutant removal and microbial community changes in subsurface wastewater infiltration system

Subsurface wastewater infiltration system (SWIS) is widely used in the treatment of rural domestic sewage. However, the nitrogen removal effect is unsatisfactory. In this study, laboratory-scale SWIS was built, and the nitrogen removal capacity of the system was optimized by a combination of intermittent operation and the shunt method. The optimal total nitrogen (TN) removal rate (56.53%) is obtained when the wet-dry ratio (Rwd) is 1:2 and the shunt ratio is 3:1. The optimal ammonia nitrogen (NH4+-N) removal rate (54.97%) is obtained when Rwd is 1:3 and shunt ratio is 2:1. In addition, SWIS achieved a good COD removal rate (>75%) and TP removal rate (>95%) during the whole operation stage. Proteobacteria, Acidobateria and Bacteroidetes are high abundance phyla in SWIS. Bacteroidetes, Chloroflexi and Nitrospirae associated with nitrogen removal are abundant at the shunt ratio of 2:1 and 3:1. At the genus level, Bosea, Bradyrhizobium and Phenylobacterium were functional genera associated with denitrification, enriching the substratum of SWIS at shunt ratios of 1:1, 2:1 and 3:1. Nitrogen (N) forms and ORP analysis showed that under the appropriate shunt ratio and Rwd, the SWIS matrix has a suitable redox microenvironment, and thus creates a well-defined nitrification area and denitrification area. This research provides a reliable theoretical basis for the construction and operation of SWIS in rural areas.

Lab‐scale data and microbial community structure suggest shortcut nitrogen removal as the predominant nitrogen removal mechanism in post‐aerobic digestion (PAD)

Implementing an aerobic digestion step after anaerobic digestion, referred to as "post aerobic digestion" (PAD), can remove ammonia without the need for an external carbon source and destroy volatile solids. While this process has been documented at the lab-scale and full-scale, the mechanism for N removal and the corresponding microbial community that carries out this process have not been established. This research gap is important to fill because the nitrogen removal pathway has implications on aeration requirements and carbon demand, that is, short-cut N-removal requires less oxygen and carbon than simultaneous nitrification-denitrification. The aims of this research were to (i) determine if nitrite (NO2 - ) or nitrate (NO3 - ) dominates following ammonia removal and (ii) characterize the microbial community from PAD reactors. Here, lab-scale PAD reactors were seeded with biomass from two different full-scale PAD reactors. The lab-scale reactors were fed with biomass from full-scale reactors and operated in batch mode to quantify nitrogen species concentrations (ammonia, NH4 + , NO2 - , and NO3 - ) over time. Experimental results revealed that NO2 - production rates were several orders of magnitude greater than NO3 - production rates. Indeed, nitrite accumulation rate (NAR) was greater than 90% at most temperatures, confirming that shortcut nitrogen removal was the dominant NH4 + removal mechanism in PAD. Microbial community analysis via 16S rRNA sequencing indicated that ammonia oxidizing bacteria (AOB) were much more abundant than nitrite oxidizing bacteria (NOB). Overall, this study suggests that aeration requirements for post-aerobic digestion should be based on NO2 - shunt and not complete simultaneous nitrification denitrification. PRACTITIONER POINTS: AOB are a key feature of PAD microbial communities NOB are present, but in much lower abundance than AOB High nitrite accumulation ratio suggests shortcut nitrite as the main mechanism for nitrogen removal Nitritation in PAD reactors is sustained at temperatures as high as 40°C No ammonia oxidation occurred at 50°C implying different mechanisms of nitrogen removal including ammonia stripping.

Application of low-intensity ultrasound to enhance simultaneous nitrification/iron-based autotrophic denitrification.

Intermittent ultrasound with an intensity of 0.2W/ml was applied during simultaneous nitrification/iron-based autotrophic denitrification to evaluate its impacts on total nitrogen (TN) removal efficiency and microbial characteristics during low carbon/nitrogen ratio (C/N) wastewater treatment. At an optimal dissolved oxygen (DO) concentration of 1.2mg/L, the TN removal rate was 91 ± 4%, and the TN concentration in the effluent decreased by 31% owing to the ultrasound treatment. In addition, the number of iron-compounds that formed in the sludge and wastewater increased by 55% and 37%, respectively. Low-intensity ultrasound caused a substantial increase in ammonia monooxygenase activity. Moreover, when the DO concentration increased to 1.2mg/L, the activities of nitrate reductase and nitrite reductase, both of which are associated with denitrification, were effectively maintained. High-throughput sequencing indicated that low-intensity ultrasound enriched ammonium oxidising bacteria (Nitrosomonas) and suppressed the growth of heterotrophic denitrifying bacteria (Zoogloea and Simplicispira). These changes benefited simultaneous nitrification and autotrophic denitrification. Thus, low-intensity ultrasound promoted the simultaneous nitrification/iron-based autotrophic denitrification process during low C/N wastewater treatment.

Nitrate removal by combining chemical and biostimulation approaches using micro-zero valent iron and lactic acid

The occurrence of nitrate is the most significant type of pollution affecting groundwater globally, being a major contributor to the poor condition of water bodies. This pollution is related to livestock-agricultural and urban activities, and the nitrate presence in drinking water has a clear impact on human health. For example, it causes the blue child syndrome. Moreover, the high nitrate content in aquifers and surface waters significantly affects aquatic ecosystems since it is responsible for the eutrophication of surface water bodies. A treatability test was performed in the laboratory to study the decrease of nitrate in the capture zone of water supply wells. For this purpose, two boreholes were drilled from which groundwater and sediments were collected to conduct the test. The goal was to demonstrate that nitrate in groundwater can be decreased much more efficiently using combined abiotic and biotic methods with micro-zero valent iron and biostimulation with lactic acid, respectively, than when both strategies are used separately. The broader implications of this goal derive from the fact that the separate use of these reagents decreases the efficiency of nitrate removal. Thus, while nitrate is removed using micro-valent iron, high concentrations of harmful ammonium are also generated. Furthermore, biostimulation alone leads to overgrowth of other microorganisms that do not result in denitrification, therefore complete denitrification requires more time to occur. In contrast, the combined strategy couples abiotic denitrification of nitrate with biostimulation of microorganisms capable of biotically transforming the abiotically generated harmful ammonium. The treatability test shows that the remediation strategy combining in situ chemical reduction using micro-zero valent iron and biostimulation with lactic acid could be a viable strategy for the creation of a reactive zone around supply wells located in regions where groundwater and porewater in low permeability layers are affected by diffuse nitrate contamination.

To what extent can soil moisture and soil Cu contamination stresses affect nitrous species emissions? Estimation through calibration of a nitrification–denitrification model

Abstract. Continental biogeochemical models are commonly used to predict the effect of land use, exogenous organic matter input or climate change on soil greenhouse gas emission. However, they cannot be used for this purpose to investigate the effect of soil contamination, while contamination affects several soil processes and concerns a large fraction of land surface. For that, in this study we implemented a commonly used model estimating soil nitrogen (N) emissions, the DeNitrification DeCompostion (DNDC) model, with a function taking into account soil copper (Cu) contamination in nitrate production control. Then, we aimed at using this model to predict N2O-N, NO2-N, NO-N and NH4-N emissions in the presence of contamination and in the context of changes in precipitations. Initial incubations of soils were performed at different soil moisture levels in order to mimic expected rainfall patterns during the next decades and in particular drought and excess of water. Then, a bioassay was used in the absence or presence of Cu to assess the effect of the single (moisture) or double stress (moisture and Cu) on soil nitrate production. Data of nitrate production obtained through a gradient of Cu under each initial moisture incubation were used to parameterise the DNDC model and to estimate soil N emission considering the various effects of Cu. Whatever the initial moisture incubation, experimental results showed a NO3-N decreasing production when Cu was added but depending on soil moisture. The DNDC-Cu version we proposed was able to reproduce these observed Cu effects on soil nitrate concentration with r2 > 0.99 and RMSE < 10 % for all treatments in the DNDC-Cu calibration range (> 40 % of the water holding capacity) but showed poor performances for the dry treatments. We modelled a Cu effect inducing an increase in NH4-N soil concentration and emissions due to a reduced nitrification activity and therefore a decrease in NO3-N, N2O-N and NOx-N concentrations and emissions. The effect of added Cu predicted by the model was larger on N2-N and N2O-N emissions than on the other N species and larger for the soils incubated under constant than variable moisture. Our work shows that soil contamination can be considered in continental biogeochemical models to better predict soil greenhouse gas emissions.

Denitrification in foraminifera has an ancient origin and is complemented by associated bacteria

Benthic foraminifera are unicellular eukaryotes that inhabit sediments of aquatic environments. Several foraminifera of the order Rotaliida are known to store and use nitrate for denitrification, a unique energy metabolism among eukaryotes. The rotaliid Globobulimina spp. has been shown to encode an incomplete denitrification pathway of bacterial origin. However, the prevalence of denitrification genes in foraminifera remains unknown, and the missing denitrification pathway components are elusive. Analyzing transcriptomes and metagenomes of 10 foraminiferal species from the Peruvian oxygen minimum zone, we show that denitrification genes are highly conserved in foraminifera. We infer the last common ancestor of denitrifying foraminifera, which enables us to predict the ability to denitrify for additional foraminiferal species. Additionally, an examination of the foraminiferal microbiota reveals evidence for a stable interaction with Desulfobacteraceae, which harbor genes that complement the foraminiferal denitrification pathway. Our results provide evidence that foraminiferal denitrification is complemented by the foraminifera-associated microbiome. The interaction of foraminifera with their resident bacteria is at the basis of foraminiferal adaptation to anaerobic environments that manifested in ecological success in oxygen depleted habitats.

Distribution and Production of N2O in the Subtropical Western North Pacific Ocean During the Spring of 2020

Nitrous oxide (N2O) is an important greenhouse gas emitted in significant volumes by the Pacific Ocean. However, the relationship between N2O dynamics and environmental drivers in the subtropical western North Pacific Ocean (STWNPO) remains poorly understood. We investigated the distribution of N2O and its production as well as the related mechanisms at the surface (0–200 m), intermediate (200–1500 m), and deep (1500–5774 m) layers of the STWNPO, which were divided according to the distribution of water masses. We applied the transit time distribution (TTD) method to determine the ventilation times, and to estimate the N2O equilibrium concentration of water parcels last in contact with the atmosphere prior to being ventilated. In the surface layer, biologically derived N2O (ΔN2O) was positively correlated with the apparent oxygen utilization (AOU) (R2 = 0.48), suggesting that surface N2O may be produced by nitrification. In the intermediate layer, ΔN2O was positively correlated with AOU and NO3− (R2 = 0.92 and R2 = 0.91, respectively) and negatively correlated with nitrogen sinks (N*) (R2 = 0.60). Hence, the highest ΔN2O value in the oxygen minimum layer suggested N2O production through nitrification and potential denitrification (up to 51% and 25% of measured N2O, respectively). In contrast, the deep layer exhibited a positive correlation between ΔN2O and AOU (R2 = 0.92), suggesting that the N2O accumulation in this layer may be caused by nitrification. Our results demonstrate that the STWNPO serves as an apparent source of atmospheric N2O (mean air−sea flux 2.0 ± 0.3 μmol m-2 d-1), and that nitrification and potential denitrification may be the primary mechanisms of N2O production in the STWNPO. We predict that ongoing ocean warming, deoxygenation, acidification, and anthropogenic nitrogen deposition in the STWNPO may elevate N2O emissions in the future. Therefore, the results obtained here are important for elucidating the relationships between N2O dynamics and environmental changes in the STWNPO and the global ocean.