Organic carbon determines nitrous oxide consumption activity of clade I and II nosZ bacteria: Genomic and biokinetic insights

Benthic clade II-type nitrous oxide reducers suppress nitrous oxide emissions in shallow lakes

Shallow lakes, recognized as hotspots for nitrogen cycling, contribute to the emission of the potent greenhouse gas nitrous oxide (N2O), but the current emission estimates for this gas have a high degree of uncertainty. However, the role of N2O-reducing bacteria (N2ORB) as N2O sinks and their contribution to N2O reduction in aquatic ecosystems in response to N2O dynamics have not been determined. Here, we investigated the N2O dynamics and microbial processes in the nitrogen cycle, which included both N2O production and consumption, in five shallow lakes spanning approximately 500 km. The investigated sites exhibited N2O oversaturation, with excess dissolved N2O concentrations (ΔN2O) ranging from 0.55 ± 0.61 to 53.17 ± 15.75 nM. Sediment-bound N2O (sN2O) was significantly positively correlated with the nitrate concentration in the overlying water (p < 0.05), suggesting that nitrate accumulation contributes to benthic N2O generation. High N2O consumption activity (RN2O) corresponded to low ΔN2O. In addition, a significant negative correlation was found between RN2O and nir/nosZ, showing that bacteria encoding nosZ contributed to N2O consumption in the benthic sediments. Redundancy analysis indicated that benthic functional genes effectively reflected the variations in RN2O and ∆N2O. qPCR analysis revealed that the clade II nosZ gene was more sensitive to ΔN2O than the clade I nosZ gene. Furthermore, four novel genera of potential nondenitrifying N2ORB were identified based on metagenome-assembled genome analysis. These genera, which are affiliated with clade II, lack genes responsible for N2O production. Collectively, benthic N2ORB, especially for clade II-type N2ORB, harnesses N2O consumption activity leading to low N2O emissions from shallow lakes. This study advances our knowledge of the role of benthic clade II-type N2ORB in regulating N2O emissions in shallow lakes.

Future directions in microbial nitrogen cycling in wastewater treatment

Discoveries in the past decade of novel reactions, processes, and micro-organisms have altered our understanding of microbial nitrogen cycling in wastewater treatment systems. These advancements pave the way for a transition toward more sustainable and energy-efficient wastewater treatment systems that also minimize greenhouse gas emissions. This review highlights these innovative directions in microbial nitrogen cycling within the context of wastewater treatment. Processes such as comammox, Feammox, electro-anammox, and nitrous oxide mitigation offer innovative approaches for sustainable, energy-efficient nitrogen removal. However, while these emerging processes show promise, advancing from laboratory research to practical applications, particularly in decentralized systems, remains a critical next step toward a sustainable and efficient wastewater management.

Using RNA-Stable isotope probing to investigate methane oxidation metabolites and active microbial communities in methane oxidation coupled to denitrification

The active denitrifying communities performing methane oxidation coupled to denitrification (MOD) were investigated using samples from an aerobic reactor (∼20% O2 and 2% CH4) and a microaerobic reactor (2% O2, 2% CH4) undertaking denitrification. The methane oxidation metabolites excreted in the reactors were acetate, methanol, formate and acetaldehyde. Using anaerobic batch experiments supplemented with exogenously supplied 13C-labelled metabolites, the active denitrifying bacteria were identified using 16S rRNA amplicon sequencing and RNA-stable isotope probing (RNA-SIP). With the aerobic reactor (AR) samples, the maximum NO3- removal rates were 0.43 mmol g-1 d-1, 0.40 mmol g-1 d-1, 0.33 mmol g-1 d-1 and 0.10 mmol g-1 d-1 for exogenously supplied acetate, formate, acetaldehyde and methanol batch treatments respectively, while with the microaerobic reactor (MR) samples, the maximum NO3- removal rates were 0.41 mmol g-1 d-1, 0.33 mmol g-1 d-1, 0.38 mmol g-1 d-1 and 0.14 mmol g-1 d-1 for exogenously supplied acetate, formate, acetaldehyde and methanol batch treatments respectively. The RNA-SIP experiments with 13C-labelled acetate, formate, and methanol identified Methyloversatilis, and Hyphomicrobium as the active methane-driven denitrifying bacteria in the AR samples, while Pseudoxanthomonas, Hydrogenophaga and Hyphomicrobium were the active MOD bacteria in the MR samples. Collectively, all the data indicate that formate is a key cross-feeding metabolite excreted by methanotrophs and consumed by denitrifiers performing MOD.

Unlocking bacterial potential to reduce farmland N2O emissions

Farmed soils contribute substantially to global warming by emitting N2O (ref. 1), and mitigation has proved difficult2. Several microbial nitrogen transformations produce N2O, but the only biological sink for N2O is the enzyme NosZ, catalysing the reduction of N2O to N2 (ref. 3). Although strengthening the NosZ activity in soils would reduce N2O emissions, such bioengineering of the soil microbiota is considered challenging4,5. However, we have developed a technology to achieve this, using organic waste as a substrate and vector for N2O-respiring bacteria selected for their capacity to thrive in soil6-8. Here we have analysed the biokinetics of N2O reduction by our most promising N2O-respiring bacterium, Cloacibacterium sp. CB-01, its survival in soil and its effect on N2O emissions in field experiments. Fertilization with waste from biogas production, in which CB-01 had grown aerobically to about 6 × 109 cells per millilitre, reduced N2O emissions by 50-95%, depending on soil type. The strong and long-lasting effect of CB-01 is ascribed to its tenacity in soil, rather than its biokinetic parameters, which were inferior to those of other strains of N2O-respiring bacteria. Scaling our data up to the European level, we find that national anthropogenic N2O emissions could be reduced by 5-20%, and more if including other organic wastes. This opens an avenue for cost-effective reduction of N2O emissions for which other mitigation options are lacking at present.

Contribution of Microorganisms with the Clade II Nitrous Oxide Reductase to Suppression of Surface Emissions of Nitrous Oxide

The sources and sinks of nitrous oxide, as control emissions to the atmosphere, are generally poorly constrained for most environmental systems. Initial depth-resolved analysis of nitrous oxide flux from observation wells and the proximal surface within a nitrate contaminated aquifer system revealed high subsurface production but little escape from the surface. To better understand the environmental controls of production and emission at this site, we used a combination of isotopic, geochemical, and molecular analyses to show that chemodenitrification and bacterial denitrification are major sources of nitrous oxide in this subsurface, where low DO, low pH, and high nitrate are correlated with significant nitrous oxide production. Depth-resolved metagenomes showed that consumption of nitrous oxide near the surface was correlated with an enrichment of Clade II nitrous oxide reducers, consistent with a growing appreciation of their importance in controlling release of nitrous oxide to the atmosphere. Our work also provides evidence for the reduction of nitrous oxide at a pH of 4, well below the generally accepted limit of pH 5.

Oxic-anoxic cycling promotes coupling between complex carbon metabolism and denitrification in woodchip bioreactors

Denitrifying woodchip bioreactors (WBRs) are increasingly used to manage the release of non-point source nitrogen (N) by stimulating microbial denitrification. Woodchips serve as a renewable organic carbon (C) source, yet the recalcitrance of organic C in lignocellulosic biomass causes many WBRs to be C-limited. Prior studies have observed that oxic-anoxic cycling increased the mobilization of organic C, increased nitrate (NO3 - ) removal rates, and attenuated production of nitrous oxide (N2 O). Here, we use multi-omics approaches and amplicon sequencing of fungal 5.8S-ITS2 and prokaryotic 16S rRNA genes to elucidate the microbial drivers for enhanced NO3 - removal and attenuated N2 O production under redox-dynamic conditions. Transient oxic periods stimulated the expression of fungal ligninolytic enzymes, increasing the bioavailability of woodchip-derived C and stimulating the expression of denitrification genes. Nitrous oxide reductase (nosZ) genes were primarily clade II, and the ratio of clade II/clade I nosZ transcripts during the oxic-anoxic transition was strongly correlated with the N2 O yield. Analysis of metagenome-assembled genomes revealed that many of the denitrifying microorganisms also have a genotypic ability to degrade complex polysaccharides like cellulose and hemicellulose, highlighting the adaptation of the WBR microbiome to the ecophysiological niche of the woodchip matrix.

High nitrous oxide (N2O) greenhouse gas reduction potential of Pseudomonas sp. YR02 under aerobic condition

Aerobic environments exist widely in wastewater treatment plants (WWTP) and are unfavorable for greenhouse gas nitrous oxide (N₂O) reduction. Here, a novel strain Pseudomonas sp. YR02, which can perform N₂O reduction under aerobic conditions, was isolated. The successful amplification of four denitrifying genes proved its complete denitrifying ability. The inorganic nitrogen (IN) removal efficiencies (NRE) were >98.0% and intracellular nitrogen and gaseous nitrogen account for 52.6-58.4% and 41.6-47.4% of input nitrogen, respectively. The priority of IN utilization was TAN > NO₃^--N > NO₂^--N. The optimal conditions for IN and N₂O removal were consistent, except for the C/N ratio, which is 15 and 5 for IN and N₂O removal, respectively. The biokinetic constants analysis indicated strain YR02 had high potential to treat high ammonia and dissolved N₂O wastewater. Strain YR02 bioaugmentation mitigated 98.7% of N₂O emission and improved 32% NRE in WWTP, proving its application potential for N₂O mitigation.

Clarifying Microbial Nitrous Oxide Reduction under Aerobic Conditions: Tolerant, Intolerant, and Sensitive

One of the major challenges for the bioremediation application of microbial nitrous oxide (N2O) reduction is its oxygen sensitivity. While a few strains were reported capable of reducing N2O under aerobic conditions, the N2O reduction kinetics of phylogenetically diverse N2O reducers are not well understood. Here, we analyzed and compared the kinetics of clade I and clade II N2O-reducing bacteria in the presence or absence of oxygen (O2) by using a whole-cell assay with N2O and O2 microsensors. Among the seven strains tested, N2O reduction of Stutzerimonas stutzeri TR2 and ZoBell was not inhibited by oxygen (i.e., oxygen tolerant). Paracoccus denitrificans, Azospirillum brasilense, and Gemmatimonas aurantiaca reduced N2O in the presence of O2 but slower than in the absence of O2 (i.e., oxygen sensitive). N2O reduction of Pseudomonas aeruginosa and Dechloromonas aromatica did not occur when O2 was present (i.e., oxygen intolerant). Amino acid sequences and predicted structures of NosZ were highly similar among these strains, whereas oxygen-tolerant N2O reducers had higher oxygen consumption rates. The results suggest that the mechanism of O2 tolerance is not directly related to NosZ structure but is rather related to the scavenging of O2 in the cells and/or accessory proteins encoded by the nos cluster. IMPORTANCE Some bacteria can reduce N2O in the presence of O2, whereas others cannot. It is unclear whether this trait of aerobic N2O reduction is related to the phylogeny and structure of N2O reductase. The understanding of aerobic N2O reduction is critical for guiding emission control, due to the common concurrence of N2O and O2 in natural and engineered systems. This study provided the N2O reduction kinetics of various bacteria under aerobic and anaerobic conditions and classified the bacteria into oxygen-tolerant, -sensitive, and -intolerant N2O reducers. Oxygen-tolerant N2O reducers rapidly consumed O2, which could help maintain the low O2 concentration in the cells and keep their N2O reductase active. These findings are important and useful when selecting N2O reducers for bioremediation applications.

Biotrickling Filtration for the Reduction of N2O Emitted during Wastewater Treatment: Results from a Long-Term In Situ Pilot-Scale Testing

Wastewater treatment plants (WWTPs) are a major source of N₂O, a potent greenhouse gas with 300 times higher global warming potential than CO₂. Several approaches have been proposed for mitigation of N₂O emissions from WWTPs and have shown promising yet only site-specific results. Here, self-sustaining biotrickling filtration, an end-of-the-pipe treatment technology, was tested in situ at a full-scale WWTP under realistic operational conditions. Temporally varying untreated wastewater was used as trickling medium, and no temperature control was applied. The off-gas from the covered WWTP aerated section was conveyed through the pilot-scale reactor, and an average removal efficiency of 57.9 ± 29.1% was achieved during 165 days of operation despite the generally low and largely fluctuating influent N₂O concentrations (ranging between 4.8 and 96.4 ppmv). For the following 60-day period, the continuously operated reactor system removed 43.0 ± 21.2% of the periodically augmented N₂O, exhibiting elimination capacities as high as 5.25 g N₂O m^-3·h^-1. Additionally, the bench-scale experiments performed abreast corroborated the resilience of the system to short-term N₂O starvations. Our results corroborate the feasibility of biotrickling filtration for mitigating N₂O emitted from WWTPs and demonstrate its robustness toward suboptimal field operating conditions and N₂O starvation, as also supported by analyses of the microbial compositions and nosZ gene profiles.

Microbial response mechanism of plants and zero valent iron in ecological floating bed: Synchronous nitrogen, phosphorus removal and greenhouse gas emission reduction

Iron-based ecological floating beds (EFBs) are often used to treat the secondary effluent from wastewater treatment plant to enhance the denitrification process. However, the impact and necessity of plants on iron-based EFBs have not been systematically studied. In this research, two iron-based EFBs with and without plants (EFB-P and EFB) were performed to investigate the response of plants on nutrient removal, GHG emissions, microbial communities and functional genes. Results showed the total nitrogen and total phosphorus removal in EFB-P was 45-79% and 48-72%, respectively, while that in EFB was 31-67% and 44-57%. Meanwhile, plants could decrease CH₄ emission flux (0-3.89 mg m^-2 d^-1) and improve CO₂ absorption (4704-22321 mg m^-2 d^-1). Plants could increase the abundance of Nitrosospira to 1.6% which was a kind of nitrifying bacteria dominant in plant rhizosphere. Among all denitrification related genera, Simplicispira (13.08%) and Novosphingobium (6.25%) accounted for the highest proportion of plant rhizosphere and iron scrap, respectively. Anammox bacteria such as Candidatus_Brocadia was more enriched on iron scraps with the highest proportion was 1.21% in EFB-P, and 2.20% in EFB. Principal co-ordinates analysis showed that plants were the critical factor determining microbial community composition. TN removal pathways were mixotrophic denitrification and anammox in EFB-P while TP removal pathways were plant uptake and phosphorus-iron coprecipitation. In general, plants play an important directly or indirectly role in iron-based EFBs systems, which could not only improve nutrients removal, but also minimize the global warming potential and alleviate the greenhouse effect to a certain extent.

Contribution of nitrous oxide to the carbon footprint of full-scale wastewater treatment plants and mitigation strategies- a critical review

Nitrous oxide (N₂O), a potent greenhouse gas, significantly contributes to the carbon footprint of wastewater treatment plants (WWTPs) and contributes significantly to global climate change and to the deterioration of the natural environment. Our understanding of N₂O generation mechanisms has significantly improved in the last decade, but the development of effective N₂O emission mitigation strategies has lagged owing to the complexity of parameter regulation, substandard monitoring activities, and inadequate policy criteria. Based on critically screened published studies on N₂O control in full-scale WWTPs, this review elucidates N₂O generation pathway identifications and emission mechanisms and summarizes the impact of N₂O on the total carbon footprint of WWTPs. In particular, a linear relationship was established between N₂O emission factors and total nitrogen removal efficiencies in WWTPs located in China. Promising N₂O mitigation options were proposed, which focus on optimizing operating conditions and implementation of innovative treatment processes. Furthermore, the sustainable operation of WWTPs has been anticipated to convert WWTPs into absolute greenhouse gas reducers as a result of the refinement and improvement of on-site monitoring activities, mitigation mechanisms, regulation of operational parameters, modeling, and policies.

Enhanced nitritation/denitritation and potential mechanism in an electrochemically assisted sequencing batch biofilm reactor treating sludge digester liquor with extremely low C/N ratios

Nitritation/denitritation is a promising strategy to treat sludge digester liquor but would be unstable and inefficient at extremely low C/N ratios. Here, a novel electrochemically assisted sequencing batch biofilm reactor (E-SBBR) was established to treat synthetic/real sludge digester liquor with decreasing C/N ratios. The results showed that the E-SBBR achieved stable nitritation and appreciable TN removal (>70 %) even at C/N < 0.5. The high-strength free ammonium (FA) (91.1-132.8 mg NH₃-N/L) and long inhibition time (>9h) magnified by electrolysis promoted the robustness of nitritation through efficient nitrite-oxidizing bacteria elimination. Meanwhile, mass balance denoted that heterotrophic denitritation dominated in the enhanced TN removal and relied on carbon supplementation from cell apoptosis/lysis stimulated by electrolysis and high-strength FA, further supported by the recovery of heterotrophic denitrifiers, fermentation bacteria, and relevant functional genes at extremely low C/N ratios. This study provides a novel nitrogen removal approach for the sludge digester liquor treatment.

Effect of nitrous oxide (N2O) on the structure and function of nitrogen-oxide reducing microbial communities

Nitrous oxide (N₂O) is a potent greenhouse gas that can be produced by nitrifying and denitrifying bacteria. Yet the effects of N₂O on microbial communities is not well understood. We used batch tests to explore the effects of N₂O on mixed denitrifying communities. Batch tests were carried out with acetate as the electron donor and with the following electron acceptors: nitrate (NO₃^-), nitrite (NO₂^-), N₂O, NO₃^- + N₂O, and NO₂^- + N₂O. Activated sludge from a municipal wastewater treatment plant was used as the inoculum. The bacteria grew readily with N₂O as the sole acceptor. When N₂O was provided along with NO₃^- or NO₂^-, it was used concurrently and resulted in higher growth rates than the same acceptors without added N₂O. The microbial communities resulting from N₂O addition were significantly different at the genus level from those with just NO₃^- or NO₂^-. Tests with N₂O as the sole added acceptor revealed a reduced diversity. Analysis of inferred gene content using PICRUSt2 indicated a greater abundance of genera with a complete denitrification pathway when growing on N₂O or NO₂^-, relative to all other tests. This suggests that specific N₂O reduction rates are high, and that N₂O alone selects for a low-diversity, fully denitrifying community. When N₂O is present with NO₂^- or NO₃^-, the microbial communities were more diverse and did not select exclusively for full denitrifiers. N₂O alone appears to select for a "generalist" community with full denitrification pathways and lower diversity. In terms of denitrification genes, the combination of acceptors with N₂O appeared to increase the number of microbes carrying nirK, while fully denitrifying bacteria appear more likely to carry nirS. Lastly, all the taxa in NO₂^- and N₂O samples were predicted to harbor nosZ. This suggests the potential for reduced N₂O emissions in denitrifying systems.

Simultaneous nitrification-denitrification in biofilm systems for wastewater treatment: Key factors, potential routes, and engineered applications

Simultaneous nitrification-denitrification (SND) is an advantageous bioprocess that allows the complete removal of ammonia nitrogen through sequential redox reactions leading to nitrogen gas production. SND can govern nitrogen removal in single-stage biofilm systems, such as the moving bed biofilm reactor and aerobic granular sludge system, as oxygen gradients allow the development of multilayered biofilms including nitrifying and denitrifying bacteria. Environmental and operational conditions can strongly influence SND performance, biofilm development and biochemical pathways. Recent advances have outlined the possibility to reduce the carbon and energy consumption of the process via the "shortcut pathway", and simultaneously remove both N and phosphorus under specific operational conditions, opening new possibilities for wastewater treatment. This work critically reviews the factors influencing SND and its application in biofilm systems from laboratory to full scale. Operational strategies to enhance SND efficiency and hints to reduce nitrous oxide emission and operational costs are provided.

Exploring the Functions of Efficient Canonical Denitrifying Bacteria as N2O Sinks: Implications from 15N Tracer and Transcriptome Analyses

In denitrifying reactors, canonical complete denitrifying bacteria reduce nitrate (NO₃^-) to nitrogen via N₂O. However, they can also produce N₂O under certain conditions. We used a ¹⁵N tracer method, in which ¹⁵N-labeled NO₃^-/nitrite (NO₂^-) and nonlabeled N₂O were simultaneously supplied with organic electron donors to five canonical complete denitrifying bacteria affiliated with either Clade I or Clade II nosZ. We calculated their NO₃^-, NO₂^-, and N₂O consumption rates. The Clade II nosZ bacterium Azospira sp. strain I13 had the highest N₂O consumption rate (3.47 ± 0.07 fmol/cell/h) and the second lowest NO₃^- consumption rate (0.20 ± 0.03 fmol/cell/h); hence, it is a N₂O sink. A change from peptone- to acetate/citrate-based organic electron donors increased the NO₃^- consumption rate by 4.8 fold but barely affected the N₂O consumption rate. Electron flow was directed to N₂O rather than NO₃^- in Azospira sp. strain I13 and Az. oryzae strain PS only exerting a N₂O sink but to NO₃^- in the Clade I nosZ N₂O-reducing bacteria Pseudomonas stutzeri strain JCM 5965 and Alicycliphilus denitrificans strain I51. Transcriptome analyses revealed that the genotype could not fully describe the phenotype. The results show that N₂O production and consumption differ among canonical denitrifying bacteria and will be useful for developing N₂O mitigation strategies.

Successful year-round mainstream partial nitritation anammox: Assessment of effluent quality, performance and N2O emissions

For two decades now, partial nitritation anammox (PNA) systems were suggested to more efficiently remove nitrogen (N) from mainstream municipal wastewater. Yet to date, only a few pilot-scale systems and even fewer full-scale implementations of this technology have been described. Process instability continues to restrict the broad application of PNA. Especially problematic are insufficient anammox biomass retention, the growth of undesired aerobic nitrite-oxidizers, and nitrous oxide (N₂O) emissions. In this study, a two-stage mainstream pilot-scale PNA system, consisting of three reactors (carbon pre-treatment, nitritation, anammox - 8 m³ each), was operated over a year, treating municipal wastewater. The aim was to test whether both, robust autotrophic N removal and high effluent quality, can be achieved throughout the year. A second aim was to better understand rate limiting processes, potentially affecting the overall performance of PNA systems. In this pilot study, excellent effluent quality, in terms of inorganic nitrogen, was accomplished (average effluent concentrations: 0.4 mgNH₄-N/L, 0.1 mgNO₂-N/L, 0.9 mgNO₃-N/L) even at wastewater temperatures previously considered problematic (as low as 8 °C). N removal was limited by nitritation rates (84 ± 43 mgNH₄-N/L/d), while surplus anammox activity was observed at all times (178 ± 43 mgN/L/d). Throughout the study, nitrite-oxidation was maintained at a low level (<2.5% of ammonium consumption rate). Unfortunately, high N₂O emissions from the nitritation stage (1.2% of total nitrogen in the influent) were observed, and, based on natural isotope abundance measurements, could be attributed to heterotrophic denitrification. In situ batch experiments were conducted to identify the role of dissolved oxygen (DO) and organic substrate availability in N₂O emission-mitigation. The addition of organic substrate, to promote complete denitrification, was not successful in decreasing N₂O emission, but increasing the DO from 0.3 to 2.9 mgO₂/L decreased N₂O emissions by a factor of 3.4.

Exploring the microbial influence on seasonal nitrous oxide concentration in a full-scale wastewater treatment plant using metagenome assembled genomes

Nitrous oxide is a highly potent greenhouse gas and one of the main contributors to the greenhouse gas footprint of wastewater treatment plants (WWTP). Although nitrous oxide can be produced by abiotic reactions in these systems, biological N₂O production resulting from the imbalance of nitrous oxide production and reduction by microbial populations is the dominant cause. The microbial populations responsible for the imbalance have not been clearly identified, yet they are likely responsible for strong seasonal nitrous oxide patterns. Here, we examined the seasonal nitrous oxide concentration pattern in Avedøre WWTP alongside abiotic parameters, the microbial community composition based on 16S rRNA gene sequencing and already available metagenome-assembled genomes (MAGs). We found that the WWTP parameters could not explain the observed pattern. While no distinct community changes between periods of high and low dissolved nitrous oxide concentrations were determined, we found 26 and 28 species with positive and negative correlations to the seasonal N₂O concentrations, respectively. MAGs were identified for 124 species (approximately 31% mean relative abundance of the community), and analysis of their genomic nitrogen transformation potential could explain this correlation for four of the negatively correlated species. Other abundant species were also analysed for their nitrogen transformation potential. Interestingly, only one full-denitrifier (Candidatus Dechloromonas phosphorivorans) was identified. 59 species had a nosZ gene predicted, with the majority identified as a clade II nosZ gene, mainly from the phylum Bacteroidota. A correlation of MAG-derived functional guilds with the N₂O concentration pattern showed that there was a small but significant negative correlation with nitrite oxidizing bacteria and species with a nosZ gene (N₂O reducers (DEN)). More research is required, specifically long-term activity measurements in relation to the N₂O concentration to increase the resolution of these findings.

Identification of nosZ-expressing microorganisms consuming trace N2O in microaerobic chemostat consortia dominated by an uncultured Burkholderiales

Microorganisms possessing N₂O reductases (NosZ) are the only known environmental sink of N₂O. While oxygen inhibition of NosZ activity is widely known, environments where N₂O reduction occurs are often not devoid of O₂. However, little is known regarding N₂O reduction in microoxic systems. Here, 1.6-L chemostat cultures inoculated with activated sludge samples were sustained for ca. 100 days with low concentration (<2 ppmv) and feed rate (<1.44 µmoles h⁻¹) of N₂O, and the resulting microbial consortia were analyzed via quantitative PCR (qPCR) and metagenomic/metatranscriptomic analyses. Unintended but quantified intrusion of O₂ sustained dissolved oxygen concentration above 4 µM; however, complete N₂O reduction of influent N₂O persisted throughout incubation. Metagenomic investigations indicated that the microbiomes were dominated by an uncultured taxon affiliated to Burkholderiales, and, along with the qPCR results, suggested coexistence of clade I and II N₂O reducers. Contrastingly, metatranscriptomic nosZ pools were dominated by the Dechloromonas-like nosZ subclade, suggesting the importance of the microorganisms possessing this nosZ subclade in reduction of trace N₂O. Further, co-expression of nosZ and ccoNO/cydAB genes found in the metagenome-assembled genomes representing these putative N₂O-reducers implies a survival strategy to maximize utilization of scarcely available electron acceptors in microoxic environmental niches.