Biogeographical distribution of denitrifying anaerobic methane oxidizing bacteria in Chinese wetland ecosystems

Vertical distribution of Candidatus Methylomirabilis and Methanoperedens in agricultural soils

Candidatus Methylomirabilis-related bacteria conduct anaerobic oxidation of methane (AOM) coupling with NO2- reduction, and Candidatus Methanoperedens-related archaea perform AOM coupling with reduction of diverse electron acceptors, including NO3-, Fe (III), Mn (IV) and SO42-. Application of nitrogen fertilization favors the growth of these methanotrophs in agricultural fields. Here, we explored the vertical variations in community structure and abundance of the two groups of methanotrophs in a nitrogen-rich vegetable field via using illumina MiSeq sequencing and quantitative PCR. The retrieved Methylomirabilis-related sequences had 91.12%-97.32% identity to the genomes of known Methylomirabilis species, and Methanoperedens-related sequences showed 85.49%-97.48% identity to the genomes of known Methanoperedens species which are capable of conducting AOM coupling with reduction of NO3- or Fe (III). The Methanoperedens-related archaeal diversity was significantly higher than Methylomirabilis-related bacteria, with totally 74 and 16 operational taxonomic units, respectively. In contrast, no significant difference in abundance between the bacteria (9.19 × 103-3.83 × 105 copies g-1 dry soil) and the archaea (1.55 × 104-3.24 × 105 copies g-1 dry soil) was observed. Furthermore, the abundance of both groups of methanotrophs exhibited a strong vertical variation, which peaked at 30-40 and 20-30 cm layers, respectively. Soil water content and pH were the key factors influencing Methylomirabilis-related bacterial diversity and abundance, respectively. For the Methanoperedens-related archaea, both soil pH and ammonium content contributed significantly to the changes of these archaeal diversity and abundance. Overall, we provide the first insights into the vertical distribution and regulation of Methylomirabilis-related bacteria and Methanoperedens-related archaea in vegetable soils. KEY POINTS: • The archaeal diversity was significantly higher than bacterial. • There was no significant difference in the abundance between bacteria and archaea. • The abundance of bacteria and archaea peaked at 30-40 and 20-30 cm, respectively.

Evolutionary ecology of denitrifying methanotrophic NC10 bacteria in the deep-sea biosphere

The NC10 phylum links anaerobic methane oxidation to nitrite denitrification through a unique O2-producing intra-aerobic methanotrophic pathway. Although numerous amplicon-based studies revealed the distribution of this phylum, comprehensive genomic insights and niche characterization in deep-sea environments were still largely unknown. In this study, we extensively surveyed the NC10 bacteria across diverse deep-sea environments, including waters, sediments, cold seeps, biofilms, rocky substrates, and subseafloor aquifers. We then reconstructed and analysed 38 metagenome-assembled genomes (MAGs), and revealed the extensive distribution of NC10 bacteria and their intense selective pressure in these harsh environments. Isotopic analyses combined with gene expression profiling confirmed that active nitrite-dependent anaerobic methane oxidation (n-DAMO) occurs within deep-sea sediments. In addition, the identification of the Wood-Ljungdahl (WL) and 3-hydroxypropionate/4-hydroxybutyrat (3HB/4HP) pathways in these MAGs suggests their capability for carbon fixation as chemoautotrophs in these deep-sea environments. Indeed, we found that for their survival in the oligotrophic deep-sea biosphere, NC10 bacteria encode two branches of the WL pathway, utilizing acetyl-CoA from the carbonyl branch for citric acid cycle-based energy production and methane from the methyl branch for n-DAMO. The observed low ratios of non-synonymous substitutions to synonymous substitutions (pN/pS) in n-DAMO-related genes across these habitats suggest a pronounced purifying selection that is critical for the survival of NC10 bacteria in oligotrophic deep-sea environments. These findings not only advance our understanding of the evolutionary adaptations of NC10 bacteria but also underscore the intricate coupling between the carbon and nitrogen cycles within deep-sea ecosystems, driven by this bacterial phylum.

Nitrite-dependent anaerobic methane oxidation (N-DAMO) in global aquatic environments: A review

The vast majority of processes in the carbon and nitrogen cycles are driven by microorganisms. The nitrite-dependent anaerobic oxidation of methane (N-DAMO) process links carbon and nitrogen cycles, offering a novel approach for the simultaneous reduction of methane emissions and nitrite pollution. However, there is currently no comprehensive summary of the current status of the N-DAMO process in natural aquatic environments. Therefore, our study aims to fill this knowledge gap by conducting a comprehensive review of the global research trends in N-DAMO processes in various aquatic environments (excluding artificial bioreactors). Our review mainly focused on molecular identification, global study sites, and their interactions with other elemental cycling processes. Furthermore, we performed a data integration analysis to unveil the effects of key environmental factors on the abundance of N-DAMO bacteria and the rate of N-DAMO process. By combining the findings from the literature review and data integration analysis, we proposed future research perspectives on N-DAMO processes in global aquatic environments. Our overarching goal is to advance the understanding of the N-DAMO process and its role in synergistically reducing carbon emissions and removing nitrogen. By doing so, we aim to make a significant contribution to the timely achievement of China's carbon peak and carbon neutrality targets.

Anaerobic oxidation of methane in terrestrial wetlands: The rate, identity and metabolism

The recent discovery of anaerobic oxidation of methane (AOM) in freshwater ecosystems has caused a great interest in "cryptic methane cycle" in terrestrial ecosystems. Anaerobic methanotrophs appears widespread in wetland ecosystems, yet, the scope and mechanism of AOM in natural wetlands remain poorly understood. In this paper, we review the recent progress regarding the potential of AOM, the diversity and distribution, and the metabolism of anaerobic methanotrophs in wetland ecosystems. The potential of AOM determined through laboratory incubation or in situ isotopic labeling ranges from 1.4 to 704.0 nmol CH4·g-1 dry soil·d-1. It appears that the availability of electron acceptors is critical in driving different AOM in wetland soils. The environmental temperature and salinity exert a significant influence on AOM activity. Reversal methanogenesis and extracellular electron transfer are likely involved in the AOM process. In addition to anaerobic methanotrophic archaea, the direct involvement of methanogens in AOM is also probable. This review presented an overview of the rate, identity, and metabolisms to unravel the biogeochemical puzzle of AOM in wetland soils.

Role and regulation of anaerobic methane oxidation catalyzed by NC10 bacteria and ANME-2d archaea in various ecosystems

Freshwater wetlands, paddy fields, inland aquatic ecosystems and coastal wetlands are recognized as important sources of atmospheric methane (CH₄). Currently, increasing evidence shows the potential importance of the anaerobic oxidation of methane (AOM) mediated by NC10 bacteria and a novel cluster of anaerobic methanotrophic archaea (ANME)-ANME-2d in mitigating CH₄ emissions from different ecosystems. To better understand the role of NC10 bacteria and ANME-2d archaea in CH₄ emission reduction, the current review systematically summarizes different AOM processes and the functional microorganisms involved in freshwater wetlands, paddy fields, inland aquatic ecosystems and coastal wetlands. NC10 bacteria are widely present in these ecosystems, and the nitrite-dependent AOM is identified as an important CH₄ sink and induces nitrogen loss. Nitrite- and nitrate-dependent AOM co-occur in the environment, and they are mainly affected by soil/sediment inorganic nitrogen and organic carbon contents. Furthermore, salinity is another key factor regulating the two AOM processes in coastal wetlands. In addition, ANME-2d archaea have the great potential to couple AOM to the reduction of iron (III), manganese (IV), sulfate, and even humics in different ecosystems. However, the study on the environmental distribution of ANME-2d archaea and their role in CH₄ mitigation in environments is insufficient. In this study, we propose several directions for future research on the different AOM processes and respective functional microorganisms.

How methanotrophs respond to pH: A review of ecophysiology

Varying pH globally affects terrestrial microbial communities and biochemical cycles. Methanotrophs effectively mitigate methane fluxes in terrestrial habitats. Many methanotrophs grow optimally at neutral pH. However, recent discoveries show that methanotrophs grow in strongly acidic and alkaline environments. Here, we summarize the existing knowledge on the ecophysiology of methanotrophs under different pH conditions. The distribution pattern of diverse subgroups is described with respect to their relationship with pH. In addition, their responses to pH stress, consisting of structure-function traits and substrate affinity traits, are reviewed. Furthermore, we propose a putative energy trade-off model aiming at shedding light on the adaptation mechanisms of methanotrophs from a novel perspective. Finally, we take an outlook on methanotrophs' ecophysiology affected by pH, which would offer new insights into the methane cycle and global climate change.

Spatial variations of activity and community structure of nitrite-dependent anaerobic methanotrophs in river sediment

Rivers are an important site for methane emissions and reactive nitrogen removal. The process of nitrite-dependent anaerobic methane oxidation (n-damo) links the global carbon cycle and the nitrogen cycle, but its role in methane mitigation and nitrogen removal in rivers is poorly known. In the present study, we investigated the activity, abundance, and community composition of n-damo bacteria in sediment of the upper, middle, and lower reaches of Wuxijiang River (Zhejiang Province, China). The ¹³CH₄ stable isotope experiments showed that the methane oxidation activity of n-damo was 0.11-1.88 nmol CO₂ g^-1 (dry sediment) d^-1, and the activity measured from the middle reaches was significantly higher than that from the remaining regions. It was estimated that 3.27 g CH₄ m^-2 year^-1 and 8.72 g N m^-2 year^-1 could be consumed via n-damo. Quantitative PCR confirmed the presence of n-damo bacteria, and their 16S rRNA gene abundance varied between 5.45 × 10⁵ and 5.86 × 10⁶ copies g^-1 dry sediment. Similarly, the abundance of n-damo bacteria was significantly higher in the middle reaches. High-throughput sequencing showed a high n-damo bacterial diversity, with totally 152 operational taxonomic units being detected at 97 % sequence similarity cut-off. In addition, the n-damo bacterial community composition also varied spatially. The inorganic nitrogen (NH₄⁺, NO₂^-, NO₃^-) level was found to be the key environmental factor controlling the n-damo activity and bacterial community composition. Overall, our results showed the spatial variations and environmental regulation of the activity and community structure of n-damo bacteria in river sediment, which expanded our understanding of the quantitative importance of n-damo in both methane oxidation and reactive nitrogen removal in riverine systems.

Distribution pattern of N-damo bacteria along an anthropogenic nitrogen input gradient from the coastal mangrove wetland to the South China sea sediments

Microbial nitrite-dependent anaerobic methane oxidation (n-damo) process is important for mitigating methane emission and anthropogenic nitrogen inputs in the marine environment. However, the distribution pattern of n-damo bacteria along an anthropogenic N-input gradient from the coastal wetland to the pristine South China Sea is poorly understood. This study investigated the diversity and abundance of n-damo bacteria in samples collected along a N-input gradient from Mai Po (MP) mangrove wetland sediments of the Pearl River Estuary (PRE) to the deep ocean sediments of the South China Sea (SCS). Retrieved 16S rDNA sequences showed a shift of n-damo community composition of complex structures with both freshwater and marine n-damo lineages in MP intertidal sediments to marine dominated characteristic in SCS sediments. The observed variation of Shannon and Chao1 indexes of n-damo bacteria shared a similar trend of a decrease at first followed by an increase along the targeting gradient with previously investigated methanogens, anaerobic methanotrophic archaea, ammonia-oxidizing archaea and ammonia-oxidizing bacteria, but had a reverse pattern with anammox bacteria. The community structure of pmoA gene sequences contained freshwater lineages only in SCS continental shelf sediments closer to the PRE, and turned to group with other marine samples in deeper and pristine sediments. Results suggested that n-damo bacteria might be a major contributor to anaerobic denitrification in the SCS sediments because their abundances were much higher than previously studied anammox bacteria in the same sample set. The distribution pattern of n-damo bacterial diversity, richness and abundance along the anthropogenic N-input gradient implies that they could be used as a bio-indicator for monitoring the anthropogenic/terrestrial inputs in marine environments.

Response of denitrifying anaerobic methane oxidation enrichment to salinity stress: Process and microbiology

Denitrifying anaerobic methane oxidation (DAMO) is a novel biological process which could decrease nitrogen pollution and methane emission simultaneously in wastewater treatment. Salinity as a key environmental factor has important effects on microbial community and activity, however, it remains unclear for DAMO microorganisms. In this study, response of the enrichment of DAMO archaea and bacteria to different salinity was investigated from the aspect of process and microbiology. The results showed that the increasing salinity from 0.14% to 25% evidently deteriorated DAMO process, with the average removal rate of nitrate and methane decreased from 1.91 mg N/(L·d) to 0.07 mg N/(L·d) and 3.22 μmol/d to 0.59 μmol/d, respectively. The observed IC₅₀ value of salinity on the DAMO culture was 1.73%. Further microbial analyses at the gene level suggested that the relative abundance of DAMO archaea in the enrichment decreased to 46%, 39%, 38% and 33% of the initial value. However, DAMO bacteria suffered less impact with the relative abundance maintaining over 75% of the initial value (except 1% salinity). In functional genes of DAMO bacteria, pmoA, decreased gradually from 100% to 86%, 43%, 15% and 2%, while mcrA (DAMO archaea) maintained at 67%-97%. This difference probably indicated DAMO bacteria appeared functional inhibition prior to community inhibition, which was opposite for the DAMO archaea. Results above-mentioned concluded that, though the process of nitrate-dependent anaerobic methane oxidation was driven by the couple of DAMO archaea and bacteria, they individually featured different response to high salinity stress. These findings could be helpful for the application of DAMO-based process in high salinity wastewater treatment, and also the understanding to DAMO microorganisms.

Potential role of nitrite-dependent anaerobic methane oxidation in methane consumption and nitrogen removal in Chinese paddy fields

Nitrite-dependent anaerobic methane oxidation (n-damo), catalyzed by bacteria closely related to Candidatus Methylomirabilis oxyfera, links the global carbon and nitrogen cycles. Currently, the contribution of n-damo in controlling methane emissions and nitrogen removal, and the key regulatory factors of this process in Chinese paddy fields are poorly known. Here, soil samples from 20 paddy fields located in different climate zones across China were collected to examine the n-damo activity and bacterial communities. The n-damo activity and bacterial abundance varied from 1.05 to 5.97 nmol CH₄ g^-1 (dry soil) d^-1 and 2.59 × 10⁵ to 2.50 × 10⁷ copies g^-1 dry soil, respectively. Based on the n-damo activity, it was estimated that approximately 0.91 Tg CH₄ and 2.17 Tg N could be consumed annually via n-damo in Chinese paddy soils. The spatial variations in n-damo activity and community structure of n-damo bacteria were significantly (p < 0.05) affected by the soil ammonium content, labile organic carbon content and pH. Furthermore, significant differences in n-damo activity, bacterial abundance and community composition were observed among different climate zones. The n-damo activity was found to be positively correlated with the mean annual air temperature. Taken together, our results demonstrated the potential importance of n-damo in both methane consumption and nitrogen removal in Chinese paddy soils, and this process was regulated by local soil and climatic factors.

Quinolone antibiotics enhance denitrifying anaerobic methane oxidation in Wetland sediments: Counterintuitive results

Denitrifying anaerobic methane oxidation (DAMO) plays an important role in the element cycle of wetlands. In recent years, the content of antibiotics in wetlands has gradually increased due to human activities. However, the impact of antibiotics on the ecological function of DAMO remains unclear. Here we studied the influence of three high-content quinolone antibiotics (QNs) on DAMO in the sediments of the Baiyangdian Wetland. The results show that QNs can significantly promote the potential DAMO rates. Moreover, the enhancement of potential DAMO rates is positively correlated with the dosage of QNs. This promotion effect of QNs on nitrate-DAMO can be attributed to the hormesis phenomenon or their inhibition of substrate competitors. As antibacterial agents, QNs inhibit nitrite-DAMO conducted by bacteria, but greatly promote nitrate-DAMO conducted by archaea. These results suggest that the short-term effect of QNs on DAMO in wetlands is promotion rather than inhibition.

Response of potential activity, abundance and community composition of nitrite-dependent anaerobic methanotrophs to long-term fertilization in paddy soils

The process of nitrite-dependent anaerobic methane oxidation (n-damo) catalysed by Candidatus Methylomirabilis oxyfera (M. oxyfera)-like bacteria is a novel pathway in regulating methane (CH₄ ) emissions from paddy fields. Nitrogen fertilization is essential to improve rice yields and soil fertility; however, its effect on the n-damo process is largely unknown. Here, the potential n-damo activity, abundance and community composition of M. oxyfera-like bacteria were investigated in paddy fields under three long-term (32 years) fertilization treatments, i.e. unfertilized control (CK), chemical fertilization (NPK) and straw incorporation with chemical fertilization (SNPK). Relative to the CK, both NPK and SNPK treatments significantly (p < 0.05) increased the potential n-damo activity (88%-110%) and the abundance (52%-105%) of M. oxyfera-like bacteria. The variation of soil organic carbon (OrgC) content and inorganic nitrogen content caused by the input of chemical fertilizers and straw returning were identified as the key factors affecting the potential n-damo activity and the abundance of M. oxyfera-like bacteria. However, the community composition and diversity of M. oxyfera-like bacteria did not change significantly by the input of fertilizers. Overall, our results provide the first evidence that long-term fertilization greatly stimulates the n-damo process, indicating its active role in controlling CH₄ emissions from paddy fields.

Denitrifying anaerobic methane oxidation and mechanisms influencing it in Yellow River Delta coastal wetland soil, China

Methane oxidation coupled to denitrification is mediated by Candidatus "Methylomirabilis oxyfera" (M. oxyfera), which belongs to the candidate phylum NC10, and plays a crucial role in the global carbon and nitrogen cycle. Using the Yellow River Delta coastal wetland as the study area, molecular biology technology and laboratory incubation were used to determine the abundance of NC10 bacteria and the denitrifying anaerobic methane oxidation (DAMO) rate in soils from different vegetation areas. The results of the electrophoresis detection show that M. oxyfera-like bacteria can be found in the four types of soils, according to the growth analysis by the system, OTU1 (SA) has been found the highest similarity to first-discovered Candidatus Methylomir-abilis oxyfera (FP565575) (over 98%); Vegetation cover significantly increased the abundance of M. oxyfera-like bacteria compared to beach areas, which abundance was significantly higher in deeper layers than in surface ones. Nitrate, nitrite, total nitrogen, and conductivity were identified as the main environmental factors affecting the DAMO rate. This study showed that both groups A and B of Candidatus M. oxyfera-like bacteria exist in the coastal wetland of the Yellow River Delta, which provides molecular biological evidence for the existence of the DAMO process therein. Moreover, it was revealed the influence mechanism of physical and chemical characteristics of each region on the DAMO rate. This is of significance for furthering our understanding of the coupled effect of the global carbon and nitrogen cycle.

NO3- is an important driver of nitrite-dependent anaerobic methane oxidation bacteria and CH4 fluxes in the reservoir riparian zone

Nitrite-dependent anaerobic methane oxidation (N-DAMO) is an important biological process that combines microbial nitrogen and carbon cycling and is mainly carried out by nitrite-dependent anaerobic methane-oxidizing bacteria. The discovery of this microbial process has changed the conventional view of methane oxidation and nitrogen loss. In this study, the abundance, diversity, and community structure of N-DAMO bacteria were investigated based on high-throughput sequencing and fluorescence quantitative PCR measurements. We examined environmental factors driving the variations of CH₄ fluxes and N-DAMO bacterial using correlation analysis and redundancy analysis. We found low CH₄ fluxes and abundant N-DAMO bacteria in the riparian zone. After decomposing the effects of single variables and exploring them, NO₃^- was the only significant factor that significantly correlated with the abundance and richness of the N-DAMO community and gas fluxes (p < 0.05). Under the influence of three different land use types, the increase in NO₃^- (grassland vs. woodland and sparse woods, + 132.81% and + 106.25%) caused structural changes in the composition of the N-DAMO bacterial community, increasing its abundance (- 9.58% and + 21.19%), thus promoting the oxidation of CH₄ and reduced CH₄ emissions (+ 4.78% and + 35.63%) from the riparian zone. Appropriate NO₃^- input helps maintain the existing low methane emission fluxes in the riparian zone of the reservoir.

Microbial dynamics and activity of denitrifying anaerobic methane oxidizers in China's estuarine and coastal wetlands

Estuarine and coastal wetlands, which act as large sources of methane (CH₄) and undergo substantial loading of anthropogenic nitrogen (N), provide ideal conditions for denitrifying anaerobic methane oxidation (DAMO) to occur. Yet the microbial mechanisms governing DAMO and the main driving factors in estuarine and coastal ecosystems remain unclear. This study investigated the spatiotemporal distribution and associated activity of DAMO microorganisms along a wide swath of China's coastline (latitudinal range: 22-41°N) using molecular assays and isotope tracing techniques. We uncovered significant spatial and seasonal variation in DAMO bacterial community structure, whereas DAMO archaeal community structure exhibited no seasonal differences. The abundance of DAMO bacterial pmoA gene (2.2 × 10⁵-1.0 × 10⁷ copies g^-1) was almost one order of magnitude higher than that of DAMO archaeal mcrA gene (8.7 × 10⁴ -1.8 × 10⁶ copies g^-1). A significant positive correlation between pmoA and mcrA gene abundances (p < 0.01) was observed, indicating that DAMO bacteria and archaea may cooperate closely and thus complete nitrate elimination. Potential DAMO rates, in the range of 0.09-23.4 nmol ¹³CO₂ g^-1 day^-1 for nitrite-DAMO and 0.03-43.7 nmol ¹³CO₂ g^-1 day^-1 for nitrate-DAMO, tended to be greater in the relatively warmer low-latitudes. Potential DAMO rates were weakly positively correlated with gene abundances, suggesting that DAMO microbial activity could not be predicted directly by gene abundance alone. The heterogeneous variability of DAMO was shaped by interactions among key environmental characteristics (sediment texture, N availability, TOC, Fe³⁺, salinity of water, and temperature). On a broader continental scale, potential N removal rates of 0.1-11.2 g N m^-2 yr^-1 were estimated via nitrite-DAMO activity in China's coastal wetlands. Overall, our results highlight the widespread distribution of DAMO microbes and their potential role in eliminating excess N inputs and reducing CH₄ emissions in estuarine and coastal ecosystems, which could help mitigate global warming.

Nitrite-dependent anaerobic methane oxidation bacteria and potential in permafrost region of Daxing'an Mountains

Nitrite-dependent anaerobic methane oxidation (n-damo) acts as a crucial link between biogeochemical carbon and nitrogen cycles. Nevertheless, very few studies have characterized n-damo microorganisms in high-latitude permafrost regions. Therefore, this study investigated the vertical distribution and diversity of n-damo bacterial communities in soil from three forest types in the permafrost regions of the Daxing'an Mountains. A total of 11 and 8 operational taxonomic units (OTUs) of n-damo 16S rRNA and pmoA genes were observed, respectively. Remarkable spatial variations in n-damo bacteria community richness, diversity, and structure were observed at different soil depths. Moreover, the abundances of n-damo bacteria (16S rRNA and pmoA genes) varied between 1.55 × 10⁴ to 1.47 × 10⁵ and 1.31 × 10³ to 3.11 × 10⁴ copies g^-1 dry soil (ds), as demonstrated by quantitative PCR analyses. ¹³CH₄ stable isotope tracer assays indicated that the potential n-damo rates varied from 0 to 1.26 nmol g^-1 day^-1, with the middle layers (20-40 cm and 40-60 cm) exhibiting significantly higher values than the upper (0-20 cm) and deeper layers (80-100 cm) in all three forest types. Redundancy analyses (RDA) indicated that total organic carbon (TOC), nitrate (NO₃^--N), and nitrite (NO₂^--N) were key modulators of the distribution of n-damo bacterial communities. This study thus demonstrated the widespread occurrence of n-damo bacteria in cold and high-latitude regions of forest ecosystems and provided important insights into the global distribution of these bacteria. KEY POINTS: • This study detected n-damo bacteria in soil samples obtained from the permafrost region of three forest types in the Daxing'an Mountains. • The community composition of n-damo bacteria was mainly affected by soil depth and not forest type. • The abundances of n-damo bacteria first increased and then decreased at higher soil depths.

Response of nitrite-dependent anaerobic methanotrophs to elevated atmospheric CO2 concentration in paddy fields

Nitrite-dependent anaerobic methane oxidation (n-damo) catalyzed by Candidatus Methylomirabilis oxyfera (M. oxyfera)-like bacteria is a new pathway for the regulation of methane emissions from paddy fields. Elevated atmospheric CO₂ concentrations (e[CO₂]) can indirectly affect the structure and function of microbial communities. However, the response of M. oxyfera-like bacteria to e[CO₂] is currently unknown. Here, we investigated the effect of e[CO₂] (ambient CO₂ + 200 ppm) on community composition, abundance, and activity of M. oxyfera-like bacteria at different depths (0-5, 5-10, and 10-20 cm) in paddy fields across multiple rice growth stages (tillering, jointing, and flowering). High-throughput sequencing showed that e[CO₂] had no significant effect on the community composition of M. oxyfera-like bacteria. However, quantitative PCR suggested that the 16S rRNA gene abundance of M. oxyfera-like bacteria increased significantly in soil under e[CO₂], particularly at the tillering stage. Furthermore, ¹³CH₄ tracer experiments showed potential n-damo activity of 0.31-8.91 nmol CO₂ g^-1 (dry soil) d^-1. E[CO₂] significantly stimulated n-damo activity, especially at the jointing and flowering stages. The n-damo activity and abundance of M. oxyfera-like bacteria increased by an average of 90.9% and 50.0%, respectively, under e[CO₂]. Correlation analysis showed that the increase in soil dissolved organic carbon content caused by e[CO₂] had significant effects on the activity and abundance of M. oxyfera-like bacteria. Overall, this study provides the first evidence for a positive response of M. oxyfera-like bacteria to e[CO₂], which may help reduce methane emissions from paddy fields under future climate change conditions.

Methanotrophs: Discoveries, Environmental Relevance, and a Perspective on Current and Future Applications

Methane is the final product of the anaerobic decomposition of organic matter. The conversion of organic matter to methane (methanogenesis) as a mechanism for energy conservation is exclusively attributed to the archaeal domain. Methane is oxidized by methanotrophic microorganisms using oxygen or alternative terminal electron acceptors. Aerobic methanotrophic bacteria belong to the phyla Proteobacteria and Verrucomicrobia, while anaerobic methane oxidation is also mediated by more recently discovered anaerobic methanotrophs with representatives in both the bacteria and the archaea domains. The anaerobic oxidation of methane is coupled to the reduction of nitrate, nitrite, iron, manganese, sulfate, and organic electron acceptors (e.g., humic substances) as terminal electron acceptors. This review highlights the relevance of methanotrophy in natural and anthropogenically influenced ecosystems, emphasizing the environmental conditions, distribution, function, co-existence, interactions, and the availability of electron acceptors that likely play a key role in regulating their function. A systematic overview of key aspects of ecology, physiology, metabolism, and genomics is crucial to understand the contribution of methanotrophs in the mitigation of methane efflux to the atmosphere. We give significance to the processes under microaerophilic and anaerobic conditions for both aerobic and anaerobic methane oxidizers. In the context of anthropogenically influenced ecosystems, we emphasize the current and potential future applications of methanotrophs from two different angles, namely methane mitigation in wastewater treatment through the application of anaerobic methanotrophs, and the biotechnological applications of aerobic methanotrophs in resource recovery from methane waste streams. Finally, we identify knowledge gaps that may lead to opportunities to harness further the biotechnological benefits of methanotrophs in methane mitigation and for the production of valuable bioproducts enabling a bio-based and circular economy.

Fundamentals and potential environmental significance of denitrifying anaerobic methane oxidizing archaea

Many properties of denitrifying anaerobic methane oxidation (DAMO) bacteria have been explored since their first discovery, while DAMO archaea have attracted less attention. Since nitrate is more abundant than nitrite not only in wastewater but also in the natural environment, in depth investigations of the nitrate-DAMO process should be conducted to determine its environmental significance in the global carbon and nitrogen cycles. This review summarizes the status of research on DAMO archaea and the catalyzed nitrate-dependent anaerobic methane oxidation, including such aspects as laboratory enrichment, environmental distribution, and metabolic mechanism. It is shown that appropriate inocula and enrichment parameters are important for the culture enrichment and thus the subsequent DAMO activity, but there are still relatively few studies on the environmental distribution and physiological metabolism of DAMO archaea. Finally, some hypotheses and directions for future research on DAMO archaea, anaerobic methanotrophic archaea, and even anaerobically metabolizing archaea are also discussed.

Specialized metabolic functions of keystone taxa sustain soil microbiome stability

BACKGROUND

The relationship between biodiversity and soil microbiome stability remains poorly understood. Here, we investigated the impacts of bacterial phylogenetic diversity on the functional traits and the stability of the soil microbiome. Communities differing in phylogenetic diversity were generated by inoculating serially diluted soil suspensions into sterilized soil, and the stability of the microbiome was assessed by detecting community variations under various pH levels. The taxonomic features and potential functional traits were detected by DNA sequencing.

RESULTS

We found that bacterial communities with higher phylogenetic diversity tended to be more stable, implying that microbiomes with higher biodiversity are more resistant to perturbation. Functional gene co-occurrence network and machine learning classification analyses identified specialized metabolic functions, especially "nitrogen metabolism" and "phosphonate and phosphinate metabolism," as keystone functions. Further taxonomic annotation found that keystone functions are carried out by specific bacterial taxa, including Nitrospira and Gemmatimonas, among others.

CONCLUSIONS

This study provides new insights into our understanding of the relationships between soil microbiome biodiversity and ecosystem stability and highlights specialized metabolic functions embedded in keystone taxa that may be essential for soil microbiome stability. Video abstract.

Structure and distribution of nitrite-dependent anaerobic methane oxidation bacteria vary with water tables in Zoige peatlands

The recently discovered nitrite-dependent anaerobic methane oxidation (n-damo) is an important methane sink in natural ecosystems performed by NC10 phylum bacteria. However, the effect of water table (WT) gradient due to global change on n-damo bacterial communities is not well studied in peatlands. Here, we analysed the vertical distribution (0-100 cm) of n-damo bacterial communities at three sites with different WTs of the Zoige peatlands in the Qinghai-Tibetan Plateau. Using an n-damo bacterial specific 16S rRNA gene clone library, we obtained 25 operational taxonomic units (OTUs) that could be divided into Groups A, B, C, D and E (dominated by A and B). The dominant group was Group B at the high (OTU14 and OTU20) and intermediate (OTU7 and OTU8) WT sites and Group A was dominant at the low WT site (OTU6 and OTU5). Using high-throughput sequencing, we observed that n-damo bacteria mainly distributed in subsurface soils (50-60 and 20-30 cm), and their relative abundances were higher at the low WT site than at the other two sites. In addition, we found that pH and nitrate were positively correlated with Group A, while total organic carbon, total nitrogen and ammonia were positively associated with Group B. Our study provides new insights into our understanding of the response of n-damo bacteria to WT gradient in peatlands, with important implications for global change.

Dissimilatory nitrate/nitrite reduction to ammonium (DNRA) pathway dominates nitrate reduction processes in rhizosphere and non-rhizosphere of four fertilized farmland soil

Nitrate (NO₃^-) reduction partitioning between denitrification, anaerobic ammonium oxidation (anammox), denitrifying anaerobic methane oxidation (DAMO), and dissimilatory nitrate reduction to ammonium (DNRA), can influence the nitrogen (N) use efficiency and crop production in arid farmland. The microbial structure, function and potential rates of denitrification, anammox, DAMO and DNRA, and their respective contributions to total NO₃^- reduction were investigated in rhizosphere and non-rhizosphere soil of four typical crops in north China by functional gene amplification, high-throughput sequencing, network analysis and isotopic tracing technique. The measured denitrification and DNRA rate varied from 0.0294 to 20.769 nmol N g^-1 h^-1and 2.4125-58.682 nmol N g^-1 h^-1, respectively, based on which DNRA pathway contributed to 84.44 ± 14.40% of dissimilatory NO₃^- reduction, hence dominated NO₃^- reduction processes compared to denitrification. Anammox and DAMO were not detected. High-throughput sequencing analysis on DNRA nrfA gene, and denitrification nirS and nirK genes demonstrated that these two processes did not correlate to corresponding gene abundance or dominant genus. RDA and Pearson's correlation analysis illustrated that DNRA rate was significantly correlated with the abundance of Chthiniobacter, as well as total organic matter (TOM); denitrification rate was significantly correlated with the abundance of Lautropia, so did TOM. Network analysis showed that the genus performed DNRA was the key connector in the microbial community of dissimilatory nitrate reducers. This study simultaneously investigated the dissimilatory nitrate reduction processes in rhizosphere and non-rhizosphere soils in arid farmland, highlighting that DNRA dominated NO₃^- reduction processes against denitrification. As denitrification results in N loss, whereas DNRA contributes to N retention, the relative contributions of DNRA versus denitrification activities should be considered appropriately when assessing N transformation processes and N fertilizer management in arid farmland fields.

Nitrate reduction in the reed rhizosphere of a riparian zone: From functional genes to activity and contribution

The increased nitrogen (N) fertilizer usage caused substantial nitrate (NO₃^-) leaching into groundwater and eutrophication in downstream aquatic systems. Riparian zones positioned as the link interfaces of terrestrial and aquatic environments are effective in NO₃^- removal. However, the microbial mechanisms regulating NO₃^- reduction in riparian zones are still unclear. In this study, four microbial NO₃^- reduction processes were explored in fine-scale riparian soil horizons by isotopic tracing technique, qPCR of functional gene, high-throughput amplicon sequencing, and phylogenetic molecular ecological network analysis. Interestingly, anaerobic ammonium oxidation (anammox) contributed to NO₃^- removal of up to 48.2% only in waterward sediments but not in landward soil. Denitrification was still the most significant contributor to NO₃^- reduction (32.0-91.8%) and N-losses (51.7-100%). Additionally, dissimilatory nitrate reduction to ammonium (DNRA) played a key role in NO₃^- reduction (4.4-67.5%) and was even comparable to denitrification. Community structure analysis of denitrifying, anammox, and DNRA bacterial communities targeting the related functional gene showed that spatial heterogeneity played a greater role than both temporal and soil type (rhizosphere and non-rhizosphere soil) variability in microbial community structuring. Denitrification and DNRA communities were diverse, and their activities did not depend on gene abundance but were significantly related to organic matter, suggesting that gene abundance alone was insufficient in assessing their activity in riparian zones. Based on networks, DNRA plays a keystone role among the microbial NO₃^- reducers. As the last line of defense in the interception of terrestrial NO₃^-, these findings contribute to our understanding of NO₃^- removal mechanisms in riparian zones, and could potentially be exploited to reduce the diffusion of NO₃^- pollution.

Denitrifying anaerobic methane oxidation in marsh sediments of Chongming eastern intertidal flat

Denitrifying anaerobic methane oxidation (DAMO) and associated microbial diversity and abundance in the marsh sediments of Chongming eastern intertidal flat, the Yangtze Estuary, were investigated using carbon-isotope tracing and molecular techniques. Co-existence of nitrate-DAMO archaea and nitrite-DAMO bacteria was evidenced, with higher biodiversity of DAMO archaea than DAMO bacteria. Abundance of DAMO archaeal mcrA gene and DAMO bacterial pmoA gene ranged from 4.2 × 10³ to 3.9 × 10¹⁰ copies g^-1 and from 4.5 × 10⁵ to 6.4 × 10⁶ copies g^-1, respectively. High DAMO potential was detected, ranging from 0.6 to 46.7 nmol ¹³CO₂ g^-1 day^-1 for nitrate-DAMO and from 1.3 to 39.9 nmol ¹³CO₂ g^-1 day^-1 for nitrite-DAMO. In addition to playing an important role as a CH₄ sink, DAMO bacteria also removed a substantial amount of reactive nitrogen (29.4 nmol N g^-1 day^-1) from the intertidal sediments. Overall, these results indicate the importance of DAMO bioprocess as methane and nitrate sinks in intertidal marshes.

Denitrification is the main microbial N loss pathway on the Qinghai-Tibet Plateau above an elevation of 5000 m

Soil nitrogen (N) deficiency is the major factor contributing to low primary productivity on the Qinghai-Tibet Plateau. However, most of our understanding of N cycling is still based on human disturbed environments, and the microbial mechanisms governing N loss in low primary productivity environment remain unclear. This study explores three microbial N loss pathways in eight wetland and dryland soil profiles from the Qinghai-Tibet Plateau, at an elevation of above 5000 m with little human activity, using ¹⁵N isotopic tracing slurry technology, quantitative PCR, and high-throughput sequencing. No denitrifying anaerobic methane oxidation was detected. Anammox occurred in two of the wetland (n = 4) and dryland (n = 4) soil profiles, while denitrification widely occurred and was the dominant N loss pathway in all samples. Where denitrification and anammox co-occurred, both abundance and activity were higher in wetland than in dryland soils and higher in surface than in subsurface soils. In comparison with non-anammox sites, nitrate levels initiate anammox-related N cycling. High-throughput sequencing and network analysis of nirK, nirS, nosZ, and hzsB gene communities showed that Bradyrhizobiaceae (a family of rhizobia) may play a dominant role in N loss pathways in this region. Given the geological evolution and relatively undisturbed habitat, these findings strongly suggest that denitrification is the dominant N loss pathway in terrestrial habitats of the Qing-Tibet Plateau with minimal anthropogenic activity.

Methane emissions from aqueous sediments are influenced by complex interactions among microbes and environmental factors: A modeling study

Methane fluxes from aqueous sediments strongly influence global atmospheric methane. However, many questions still puzzle researchers; for example, why are some unstable sediments atmospheric methane sinks? In this study, a biofilm model originally developed for wastewater treatment was modified to simulate the microbial kinetics and substance conversions in aqueous surface sediments. The model was validated by the experimental data and could predict chemical profiles and microbial distributions in sediments. The model revealed complicated interactions between different microbial communities and environmental factors, including competition between aerobic methane-oxidizing bacteria, nitrite-dependent anaerobic methane-oxidizing bacteria, and anaerobic ammonia-oxidizing bacteria. The results of model simulations showed that the effects of environmental factors, especially dissolved oxygen and ammonia in overlying water, on methane fluxes are very complicated. Rapid environmental changes (which can be caused by tide, day-night alternation, or zoobenthic and human activity) and intensive competition between microbes greatly affected methane fluxes and resulted in alternation between atmospheric methane source and sink in unstable sediments. This study extends the application of a wastewater treatment model to ecological studies of microbial interactions in natural sediments and explains some problems that might be difficult to resolve by using experimental methods.

Denitrifying Anaerobic Methane Oxidation: A Previously Overlooked Methane Sink in Intertidal Zone

The intertidal zone is an open ecosystem rich in organic matter and plays an important role in global biogeochemical cycles. It was previously considered that methane was mainly removed by sulfate-dependent anaerobic methane oxidation (sulfate-AOM) process in marine ecosystems while other anaerobic methane oxidation processes were ignored. Recent researches have demonstrated that denitrifying anaerobic methane oxidation (DAMO), consisting of nitrite-dependent anaerobic methane oxidation (nitrite-AOM) and nitrate-dependent anaerobic methane oxidation (nitrate-AOM), can also oxidize methane. In this work, the community structure, quantity and potential methane oxidizing rate of DAMO archaea and bacteria in the intertidal zone were studied by high-throughput sequencing, qPCR and stable isotope tracing method. The results showed that nitrate-AOM and nitrite-AOM were both active in the intertidal zone and showed approximate methane oxidation rates. The copy number of 16S rRNA gene of DAMO archaea and DAMO bacteria were 10⁴ ∼ 10⁵ copies g^-1 (dry sediment), whereas NC10 bacteria were slightly higher. The contribution rate of DAMO process to total anaerobic methane removal in the intertidal zone reached 65.6% ∼ 100%, which indicates that DAMO process is an important methane sink in intertidal ecosystem. Laboratory incubations also indicated that DAMO archaea were more sensitive to oxygen and preferred a more anoxic environment. These results help us draw a more complete picture of methane and nitrogen cycles in natural habitats.

Diversity of NC10 bacteria associated with sediments of submerged Potamogeton crispus (Alismatales: Potmogetonaceae)

Background

The nitrite-dependent anaerobic methane oxidation (N-DAMO) pathway, which plays an important role in carbon and nitrogen cycling in aquatic ecosystems, is mediated by “Candidatus Methylomirabilis oxyfera” (M. oxyfera) of the NC10 phylum. M. oxyfera-like bacteria are widespread in nature, however, the presence, spatial heterogeneity and genetic diversity of M. oxyfera in the rhizosphere of aquatic plants has not been widely reported.

Method

In order to simulate the rhizosphere microenvironment of submerged plants, Potamogeton crispus was cultivated using the rhizobox approach. Sediments from three compartments of the rhizobox: root (R), near-rhizosphere (including five sub-compartments of one mm width, N1–N5) and non-rhizosphere (>5 mm, Non), were sampled. The 16S rRNA gene library was used to investigate the diversity of M. oxyfera-like bacteria in these sediments.

Results

Methylomirabilis oxyfera-like bacteria were found in all three sections, with all 16S rRNA gene sequences belonging to 16 operational taxonomic units (OTUs). A maximum of six OTUs was found in the N1 sub-compartment of the near-rhizosphere compartment and a minimum of four in the root compartment (R) and N5 near-rhizosphere sub-compartment. Indices of bacterial community diversity (Shannon) and richness (Chao1) were 0.73–1.16 and 4–9, respectively. Phylogenetic analysis showed that OTU1-11 were classified into group b, while OTU12 was in a new cluster of NC10.

Discussion

Our results confirmed the existence of M. oxyfera-like bacteria in the rhizosphere microenvironment of the submerged plant P. crispus. Group b of M. oxyfera-like bacteria was the dominant group in this study as opposed to previous findings that both group a and b coexist in most other environments. Our results indicate that understanding the ecophysiology of M. oxyfera-like bacteria group b may help to explain their existence in the rhizosphere sediment of aquatic plant.

The Occurrence of Putative Nitric Oxide Dismutase (Nod) in an Alpine Wetland with a New Dominant Subcluster and the Potential Ability for a Methane Sink

Recently, a new oxygenic pathway has been proposed based on the disproportionation of NO with putative NO dismutase (Nod). In addition to a new process in nitrogen cycling, this process provides ecological advantages for the degradation of substrates in anaerobic conditions, which is of great significance for wastewater treatment. However, the Nod distribution in aquatic environments is rarely investigated. In this study, we obtained the nod genes with an abundance of 2.38 ± 0.96 × 10⁵ copies per gram of dry soil from the Zoige wetland and aligned the molecular characteristics in the corresponding Nod sequences. These Nod sequences were not only found existing in NC10 bacteria, but were also found forming some other clusters with Nod sequences from a WWTP reactor or contaminated aquifers. Moreover, a new subcluster in the aquifer-similar cluster was even dominant in the Zoige wetland and was named the Z-aquifer subcluster. Additionally, soils from the Zoige wetland showed a high potential rate (10.97 ± 1.42 nmol of CO₂ per gram of dry soil per day) for nitrite-dependent anaerobic methane oxidation (N-DAMO) with low abundance of NC10 bacteria, which may suggest a potential activity of Nod in other clusters when considering the dominance of the Z-aquifer subcluster Nod. In conclusion, we verified the occurrence of Nod in an alpine wetland for the first time and found a new subcluster to be dominant in the Zoige wetland. Moreover, this new subcluster of Nod may even be active in the N-DAMO process in this alpine wetland, which needs further study to confirm.

Different clusters of Candidatus 'Methanoperedens nitroreducens'-like archaea as revealed by high-throughput sequencing with new primers

The newly discovered Candidatus ‘Methanoperedens nitroreducens’ (M. nitroreducens), mediating nitrate-dependent anaerobic oxidation of methane, is an important microorganism in linking carbon and nitrogen cycles. In order to explore the diversity of M. nitroreducens-like archaea in various environmental niches with advanced high-throughput sequencing, new primers based on alpha subunit of methyl-coenzyme M reductase gene were designed. The PCR results demonstrated that the new primers could effectively detect M. nitroreducens-like archaea from an enrichment culture dominated by M. nitroreducens as well as samples collected from a natural freshwater lake and a full-scale wastewater treatment plant (WWTP). By high-throughput sequencing, more than 30,000 M. nitroreducens-like sequences were obtained. Phylogenetic analysis of these sequences along with published sequences showed that M. nitroreducens-like archaea could be divided into three sub-branches (named as Group A, Group B and Group C in this study). Clear geographical difference was observed, with Group A and Group B dominating samples in Queensland (Australia) and in European ecosystems, respectively. Further quantitative PCR revealed that the M. nitroreducens-like archaea were more abundant in WWTP than the freshwater lake. The study provided a large number of sequences for M. nitroreducens-like archaeal communities, thus expanded our understanding on the ecological diversity of M. nitroreducens-like archaea.

New molecular method to detect denitrifying anaerobic methane oxidation bacteria from different environmental niches

The denitrifying anaerobic methane oxidation is an ecologically important process for reducing the potential methane emission into the atmosphere. The responsible bacterium for this process was Candidatus Methylomirabilis oxyfera belonging to the bacterial phylum of NC10. In this study, a new pair of primers targeting all the five groups of NC10 bacteria was designed to amplify NC10 bacteria from different environmental niches. The results showed that the group A was the dominant NC10 phylum bacteria from the sludges and food waste digestate while in paddy soil samples, group A and group B had nearly the same proportion. Our results also indicated that NC10 bacteria could exist in a high pH environment (pH9.24) from the food waste treatment facility. The Pearson relationship analysis showed that the pH had a significant positive relationship with the NC10 bacterial diversity (p<0.05). The redundancy analysis further revealed that the pH, volatile solid and nitrite nitrogen were the most important factors in shaping the NC10 bacterial structure (p=0.01) based on the variation inflation factors selection and Monte Carlo test (999 times). Results of this study extended the existing molecular tools for studying the NC10 bacterial community structures and provided new information on the ecological distributions of NC10 bacteria.

Microbiological and environmental significance of metal-dependent anaerobic oxidation of methane

Anaerobic oxidation of methane (AOM) can be coupled to the reduction of sulfate, nitrate and nitrite, which effectively reduces methane emission into the atmosphere. Recently, metal-dependent AOM (metal-AOM, AOM coupled to metal reduction) was demonstrated to occur in both environmental samples and enrichment cultures. Anaerobic methanotrophs are capable of respiration using Fe(III) or Mn(IV), whether they are in the form of soluble metal species or insoluble minerals. Given the wide distribution of Fe(III)/Mn(IV)-bearing minerals in aquatic methane-rich environments, metal-AOM is considered to be globally important, although it has generally been overlooked in previous studies. In this article, we discuss the discovery of this process, the microorganisms and mechanisms involved, environmental significance and factors influencing metal-AOM. Since metal-AOM is poorly studied to date, some discussion is included on the present understanding of sulfate- and nitrate-AOM and traditional metal reduction processes using organic substrates or hydrogen as electron donors. Metal-AOM is a relatively new research field, and therefore more studies are needed to fully characterize the process. This review summarizes current studies and discusses the many unanswered questions, which should be useful for future research in this field.

Aerobic and anaerobic methanotrophic communities in urban landscape wetland

Both aerobic methane-oxidizing bacteria (MOB) and nitrite-dependent anaerobic methane oxidation (n-damo) organisms can be important methane sinks in a wetland. However, the influences of the vegetation type on aerobic MOB and n-damo communities in wetland, especially in constructed wetland, remain poorly understood. The present study investigated the influences of the vegetation type on both aerobic MOB and n-damo organisms in a constructed urban landscape wetland. Sediments were collected from eight sites vegetated with different plant species. The abundance (1.19-3.27 × 10⁷ pmoA gene copies per gram dry sediment), richness (Chao1 estimator = 16.3-81.5), diversity (Shannon index = 2.10-3.15), and structure of the sediment aerobic MOB community were found to vary considerably with sampling site. In contrast, n-damo community abundance (8.74 × 10⁵-4.80 × 10⁶ NC10 16S rRNA gene copies per gram dry sediment) changed slightly with the sampling site. The richness (Chao1 estimator = 1-11), diversity (Shannon index = 0-0.78), and structure of the NC10 16S rRNA gene-based n-damo community illustrated slight site-related changes, while the spatial changes of the pmoA gene-based n-damo community richness (Chao1 estimator = 1-8), diversity (Shannon index = 0-0.99), and structure were considerable. The vegetation type could have a profound impact on the wetland aerobic MOB community and had a stronger influence on the pmoA-based n-damo community than on the NC10 16S-based one in urban wetland. Moreover, the aerobic MOB community had greater abundance and higher richness and diversity than the n-damo community. Methylocystis (type II MOB) predominated in urban wetland, while no known type I MOB species was detected. In addition, the ratio of total organic carbon to total nitrogen (C/N) might be a determinant of sediment n-damo community diversity and aerobic MOB richness.

Cooccurrence and potential role of nitrite- and nitrate-dependent methanotrophs in freshwater marsh sediments

Nitrite- and nitrate-dependent anaerobic methane oxidation are mediated by the NC10 bacteria closely related to "Candidatus Methylomirabilis oxyfera" (M. oxyfera) and the ANME-2d archaea closely related to "Candidatus Methanoperedens nitroreducens" (M. nitroreducens), respectively. Here, we investigated the occurrence and activity of both M. oxyfera-like bacteria and M. nitroreducens-like archaea in the sediment of freshwater marshes in Eastern China. The presence of diverse M. oxyfera-like bacteria (>87% identity to M. oxyfera) and M. nitroreducens-like archaea (>96% identity to M. nitroreducens) was confirmed by using Illumina-based total bacterial and archaeal 16S rRNA gene sequencing, respectively. The recovered M. oxyfera-like bacterial sequences accounted for 1.6-4.3% of the total bacterial 16S rRNA pool, and M. nitroreducens-like archaeal sequences accounted for 0.2-1.8% of the total archaeal 16S rRNA pool. The detected numbers of OTUs of the 16S rRNA genes of M. oxyfera-like bacteria and M. nitroreducens-like archaea were 78 and 72, respectively, based on 3% sequence difference. Quantitative PCR showed that the 16S rRNA gene abundance of M. oxyfera-like bacteria (6.1 × 10⁶-3.2 × 10⁷ copies g^-1 sediment) was 2-4 orders of magnitude higher than that of M. nitroreducens-like archaea (1.4 × 10³-3.2 × 10⁴ copies g^-1 sediment). Stable isotope experiments showed that the addition of both nitrite and nitrate stimulated the anaerobic methane oxidation, while the stimulation by nitrite is more significant than nitrate. Our results provide the first evidence that the M. oxyfera-like bacteria play a more important role than the M. nitroreducens-like archaea in methane cycling in wetland systems.

Faunal Burrows Alter the Diversity, Abundance, and Structure of AOA, AOB, Anammox and n-Damo Communities in Coastal Mangrove Sediments

In the present work, the diversity, community structures, and abundances of aerobic ammonia-oxidizing archaea (AOA) and bacteria (AOB), anaerobic ammonium-oxidizing (anammox) bacteria, and denitrifying anaerobic methane oxidization (n-damo) bacteria were unraveled in the bioturbated areas of the coastal Mai Po mangrove sediments. Results indicated that the bioturbation by burrowing in mangrove sediments was associated with higher concentration of NH₄⁺ but lower concentrations of both NO₂^- and NO₃^-, and increase in diversity and richness of both AOA and AOB, but relatively lower diversity and richness of n-damo bacteria. The phylotypes of anammox bacterial community were significantly increased while their phylogenetic lineages observed in the less bioturbated areas were also maintained. Infauna also showed a great impact on the composition of n-damo bacterial phylotypes and burrowing activity altered the n-damo community structure profoundly in the sampled areas. The communities of n-damo bacteria in the surrounding bulk sediments showed similar structures to marine n-damo communities, but those on the burrow wall and in the ambient surface layer had a freshwater pattern, which was different from previous findings in Mai Po wetland. On the other hand, the abundances of AOA, AOB, and n-damo bacteria were greatly stimulated on burrow walls while the abundance of anammox bacteria remained unchanged. Infaunal burrows and mangrove roots affected the relative abundance of AOA and AOB. The benthic infauna stimulated the abundances of AOA, AOB, anammox, and n-damo bacteria. Furthermore, NH₄⁺ and NO₂^- were important environmental factors changing the structure of each group. The communities of anammox and n-damo bacteria in bioturbated areas showed a competitive relationship.

A New Primer to Amplify pmoA Gene From NC10 Bacteria in the Sediments of Dongchang Lake and Dongping Lake

Nitrite-dependent anaerobic methane oxidation (n-damo) is catalyzed by the NC10 phylum bacterium "Candidatus Methylomirabilis oxyfera" (M. oxyfera). Generally, the pmoA gene is applied as a functional marker to test and identify NC10-like bacteria. However, it is difficult to detect the NC10 bacteria from sediments of freshwater lake (Dongchang Lake and Dongping Lake) with the previous pmoA gene primer sets. In this work, a new primer cmo208 was designed and used to amplify pmoA gene of NC10-like bacteria. A newly nested PCR approach was performed using the new primer cmo208 and the previous primers cmo182, cmo682, and cmo568 to detect the NC10 bacteria. The obtained pmoA gene sequences exhibited 85-92% nucleotide identity and 95-97% amino acid sequence identity to pmoA gene of M. oxyfera. The obtained diversity of pmoA gene sequences coincided well with the diversity of 16S rRNA sequences. These results indicated that the newly designed pmoA primer cmo208 could give one more option to detect NC10 bacteria from different environmental samples.

Nitrate- and nitrite-dependent anaerobic oxidation of methane

Microbial methane oxidation is an important process to reduce the emission of the greenhouse gas methane. Anaerobic microorganisms couple the oxidation of methane to the reduction of sulfate, nitrate and nitrite, and possibly oxidized iron and manganese minerals. In this article, we review the recent finding of the intriguing nitrate- and nitrite-dependent anaerobic oxidation of methane (AOM). Nitrate-dependent AOM is catalyzed by anaerobic archaea belonging to the ANME-2d clade closely related to Methanosarcina methanogens. They were named 'Candidatus Methanoperedens nitroreducens' and use reverse methanogenesis with the key enzyme methyl-coenzyme M (methyl-CoM) reductase for methane activation. Their major end product is nitrite which can be taken up by nitrite-dependent methanotrophs. Nitrite-dependent AOM is performed by the NC10 bacterium 'Candidatus Methylomirabilis oxyfera' that probably utilizes an intra-aerobic pathway through the dismutation of NO to N₂ and O₂ for aerobic methane activation by methane monooxygenase, yet being a strictly anaerobic microbe. Environmental distribution, physiological and biochemical aspects are discussed in this article as well as the cooperation of the microorganisms involved.

A novel denitrifying methanotroph of the NC10 phylum and its microcolony

The NC10 phylum is a candidate phylum of prokaryotes and is considered important in biogeochemical cycles and evolutionary history. NC10 members are as-yet-uncultured and are difficult to enrich, and our knowledge regarding this phylum is largely limited to the first species ‘Candidatus Methylomirabilis oxyfera’ (M. oxyfera). Here, we enriched NC10 members from paddy soil and obtained a novel species of the NC10 phylum that mediates the anaerobic oxidation of methane (AOM) coupled to nitrite reduction. By comparing the new 16S rRNA gene sequences with those already in the database, this new species was found to be widely distributed in various habitats in China. Therefore, we tentatively named it ‘Candidatus Methylomirabilis sinica’ (M. sinica). Cells of M. sinica are roughly coccus-shaped (0.7–1.2 μm), distinct from M. oxyfera (rod-shaped; 0.25–0.5 × 0.8–1.1 μm). Notably, microscopic inspections revealed that M. sinica grew in honeycomb-shaped microcolonies, which was the first discovery of microcolony of the NC10 phylum. This finding opens the possibility to isolate NC10 members using microcolony-dependent isolation strategies.

Anaerobic methane oxidation coupled to nitrite reduction can be a potential methane sink in coastal environments

In the current study, we investigated nitrite-dependent anaerobic methane oxidation (N-DAMO) as a potential methane sink in the Hangzhou Bay and the adjacent Zhoushan sea area. The potential activity of the N-DAMO process was primarily observed in Hangzhou Bay by means of (13)C-labeling experiments, whereas very low or no potential N-DAMO activity could be detected in the Zhoushan sea area. The measured potential N-DAMO rates ranged from 0.2 to 1.3 nmol (13)CO2 g(-1) (dry sediment) day(-1), and the N-DAMO potentially contributed 2.0-9.4 % to the total microbial methane oxidation in the examined sediments. This indicated that the N-DAMO process may be an alternative pathway in the coastal methane cycle. Phylogenetic analyses confirmed the presence of Candidatus Methylomirabilis oxyfera-like bacteria in all the examined sediments, while the group A members (the dominant bacteria responsible for N-DAMO) were found mainly in Hangzhou Bay. Quantitative PCR showed that the 16S rRNA gene abundance of Candidatus M. oxyfera-like bacteria varied from 5.4 × 10(6) to 5.0 × 10(7) copies g(-1) (dry sediment), with a higher abundance observed in Hangzhou Bay. In addition, the overlying water NO3 (-) concentration and salinity were identified as the most important factors influencing the abundance and potential activity of Candidatus M. oxyfera-like bacteria in the examined sediments. This study showed the evidence of N-DAMO in coastal environments and indicated the importance of N-DAMO as a potential methane sink in coastal environments.

Comparison of community structures of Candidatus Methylomirabilis oxyfera-like bacteria of NC10 phylum in different freshwater habitats

Methane oxidation coupled to nitrite reduction is mediated by 'Candidatus Methylomirabilis oxyfera' (M. oxyfera), which belongs to the NC10 phylum. In this study, the community composition and diversity of M. oxyfera-like bacteria of NC10 phylum were examined and compared in four different freshwater habitats, including reservoir sediments (RS), pond sediments (PS), wetland sediments (WS) and paddy soils (PAS), by using Illumina-based 16S rRNA gene sequencing. The recovered NC10-related sequences accounted for 0.4-2.5% of the 16S rRNA pool in the examined habitats, and the highest percentage was found in WS. The diversity of NC10 bacteria were the highest in RS, medium in WS, and lowest in PS and PAS. The observed number of OTUs (operational taxonomic unit; at 3% cut-off) were 97, 46, 61 and 40, respectively, in RS, PS, WS and PAS. A heterogeneous distribution of NC10 bacterial communities was observed in the examined habitats, though group B members were the dominant bacteria in each habitat. The copy numbers of NC10 bacterial 16S rRNA genes ranged between 5.8 × 10(6) and 3.2 × 10(7) copies g(-1) sediment/soil in the examined habitats. These results are helpful for a systematic understanding of NC10 bacterial communities in different types of freshwater habitats.

Improved PCR primers to amplify 16S rRNA genes from NC10 bacteria

Anaerobic oxidation of methane (AOM) coupled to nitrite reduction (AOM-NIR) is ecologically significant for mitigating the methane-induced greenhouse effect. The microbes responsible for this reaction, NC10 bacteria, have been widely detected in diverse ecosystems. However, some defects were discovered in the commonly used NC10-specific primers, 202F and qP1F. In the present work, the primers were redesigned and improved to overcome the defects found in the previous primers. A new nested PCR method was developed using the improved primers to amplify 16S ribosomal RNA (rRNA) genes from NC10 bacteria. In the new nested PCR method, the qP1mF/1492R and 1051F/qP2R primer sets were used in the first and second rounds, respectively. The PCR products were sequenced, and more operational taxonomic units (OTUs) of the NC10 phylum were obtained using the new primers compared to the previous primers. The sensitivity of the new nested PCR was tested by the serial dilution method, and the limit of detection was approximately 10(3) copies g(-1) dry sed. for the environmental samples compared to approximately 10(5) copies g(-1) dry sed. by the previous method. Finally, the improved primer, qP1mF, was used in quantitative PCR (qPCR) to determine the abundance of NC10 bacteria, and the results agreed well with the activity of AOM-NIR measured by isotope tracer experiments. The improved primers are able to amplify NC10 16S rRNA genes more efficiently than the previous primers and useful to explore the microbial community of the NC10 phylum in different systems.

Nitrite-dependent anaerobic methane oxidizing bacteria along the water level fluctuation zone of the Three Gorges Reservoir

The nitrite-dependent anaerobic methane oxidation (n-damo) mediated by "Candidatus Methylomirabilis oxyfera" connects the biogeochemical carbon and nitrogen cycles in a novel way. Many environments have been reported to harbor such organism being slow-growing and oxygen-sensitive anaerobes. Here, we focused on the population of n-damo bacteria in a fluctuating habitat being the wetland in the water level fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR) in China. A molecular approach demonstrated positive amplifications when targeting the functional pmoA gene only in the lower sites which endured longer flooding time in an elevation gradient. Only 1 operational taxonomic unit (OTU) in the lower elevation zone targeting the 16S ribosomal RNA (rRNA) gene was clustering into the NC-10 group a, which is presumed to be the true n-damo group. Moreover, a relatively low level of diversity was observed in this study. The abundances were as low as 4.7 × 10(2) to 1.5 × 10(3) copies g(-1) dry soil (ds) in the initial stage, which were almost the lowest reported. However, an increase was observed (3.2 × 10(3) to 5.3 × 10(4) copies g(-1) ds) after nearly 6 months of flooding. Intriguingly, the abundance of n-damo bacteria correlated positively with the accumulated flooding time (AFT). The current study revealed that n-damo bacteria can be detected in a fluctuating environment and the sites with longer flooding time seem to be preferred habitats. The water flooding may be the principal factor in this ecosystem by creating anoxic condition. The wide range of such habitats suggests a high potential of n-damo bacteria to play a key role in natural CH4 consumption.