Anaerobic oxidation of methane (AOM) coupled to nitrite reduction is a novel AOM process that is mediated by denitrifying methanotrophs. To date, enrichments of these denitrifying methanotrophs have been confined to freshwater systems; however, the recent findings of 16S rRNA and pmoA gene sequences in marine sediments suggest a possible occurrence of AOM coupled to nitrite reduction in marine systems. In this research, a marine denitrifying methanotrophic culture was obtained after 20 months of enrichment. Activity testing and quantitative PCR (qPCR) analysis were then conducted and showed that the methane oxidation activity and the number of NC10 bacteria increased correlatively during the enrichment period. 16S rRNA gene sequencing indicated that only bacteria in group A of the NC10 phylum were enriched and responsible for the resulting methane oxidation activity, although a diverse community of NC10 bacteria was harbored in the inoculum. Fluorescence in situ hybridization showed that NC10 bacteria were dominant in the enrichment culture after 20 months. The effect of salinity on the marine denitrifying methanotrophic culture was investigated, and the apparent optimal salinity was 20.5‰, which suggested that halophilic bacterial AOM coupled to nitrite reduction was obtained. Moreover, the apparent substrate affinity coefficients of the halophilic denitrifying methanotrophs were determined to be 9.8 ؎ 2.2 M for methane and 8.7 ؎ 1.5 M for nitrite.A naerobic oxidation of methane (AOM) occurs extensively in natural ecosystems and is a crucial biological sink in the global methane cycle that maintains the balance of greenhouse gas content in the atmosphere (1). To date, five electron acceptors that support AOM, including AOM coupled to sulfate reduction (2), nitrite reduction (3), nitrate reduction (4), iron reduction (5), and manganese reduction (5), have been discovered in natural settings. Moreover, on the basis of bioenergetic calculations, researchers have speculated that several other types of AOM (e.g., AOM coupled to perchlorate reduction, arsenate reduction, and selenate reduction) may exist in nature (6); however, these possible types have not yet been confirmed. In particular, AOM coupled to nitrite reduction has been a predominant research focus in the past several years. AOM coupled to nitrite reduction was also called nitrite-dependent anaerobic methane oxidation (ndamo) in previous reports. It has been demonstrated that AOM coupled to nitrite reduction is mediated by the bacterium "Candidatus Methylomirabilis oxyfera" (denitrifying methanotroph) (7), which is affiliated with the candidate NC10 phylum (8). This candidate division (NC10 phylum) was first defined by classification of environmental sequences retrieved from aquatic microbial formations in flooded caves (9). To date, our knowledge regarding the NC10 phylum has stemmed largely from research into "Ca. Methylomirabilis oxyfera" and "Ca. Methylomirabilis oxyfera"-like bacteria. According to the phylogenetic affiliations of 16S rRNA gene sequ...

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.

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 10 4 ∼ 10 5 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.

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Denitratation (nitrite produced from nitrate), has the potential applications in wastewater treatment by combining with ANAMMOX process. The occurrence of denitratation has been shown to be effected qualitatively by various parameters in the environment. A more quantitative understanding can be obtained using enrichment cultures in lab-scale experiments, yet information on the enrichment of functional microorganisms responsible for denitratation is lacking. In this study, a stable denitratation-dominated culture was obtained from methylotrophic denitrifying culture. The results showed that, besides the substitution of acetate for methanol, the lasting starvation following saturation of electron donor was another pivotal selection pressure that favored the growth of denitratating bacteria, which was supported by the distinctive physiological strategy involving the higher growth rate combining with larger poly-hydroxybutyrate (PHB) accumulation at sufficient electron donor situation and then manage the stress of electron donor starvation by consumpiton of the PHB. High-throughput 16S rRNA gene sequencing analysis indicated that non-methylotrophic Halomonas campisalis (48.1 %) and Halomonas campaniensis (30.4 %) dominated in the denitratating community. Moreover the denitratation was driven by the nitrate inhibiting the nirS transcription in the Halomonas species.