Methane-driven microbial nitrous oxide production and reduction in denitrifying anaerobic methane oxidation cultures

Methane (CH 4 ) and nitrous oxide (N 2 O) are two important greenhouse gases (GHGs), and it is very important to achieve the reduction of both gases in the case of increasingly serious climate change. The study confirms the simultaneous mitigation of potent GHGs N 2 O and CH 4 in denitrifying anaerobic methane oxidation (DAMO) enrichment, supplementing the understanding that DAMO microorganisms do not produce and reduce N 2 O. We observed rapid accumulation and consumption of N 2 O in DAMO cultures subjected to high nitrate and nitrite loadings. Moreover, when N 2 O served as the sole electron acceptor, a significant correlation was found between the flux of N 2 O reduction and CH 4 oxidation. Inhibition of the particulate methane monooxygenase (pMMO) led to a notable decrease of 95 % in N 2 O reduction and 50 % in CH 4 oxidation. Metagenomic analysis revealed that Candidatus Methanoperedens and Candidatus Methylomirabilis were the dominant genera, with all functional genes related to denitrification and methane oxidation identified, including NO reductase ( nor ) of Ca. Methylomirabilis. Transcriptomic analysis further confirmed that the transcripts of nor were one order of magnitude higher than other functional genes. These results indicate that when the substrate NO 2 - surpasses the dismutation capacity of NO dismutase ( nod ), DAMO bacteria can alternatively induce nor to reduce NO to N 2 O. However, considering that N 2 O reductase ( nosZ ) is still missing in Ca . Methylomirabilis, the partner Methyloversatilis may be responsible for N 2 O reduction. Our results provide new insights into the link between GHG emissions in DAMO systems under anaerobic conditions. • DAMO cultures fed with high loading of NO 3 - /NO 2 - will produce N 2 O accumulation. • There is a significant correlation between N 2 O reduction and CH 4 oxidation. • The gene nor of DAMO bacteria is transcribed by an order of magnitude higher. • DAMO bacteria utilize NO reductase to reduce NO to N 2 O.