Obligate aerobic methanotrophs have been proven to oxidize methane and participate in denitrification under hypoxic conditions. However, this phenomenon and its metabolic mechanism have not been investigated in detail in aerobic methane oxidation coupled to denitrification (AME-D) process. In this study, a type of hypoxic AME-D consortium was enriched and operated for a long time in a CH4-cycling bioreactor with strict anaerobic control and the nitrite removal rate reached approximately 50 mg N/L/d. Metagenomics combined with DNA stable-isotope probing demonstrated that the genus Methylomonas, which constitutes type I aerobic methanotrophs, was the dominant member and contributed to methane oxidation and partial denitrification. Metagenomic binning recovered a near-complete (98%) draft genome affiliated with the family Methylococcaceae containing essential genes that encode nitrite reductase (nirK), nitric oxide reductase (norBC) and hydroxylamine dehydrogenase (hao). Metabolic reconstruction of the selected Methylococcaceae genomes also revealed a potential link between methanotrophy and partial denitrification.
Image and operational performance of the laboratory-scale CH4-cycling SBR system used for the enrichment of denitrifying AOM-like consortium under O2-depleted conditions. (a) Long-term nitrite removal profile at different operational periods; (b and c) CH4 consumption and N2O production during the batch test on day 590. M and C represent samples with and without methane in the 120 mL serum bottles, respectively.