Newly Isolated Strain Methylocystis sp. L03 Oxidizes Methane with Nitrite as Terminal Electron Acceptor

Methane oxidation mediated by methanotrophs is limited, under anoxic condition, by electron acceptors availability, such as oxygen, and ineffective enrichment of microbes. Methylocystis sp., as a typical type II methanotroph, uses nitrite as a terminal electron acceptor and flexibly couples with methane oxidation. This special electron transfer process potentially accelerates methane anoxic oxidation. In this study, two lab-scaled bioreactors were inoculated with reservoir sediment. Both control and treatment groups were fed with ${\mathrm{CH}}_4$, and the treatment group was also supplemented with nitrite as an electron acceptor to enrich effective methanotrophs. The result indicated that Methylocystis sp. performs a major role in methane oxidation and denitrification; 33 key proteins critical for methane metabolism and denitrification were significantly upregulated. The Methylocystis sp.–initialized methane oxidation encoded by particulate methane monooxygenase (pmoABC) then metabolized the product to ${\mathrm{CO}}_2$ in the formaldehyde oxidation VI pathway (${\mathrm{H}}_4\mathrm{MPT}$) and reduced nitrite to nitrogen. Subsequently, ${\mathrm{CO}}_2$ and nitrogen were further transformed into bicarbonate and ammonia in enzymes encoded by cynT and nifK, respectively, both of which were reused by Bacillus sp., Caenimonas sp., Methylocella sp., and other coexisting microorganisms. The strain, Methylocystis sp. L03, was isolated and found to independently reduce nitrite and oxidize methane in an anoxic environment. This study revealed that the unexpectedly flexible methane metabolism by aerobic methanotrophs under nitrite-rich anoxic environments may act as an important and overlooked methane sink, constituting a unique link between the two global nutrient cycles of carbon and nitrogen.