Formate As an Alternative Electron Donor for the Anaerobic Methanotrophic Archaeon Candidatus 'methanoperedens Nitroreducens'

A metagenomic survey predicted that some anaerobic methanotrophic archaea (ANME) belonging to the family Methanoperedenaceae can use formate as an alternative electron donor, although this process has yet to be experimentally verified. In this study, formate was fed to an enrichment culture dominated by a member of Methanoperedenaceae, Candidatus 'Methanoperedens nitroreducens'. The culture indeed oxidized formate and reduced nitrate simultaneously in the absence or presence of methane. The fdhAB genes for formate metabolism were identified in the genome of Ca. 'M. nitroreducens', and RTqPCR results showed their expression levels increased 2-6-fold when formate was consumed. This observation was in line with the results of metaproteomic analysis, which showed that the expression levels of formate dehydrogenase (FdhAB) was increased in the presence of formate. Together, the findings strongly suggested that formate can be an alternative electron donor for Ca. 'M. nitroreducens', revealing its versatile metabolic potential. As formate is a product or intermediate compound of many microbial processes, the capability to utilize formate could aid ANME in coping with the dynamic availability of methane in natural or anthropogenic environments. Our findings demonstrate the need for additional research on the feasibility of Ca. 'M. nitroreducens' and other ANME utilizing alternative electron donors other than methane.