A microbial consortium couples anaerobic methane oxidation to denitrification

Although much speculated on, no microorganisms had been shown capable of anaerobic methane oxidation using nitrate as the sole electron acceptor. Now this reaction has been demonstrated in the laboratory in a microbial community with two members, one a slow-growing bacterium of a type that has not cultured before, and one an archaeal organism. Nucleic acid markers characteristic of both are present in freshwater samples worldwide, suggesting that this reaction is important in the biological methane and nitrogen cycles. It also has the potential to be used to counteract the increases in methane production associated with intensive agriculture. Although much speculated on, this is the first unambiguous report demonstrating the isolation of a consortium of microorganisms capable of anaerobic methane oxidation using nitrate as the sole electron acceptor. Modern agriculture has accelerated biological methane and nitrogen cycling on a global scale1,2. Freshwater sediments often receive increased downward fluxes of nitrate from agricultural runoff and upward fluxes of methane generated by anaerobic decomposition3. In theory, prokaryotes should be capable of using nitrate to oxidize methane anaerobically, but such organisms have neither been observed in nature nor isolated in the laboratory4,5,6,7,8. Microbial oxidation of methane is thus believed to proceed only with oxygen or sulphate9,10. Here we show that the direct, anaerobic oxidation of methane coupled to denitrification of nitrate is possible. A microbial consortium, enriched from anoxic sediments, oxidized methane to carbon dioxide coupled to denitrification in the complete absence of oxygen. This consortium consisted of two microorganisms, a bacterium representing a phylum without any cultured species and an archaeon distantly related to marine methanotrophic Archaea. The detection of relatives of these prokaryotes in different freshwater ecosystems worldwide11,12,13,14 indicates that the reaction presented here may make a substantial contribution to biological methane and nitrogen cycles.