Methane-Consuming Archaea Revealed by Directly Coupled Isotopic and Phylogenetic Analysis

Microorganisms living in anoxic marine sediments consume more than 80% of the methane produced in the world's oceans. In addition to single-species aggregates, consortia of metabolically interdependent bacteria and archaea are found in methane-rich sediments. A combination of fluorescence in situ hybridization and secondary ion mass spectrometry shows that cells belonging to one specific archaeal group associated with theMethanosarcinales were all highly depleted in13C (to values of –96‰). This depletion indicates assimilation of isotopically light methane into specific archaeal cells. Additional microbial species apparently use other carbon sources, as indicated by significantly higher13C/12C ratios in their cell carbon. Our results demonstrate the feasibility of simultaneous determination of the identity and the metabolic activity of naturally occurring microorganisms.