Methane-consuming archaea revealed by directly coupled isotopic and phylogenetic analysis

Microorganisms Living in anoxic marine sediments consume more than 80% of t he methane produced in the world's oceans. In addition to single-species ag gregates, consortia of metabolically interdependent bacteria and archaea ar e found in methane-rich sediments. A combination of fluorescence in situ hy bridization and secondary ion mass spectrometry shows that cells belonging to one specific archaeal group associated with the Methanosarcinales were a ll highly depleted in C-13 (to values of -96 parts per thousand). This depl etion indicates assimilation of isotopically Light methane into specific ar chaeal cells. Additional microbial species apparently use other carbon sour ces, as indicated by significantly higher C-13/C-12 ratios in their cell ca rbon. Our results demonstrate the feasibility of simultaneous determination of the identity and the metabolic activity of naturally occurring microorg anisms.