Biomarker evidence for widespread anaerobic methane oxidation in Mediterranean sediments by a consortium of methanogenic archaea and bacteria

Although abundant geochemical data indicate that anaerobic methane oxidatio n occurs in marine sediments, the linkage to specific microorganisms remain s unclear, In order to examine processes of methane consumption and oxidati on, sediment samples from mud volcanoes at two distinct sites on the Medite rranean Ridge were collected via the submersible Nautile. Geochemical data strongly indicate that methane is oxidized under anaerobic conditions, and compound-specific carbon isotope analyses indicate that this reaction is fa cilitated by a consortium of archaea and bacteria. Specifically, these meth ane-rich sediments contain high abundances of methanogen-specific biomarker s that are significantly depleted in C-13 (delta(13)C values are as low as -95 parts per thousand). Biomarkers inferred to derive from sulfate-reducin g bacteria and other heterotrophic bacteria are similarly depleted. Consist ent,vith previous work, such depletion can be explained by consumption of C -13-depleted methane by methanogens operating in reverse and as part a cons ortium of organisms in which sulfate serves as the terminal electron accept or. Moreover, our results indicate that this process is widespread in Medit erranean mud volcanoes and in some localized settings is the predominant mi crobiological process.