Methyl sulfides as intermediates in the anaerobic oxidation of methane

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Abstract
While it is clear that microbial consortia containing Archaea and sulfate-reducing bacteria (SRB) can mediate the anaerobic oxidation of methane (AOM), the interplay between these microorganisms remains unknown. The leading explanation of the AOM metabolism is ‘reverse methanogenesis’ by which a methanogenesis substrate is produced and transferred between species. Conceptually, the reversal of methanogenesis requires low H2 concentrations for energetic favourability. We used 13C-labelled CH4 as a tracer to test the effects of elevated H2 pressures on incubations of active AOM sediments from both the Eel River basin and Hydrate Ridge. In the presence of H2, we observed a minimal reduction in the rate of CH4 oxidation, and conclude H2 does not play an interspecies role in AOM. Based on these results, as well as previous work, we propose a new model for substrate transfer in AOM. In this model, methyl sulfides produced by the Archaea from both CH4 oxidation and CO2 reduction are transferred to the SRB. Metabolically, CH4 oxidation provides electrons for the energy-yielding reduction of CO2 to a methyl group (‘methylogenesis’). Methylogenesis is a dominantly reductive pathway utilizing most methanogenesis enzymes in their forward direction. Incubations of seep sediments demonstrate, as would be expected from this model, that methanethiol inhibits AOM and that CO can be substituted for CH4 as the electron donor for methylogenesis.