Identification of the microbial methane sink in anoxic marine sediments

Kai Hinrichs, Ph.D.
Woods Hole Oceanographic Institution

Wednesday, August 11, 1999
3:00 p.m.—Pacific Forum

Several geochemical studies have demonstrated the importance of anaerobic oxidation of methane in marine sediments. Of the 85 Tg methane that are produced annually in marine sediments, however, all but 10 Tg are consumed before they can reach the atmosphere, mostly anaerobically. The most plausible mechanism involves a consortium of methanogenic and sulfate-reducing bacteria, with the methanogens working in a reversed mode and sulfate-reducers utilizing products of methane oxidation. However, all attempts to reproduce natural conditions for anaerobic oxidation of methane (AOM) in laboratory experiments have failed. Furthermore, attempts to culture members of the AOM consortium have also failed.

A new mode of investigation, involving the combined study of molecular phylogenies and isotopic compositions of microbial biomarkers in methane seep sediments, has revealed for the first time specific information on the identity of members of the microbial community in a natural "reactor" for anaerobic oxidation of methane.

Results are consistent with a syntrophic community consisting of a novel group of methane-consuming archaea and novel eubacterial sulfate-reducers or hydrogen-oxidizers. The archaea are considered to be either methanogens capable of reversing their metabolism from methanogenesis to methanotrophy or obligately methanotrophic archaea. Chemotaxonomy indicates that the psychrophilic sulfate-reducer/hydrogen-oxidizer has closest relatives among the thermophilic deeply-branching eubacteria.

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