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Anaerobic Methane-oxidizing Archaea (ANME-2)
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Anaerobic methane oxidation (AOM) is a globally important process that significantly reduces methane flux from the ocean to the atmosphere (1, 2). Combined geochemical and biological evidence indicate that microbial consortia, largely composed of archaea and sulfate-reducing bacteria (SRB), can couple methane oxidation to sulfate reduction (3, 4). Phylogenetic, isotopic, and biochemical analyses indicate that several different methanogen-related archaeal groups are involved in AOM (5-8). Two groups of putative anaerobic methane-oxidizing Archaea (ANME-1 and ANME-2) (5, 6) and several SRB groups typically co-occur in methane-rich marine sediments (5, 9-12). The extent to which ANME and SRB groups cooperate in AOM is uncertain, but specific physical associations between them have been observed (6, 10).
Initial studies of the microbial communities associated with AOM identified methyl-coenzyme M reductase (mcr) as a potential effector of this process (12). In methanogenic archaea, mcr catalyzes the terminal step in biogenic methane production (13, 14). The identification of mcr homologs in methane oxidizing archaea suggests that ANME groups may have evolved by co-opting steps in the methanogenic pathway, running key steps in reverse to provide cellular carbon and energy.
Accessing the genetic diversity of ANME and SRB groups in order to identify the metabolic machinery responsible for AOM is complicated by the fact that no methane oxidizing systems have been cultured in isolation, hindering conventional metabolic and genetic approaches. To address the metabolic capacities and genomic diversity of microbial groups associated with AOM, we are conducting random whole genome shotgun sequencing (WGS) and directed sequencing of large insert fosmid DNA derived from methane oxidizing marine sediments.
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