Deep seawater circulation promotes microbial anaerobic methane oxidation at ∼400 meters below seafloor in the Nankai Trough

Microbially-mediated anaerobic oxidation of methane (AOM) in the seafloor significantly impacts the marine carbon cycle, and limits methane flux to the deep oceans. AOM mediated by microbial consortia predominantly utilizes sulfate as the terminal oxidant, although recent studies have demonstrated the potential for methane oxidation coupled to iron, manganese and nitrate reduction. In the Nankai Trough, aqueous biogeochemical data obtained during IODP Expedition 322 revealed concomitant peaks of methane and sulfide, indicative of a deeply buried sulfate-dependent methane oxidation zone located within a depth interval of intercalated mudstone and sandstone 370-450 meters below the seafloor at IODP Site C0012. Site C0012 is located on the west-northwest flank of a basement high (the Kashinosaki Knoll). Preliminary calculations suggest a ∼20% seawater contribution to sedimentary pore fluids at the depth interval of the observed AOM zone, reflecting deep circulation from possibly distant recharge areas. Methane (∼200 uM) is present and the resulting hydrogeochemical conditions promote anaerobic methane-oxidation and sulfide production. Extracted DNA from SMTZ sediments was nearly five times the concentration of any other depth, although comparison of biomass data obtained via fluorescent cell counts and total extracted DNA concentrations across all depths suggest significant challenges to extracting DNA, possibly related to the relative clay content of sediments. Regardless, the biomass associated with the deep AOM zone is very small, and possibly occupies a very transient ecological niche supported by deep hydrogeologic circulation. Ongoing 16S rDNA and dissimilatory sulfite reductase gene sequencing has thus far revealed several Archaeal and delta-Proteobacteria phylotypes closely related to known AOM consortium members.