Microbial activities of methanogenesis and methane oxidation in terrestrial subsurface environment of the Kanto Plain in Japan

Methane is a relatively potent greenhouse gas compared with carbon dioxide. Although the concentration is low in the atmosphere, recent rise in concentration makes it an important issue to understand methane flux in the surface environments and restriction of methane emission to the atmosphere. Methane is mainly produced by thermal degradation of organic matter in deep region or by methanogens in anaerobic condition of subsurface area. In marine environments, methane is mostly consumed by anaerobic methanotrophic archaea (ANME) and/or aerobic methane-oxidizing bacteria before it diffuses to the atmosphere. On the other hand, there were only a few reports of anaerobic methane-oxidizing activities in terrestrial region. In previous study, we found biogenic methane was accumulated in terrestrial sediments in the Kanto Plain in Japan and freshwater-adapted anaerobic methane-oxidizing archaea subgroup (ANME-1) dominates in the sediments (Takeuchi et al. 2011). To make it clear whether the biogenic methane is produced or consumed, we analyzed stable carbon isotopic compositions of methane and dissolved inorganic carbon (DIC) in the sediments and measured microbial activities of methane production by radiotracer experiments and methane oxidation by tracer experiment using 13C-labeled methane. Methane was mainly detected in the Holocene mud layer and the stable carbon isotopic compositions increased from -84 ‰ at the top (depth of 4 m) of the mud layer to -75 ‰ at depth of 10 m and showed almost constant value in the lower part. Carbon isotopic compositions of DIC showed a similar trend: increasing from -13‰ at the top of the mud layer to -5 ‰ at depth of 10 m and showing almost constant value in the lower part. The constant value of difference between the two isotopic compositions indicated that the biogenic methane was produced by microbial methanogenesis through carbonate reduction pathway and there was no obvious evidence of methane oxidation. Results of activity experiments showed both methane oxidation and production activities occurred in the sediments. The apparent contradiction between results of geochemical study and microbial study may imply that the methane accumulated in the present sediments has been produced through geological time or during sedimentation period, and ANME present in the sediments has grown after interstitial water in the sediments has exchanged with fresh water relatively recently. Sulfate concentration in the interstitial water was mostly lower than 10 micromole per liter. Therefore, methane oxidation in the sediments might have been controlled by amount of sulfate in the interstitial water.