Microbial production and oxidation of methane in deep subsurface

The goal of this review is to summarize present studies on microbial production and oxidation of methane in the deep subterranean environments. Methane is a long-living gas causing the greenhouse effect in the planet's atmosphere. Earlier, the deep organic carbon poor subsurface was not considered as a source of biogenic methane. Evidence of active methanogenesis and presence of viable methanogens including autotrophic organisms were obtained for some subsurface environments including water-flooded oil-fields, deep sandy aquifers, deep sea hydrothermal vents, the deep sediments and granitic groundwater at depths of 10 to 2000 in below sea level. As a rule, the deep subterranean microbial populations dwell at more or less oligotrophic conditions. Molecular. hydrogen has been found in a variety of subsurface environments, where its concentrations were significantly higher than in the tested surface aquatic environments. Chemolithoautotrophic microorganisms from deep aquifers that could grow on hydrogen and carbon dioxide can act as primary producers of organic carbon, initiating heterotrophic food chains in the deep subterranean environments independent of photosynthesis. Biogenic methane has been found all over the world. On the basis of documented occurrences, gases in reservoirs and older sediments are similar and have the isotopic character of methane derived from CO2 reduction. Groundwater representing the methanogenic end member are characterized by a relative depletion of dissolved organic carbon (DOC) in combination with an enrichment in C-13 in inorganic carbon, which is consistent with the preferential reduction of (CO2)-C-12 by autotrophic methanogens or acetogens. The isotopic composition of methane formed via CO2 reduction is controlled by the delta(13)C of the original CO2 substrate. Literature data shows that CH4 as heavy as -40parts per thousand or -50parts per thousand can be produced by the microbial reduction of isotopically heavy CO2. Produced methane may be oxidized microbially to carbon dioxide. Microbial methane oxidation is a biogeochemical process that limits the release of methane, a greenhouse gas from anaerobic environments. Anaerobic methane oxidation plays an important role in marine sediments. Similar processes may take place in deep subsurface and thus fuel the deep microbial community. Organisms or consortia responsible for anaerobic methane oxidation have not yet been cultured, although diverse aerobic methanotrophs have been isolated from a variety of underground niches. The presence of aerobic methanotrophs in the anoxic subsurface remains to be explained. The presence of methane in the deep subsurface have been shown all over the world. The flux of gases between the deep subsurface and the atmosphere is driven by the concentration gradient from depth to the atmosphere. However, methane is consumed by methanotrophs on the way of its evolution in oxidized environments and is transformed to organic form, available for further microbial processing. When the impact of subsurface environments to global warming is estimated, it is necessary to take into account the activity of methane-producing Archaea and methane-oxidizing biofilters in groundwater. Microbial production and oxidation of methane is involved in the carbon cycle in the deep subsurface environments. (C) 2002 Elsevier Science B.V. All rights reserved.