ANAEROBIC OXIDATION OF METHANE MEDIATED BY A MICROBIAL CONSORTIUM ABOVE MARINE GAS HYDRATE

The anaerobic oxidation of methane (AOM) in marine sediments is a significant sink in the global methane cycle. The controls on both methane production and consumption are consequently important in the evaluation of past and future climate change. Although there are now sufficient geochemical methods available to locate the hot spots of AOM, and to crudely estimate its contribution to the methane cycle, a fundamental understanding of the associated microbiology is still lacking, consequently preventing a thorough biogeochemical understanding of this important process of the global carbon cycle. Cold seeps and surficial gas hydrates are focussed sources of methane from deep reservoirs. At these sites some of the highest sulfate reduction rates have been measured, indicating that a significant fraction of methane is oxidized microbially before it can escape to the water column. The sulfate reduction rates above gas hydrate can reach >100 mmol/m2/d, a significant fraction of which accounts for AOM. A combination of biomarker studies involving stable isotope analysis and fluorescence in situ hybridization shows that a consortium of archaea and sulfate reducing bacteria is responsible for AOM in these and other methane-rich sediments. This consortium may occur in extremely high numbers of >10^10 cells per ml sediment. The current hypothesis is that methanogens (Archaea) operate in reverse to oxidize methane and produce an intermediate which is utilized by the sulfate reducers, thereby maintaining conditions which allow AOM to proceed exergonically. This hypothesis is supported by stable isotope analysis of lipid biomarkers. Furthermore, such consortia may be involved in the formation of carbonate structures above gas hydrates. This contribution gives an overview on very recent progress in the study of AOM, combining new insights into the microbiology and molecular ecology of AOM with biogeochemical estimates of its significance above marine gas hydrate.