Carbon isotope fractionation during the anaerobic degradation of acetate

In methanogenic environments Methanosarcinaceae are beside Methanosaetaceae the only acetate-consuming family of archaea and thus important contributors to the formation of the greenhouse gases methane and carbon dioxide. In this study, the carbon isotope fractionation during this process was determined for two species of the Methanosarcinaceae family, M. barkeri and M. acetivorans. The calculated isotope enrichment factors (ε) associated with acetoclastic methanogenesis differed slightly compared to literature data for M. barkeri where εac and εCH4 typically range from –21 and –27‰. Our experiments showed partially stronger fractionation yielding εac = –30.5‰ and εCH4 = –27.4‰ for M. barkeri and εac = –36.9‰ and εCH4 = –23.8‰ for M. acetivorans. Since fractionation varied during the catabolism of acetate a new approach is shown which allows to differentiate isotope fractionation at different stages of acetate consumption. Introduction Methane (CH4) is the most abundant organic gas in the earth’s atmosphere (Cicerone and Oremland, 1988) and an important greenhouse gas with a high global warming potential (approx. 25 times higher than carbon dioxide; IPCC, 2001). It is suggested that the contribution of CH4 to the greenhouse effect will even increase in future. This has made it necessary and more urgent to understand natural processes which lead to the production of CH4. Methanogenesis, the microbial formation of CH4, is the final step in the degradation of organic matter in anoxic environments like natural wetlands, lake sediments, and flooded rice fields. The most important precursors for the production of CH4 are acetate (Eqn. 1) and CO2 (Eqn. 2) with the following reactions (Conrad, 1989): CH3COOH → CO2 + CH4 (1) CO2 + 4H2 → 2H2O + CH4 (2)