CARBON-ISOTOPE EFFECTS ASSOCIATED WITH ACETICLASTIC METHANOGENESIS

The carbon isotope effects associated with synthesis of methane from a cetate have been determined for Methanosarcina barkeri 227 and for met hanogenic archaea in sediments of Wintergreen Lake, Michigan. At 37 de grees C, the C-13 isotope effect for the reaction acetate (methyl carb on) --> methane, as measured in replicate experiments with M. barkeri, was -21.3 parts per thousand +/- 0.3 parts per thousand. The isotope effect at the carboxyl portion of acetate was essentially equal, indic ating participation of both positions in the rate-determining step, as expected for reactions catalyzed by carbon monoxide dehydrogenase. A similar isotope effect, -19.2 parts per thousand +/- 0.3 parts per tho usand, was found for this reaction in the natural community (temperatu re = 20 degrees C). Given these observations, it has been possible to model the flow of carbon to methane within lake sediment communities a nd to account for carbon isotope compositions of evolving methane. Ext ension of the model allows interpretation of seasonal fluctuations in C-13 contents of methane in other systems.