Carbon isotope fractionation during bacterial methanogenesis by CO2 reduction

The carbon isotope fractionation between CO2 and CH4 was studied during open system (related to gas flow) culture experiments with CO2-reducing methanogenic Archaea. To study the temperature dependence of isotope fractionation during biological methanogenesis, three representatives of the order Methanococcales were cultivated in the temperature range 35-85°C. In the stationary growth phase, the carbon isotope fractionation factor between CO2 and CH4 was found to range between 1.048 and 1.079, depending on the growth temperature and on the type of fermentor. In contrast to published data derived from culture experiments, our results fall in the range of naturally occurring carbon isotope fractionations of coexisting CO2---CH4 pairs in marine sediments. Moreover, the fractionation closely approached the thermodynamic equilibrium between both gases, although thermal isotope exchange processes are unlikely to occur below 200°C. Our findings suggest that flow-through culture experiments are useful when studying biological methanogenesis and associated (carbon-) isotope fractionation as a means of deciphering complex methanogenic processes in sediments.