CARBON-ISOTOPE FRACTIONATION DURING BACTERIAL METHANOGENESIS BY CO2 REDUCTION

The carbon isotope fractionation between CO2 and CH4 was studied durin g open system (related to gas flow) culture experiments with CO2-reduc ing methanogenic Archaea. To study the temperature dependence of isoto pe fractionation during biological methanogenesis, three representativ es of the order Methanococcales were cultivated in the temperature ran ge 35-85 degrees 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 fermenter. In contrast to published data derived from culture experim ents, our results fall in the range of naturally occurring carbon isot ope fractionations of coexisting CO2-CH4 pairs in marine sediments. Mo reover, the fractionation closely approached the thermodynamic equilib rium between both gases, although thermal isotope exchange processes a re unlikely to occur below 200 degrees C. Our findings suggest that fl ow-through culture experiments are useful when studying biological met hanogenesis and associated (carbon-) isotope fractionation as a means of deciphering complex methanogenic processes in sediments. (C) 1997 E lsevier Science Ltd.