Carbon and hydrogen isotope fractionation by moderately thermophilic methanogens

A series of laboratory studies were conducted to increase understanding of stable carbon ( super 13C/ super 12C) and hydrogen (D/H) isotope fractionation arising from methanogenesis by moderately thermophilic acetate- and hydrogen-consuming methanogens. Studies of the aceticlastic reaction were conducted with two closely related strains of Methanosaeta thermophila. Results demonstrate a carbon isotope fractionation of only 7ppm ( alpha = 1.007) between the methyl position of acetate and the resulting methane. Methane formed by this process is enriched in super 13C when compared with other natural sources of methane; the magnitude of this isotope effect raises the possibility that methane produced at elevated temperature by the aceticlastic reaction could be mistaken for thermogenic methane based on carbon isotopic content. Studies of H sub 2/CO sub 2 methanogenesis were conducted with Methanothermobacter marburgensis. The fractionation of carbon isotopes between CO sub 2 and CH sub 4 was found to range from 22 to 58ppm (1.023 < = alpha < = 1.064). Greater fractionation was associated with low levels of molecular hydrogen and steady-state metabolism. The fractionation of hydrogen isotopes between source H sub 2O and CH sub 4 was found to range from 127 to 275ppm (1.16 < = alpha < = 1.43). Fractionation was dependent on growth phase with greater fractionation associated with later growth stages. The maximum observed fractionation factor was 1.43, independent of the delta D-H sub 2 supplied to the culture. Fractionation was positively correlated with temperature and/or metabolic rate. Results demonstrate significant variability in both hydrogen and carbon isotope fractionation during methanogenesis from H sub 2/CO sub 2. The relatively small fractionation associated with deuterium during H sub 2/CO sub 2 methanogenesis provides an explanation for the relatively enriched deuterium content of biogenic natural gas originating from a variety of thermal environments. Results from these experiments are used to develop a hypothesis that differential reversibility in the enzymatic steps of the H sub 2 /CO sub 2 pathway gives rise to variability in the observed carbon isotope fractionation. Results are further used to constrain the overall efficiency of electron consumption by way of the hydrogenase system in M. marburgensis, which is calculated to be less than 55 percent.