POSSIBLE INFLUENCE OF HYDROGEN CONCENTRATION ON MICROBIAL METHANE STABLE HYDROGEN ISOTOPIC COMPOSITION

Factors affecting the stable hydrogen isotopic composition (deltaD) of important sources of microbial methane to the atmosphere include oxidation, methanogenic precursor (e.g., acetate vs. CO2/H-2), and the deltaD of the environmental water. Variations in hydrogen gas concentrations or rates of interspecies hydrogen transfer resulting from variations in organic matter degradation rates may also affect deltaD-CH4. Methane produced via CO2 reduction by laboratory cultures (Balabane et al., 1987) was about 150 parts per thousand more D-depleted than methane produced in shallow marine sediments from CO2/H-2. ne dissolved hydrogen gas concentration in that laboratory culture was about 1 mM; whereas, hydrogen concentrations in methane-zone sediments typically range between 10 and 100 nM (e.g., Conrad, 1989). The relatively greater hydrogen isotope fractionation in the culture appears to result from the incorporation into methane of protons that are produced intracellularly from the hydrogenase-catalyzed oxidation of hydrogen gas. As rumen basal hydrogen concentrations are typically greater than 1 muM (Smolenski and Robinson, 1988), this mechanism may explain why ruminant methane is more D-depleted (Wahlen et al., 1990) than methane produced via CO2 reduction in natural sedimentary environments. This process may also contribute to the wide variation in sedimentary deltaD-CH4 observed in natural wetlands.