Abstract |
Factors affecting the stable hydrogen isotopic composition (delta D) of important sources of microbial methane to the atmosphere include oxidation, methanogenic precursor (e.g., acetate vs. CO2/H2), and the delta D 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 delta D-CH4. Methane produced via CO2 reduction by laboratory cultures was more D-depleted than methane produced in shallow marine sediments from CO2/H2. The dissolved hydrogen gas concentration in that laboratory culture was about 1mM; whereas, hydrogen concentrations in methane-zone sediments typically range between 10 and 100 nM. 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. |