Hydrogen metabolism and sulfate-dependent inhibition of methanogenesis in a eutrophic lake sediment (Lake Mendota)

Hydrogen metabolism was studied in anoxic sediments of the stratified Lake Mendota; using a method which allowed the measurement of in situ H2 concentrations and the headspace-free analysis of turnover of dissolved H2. Addition of sulfate resulted in partial but immediate inhibition of H2-dependent methanogenesis. Sulfate addition did not result in an immediate decrease in the steady state concentration of dissolved H2, nor did it significantly stimulate the rate constant of H2 turnover. Sulfate-induced decrease in dissolved H2 was only observed after prolonged incubation or when endogenous H2 production was stimulated by added glucose. The turnover of the in situ H2 accounted for only 14% of the H2-dependent methanogenesis from bicarbonate. While rates of methanogenesis increased during the season, rates of H2 turnover decreased, accounting for only 2% of the H2-dependent methanogenesis at the end of summer stratification. These observations indicate that increasing proportions of CH4 were formed from H2 being directly transferred in syntrophic methanogenic associations. The rapid inhibition of H2-dependent methanogenesis by exogenous sulfate may be explained at least partially by assuming methanogenic associations in which syntrophic sulfate reducers change their metabolism from fermentative H2 production to sulfate reduction.