Thermodynamics of methanogenic intermediary metabolism in littoral sediment of Lake Constance
In anoxic methanogenic sediments organic matter is degraded to CH4 and CO2 via intermediary metabolites. When CH4 production in slurries of littoral sediment was inhibited by chloroform, acetate accumulated with a rate (2.26 μM h−1) similar to the turnover rate (2.09 μM h−1) of [2−14C]acetate. Addition of chloroform resulted also in accumulation of propionate > 2-propanol > caproate > valerate > H2. Accumulation of H2 was small but sufficient to thermodynamically inhibit consumption of caproate and valerate by H+-reducing bacteria. Consumption stopped when the available Gibbs free energy had increased from about −16 to about −9 kJ mol−1 H2 produced. 2-Propanol increased probably mainly because of the accumulation of acetate with the available δG increasing from about −13 to −3 kJ mol−1 of 2-propanol consumed. Propionate accumulation, however, could not be explained by thermodynamic inhibition of propionate consumption since the Gibbs free energy of this reaction was generally very low (δG ≈ −3 kJ mol−1). Bacterial enrichment cultures on cellulose resulted in the production of similar metabolites as observed during the accumulation experiments. Assuming that propionat, 2-propanol, caproate and valerate were converted via acetate and H2 to CH4, their accumulation rates plus that of acetate accounted for 134% of the rate of CH4 production. Carbon flow through acetate accounted for 80–87% of the total carbon flow to CH4. This relatively high percentage may be due to the relative importance of either homoacetogenesis or of acetate-rich organic matter (e.g., chitin) in littoral sediment of Lake Constance.