Thermodynamics of methanogenic intermediary metabolism in littoral sediment of Lake Constance

In anoxic methanogenic sediments organic matter is degraded to CH 4 and CO 2 via intermediary metabolites. When CH 4 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− 14 C]acetate. Addition of chloroform resulted also in accumulation of propionate > 2‐propanol > caproate > valerate > H 2 . Accumulation of H 2 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 H 2 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 H 2 to CH 4 , their accumulation rates plus that of acetate accounted for 134% of the rate of CH 4 production. Carbon flow through acetate accounted for 80–87% of the total carbon flow to CH 4 . 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.