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 wit h a rate (2.26 muM h-1) similar to the turnover rate (2.09 muM h-1) of [2-C-14]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 consumpti on of caproate and valerate by H+-reducing bacteria. Consumption stopp ed when the available Gibbs free energy had increased from about -16 t o about -9 kJ mol-1 H-2 produced. 2-Propanol increased probably mainly because of the accumulation of acetate with the available DELTAG incr easing from about -13 to -3 kJ mol-1 of 2-propanol consumed. Propionat e accumulation, however, could not be explained by thermodynamic inhib ition of propionate consumption since the Gibbs free energy of this re action was generally very low (DELTAG almost-equal-to -3 kJ mol-1). Ba cterial enrichment cultures on cellulose resulted in the production of similar metabolites as observed during the accumulation experiments. Assuming that propionate, 2-propanol, caproate and valerate were conve rted via acetate and H-2 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., c hitin) in littoral sediment of Lake Constance.