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Abstract

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 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 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 DELTAG 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 (DELTAG almost-equal-to -3 kJ mol-1).
Bacterial 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
converted 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., chitin)
in littoral sediment of Lake Constance.