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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.
