Thermodynamics of methanogenic intermediary metabolism in littoral sediment of Lake Constance

Abstract

In anoxic methanogemc sediments organic matter is degraded to CH 4 and CO 2 via intermediary metabohtes. When CH 4 production in slurries of hnoral sediment was inhibited by chloroform, acetate accumulated with a rate (2.26/zM h-1) simdar to the turnover rate (2.09 p.M h -1) of [2-14C]acetate. Addition of chloroform resulted also in accumulation of propionate > 2-propanol > caproate > valerate > H 2. Accumulation of H2 was small but sufficient to thermodynamically inhibit consumption of caproate and valerate by H+-reduciug bacteria. Consumption stopped when the available Gibbs free energy had increased from about -16 to about -9 kJ mol -I H 2 produced. 2-Propanol mcreased probably mainly because of the accumulation of acetate with the avadable AG mcreaslng from about -13 to -3 kJ mol-I of 2-propanol consumed Propionate accumulation, however, could not be explained by thermodynamic inhibition of proptonate consumption since the Gibbs free energy of this reaction was generally very low (AG =-3 kJ tool-l). Bacterial enrichment cultures on cellulose resulted m the production of simdar metabolites as observed during the accumulation experiments. Assuming that proplonate, 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 CH 4 production. Carbon flow through acetate accounted for 80-87% of the total carbon flow to CH 4 This relatwely high percentage may be due to the relative importance of either homoacetogenesis or of acetate-rich orgamc matter (e.g., chitin) in httoral sediment of Lake Constance.

Turnover of propionate in methanogenic paddy soil

“…Similar small values (i.e. ΔG of −3 to −15 kJ mol −1 propionate) during phases in which propionate was degraded were reported earlier in anoxic paddy soil [11], natural wetlands [38, 39], and methanogenic digesters [40, 41].…”

A snapshot of CO2 and CH4 evolution in a thermokarst pond near Igarka, northern Siberia

“…The reasons for the high H 2 levels cannot be clarified. In absence of alternative electron acceptors hydrogenotrophic methanogenesis often operates at lower H 2 concentration levels, resulting in a ΔG r −20 to −40 kJ mol −1 (CH 4 ) in experiments with freshwater soils and sediments [ Chin and Conrad , 1995; Conrad , 1999; Rothfuss and Conrad , 1993]. In their presence and with rapid utilization, hydrogen concentrations are typically lowered to steady state values of <0.1 to 2 nmol L −1 thus resulting in a positive ΔG r of hydrogenotrophic methanogenesis [ Heimann et al , 2007; Lovley and Goodwin , 1988].…”

Methanogenesis and methanogenic pathways in a peat from subarctic permafrost

“…Hence, other substrates like ethanol or caproate (Fig. 3) might have been additional precursors of CH 4 (Stieb and Schink, 1987; Rothfuss and Conrad, 1993; Metje and Frenzel, 2005). Taken together, syntrophic oxidation of ethanol, butyrate and caproate contributed to 24% of acetoclastic methanogenesis and to 18% of hydrogenotrophic methanogenesis.…”