Thermodynamics of H2-consuming and H2-producing metabolic reactions in diverse methanogenic environments under in situ conditions

Abstract

In situ concentrations of hydrogen and other metabolites involved in H2-consuming and H 2-producing reactions were measured in anoxic methanogenic lake sediments, sewage sludge and fetid liquid of cottonwood. The data were used to calculate the Gibbs free energies of the metabolic reactions under the conditions prevailing in situ. The thermodynamics of most of the reactions studied were exergonic with Gibbs free energies being more negative for H2-dependent sulfate re-duction > methanogenesis > acetogenesis and for H2-producing lactate fermentation > ethanol fer-mentation. Butyrate and propionate fermentation, on the other hand, were endergonic under in situ conditions. This observation is interpreted by sug-gesting that butyrate and propionate is degraded within microbial clusters which shield the ferment-ing bacteria from the outside H 2 (and acetate) pool.