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 H₂-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 reduction methanogenesis acetogenesis and for H₂-producing lactate fermentation ethanol fermentation. Butyrate and propionate fermentation, on the other hand, were endergonic under in situ conditions. This observation is interpreted by suggesting that butyrate and propionate is degraded within microbial clusters which shield the fermentating bacteria from the outside H₂ (and acetate) pool.