1. SUMMARY 2. INTRODUCTION
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 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 fermenting
bacteria from the outside H 2 (and acetate)
pool.