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.
* To whom correspondence should be addressed.
The anaerobic degradation of organic compounds
such as carbohydrates to methane and
carbon dioxide is an exergonic process. However,
there is no single microorganism able to perform
this reaction all on its own. Instead, the degradation
is brought about by a complex metabolic
food chain which ends in the production of
methane [1-3]. In this mineralization process, H 2
is next to acetate the most important intermediate
utilized by methanogenic bacteria in freshwater
environments. In addition, H 2 may be consumed
by chemolithotrophic homoacetogenic bacteria
and, in the presence of sulfate, by chemolithotrophic
sulfate-reducing bacteria. On the other
hand, H 2 is an obligatory product of bacteria
which degrade reduced fermentation intermediates
such as alcohols or fatty acids to acetate.