Hydrogen is a key intermediary product during microbial degradation of organic matter in anaerobic ecosystems. The recent development of highly sensitive methods for detecting hydrogen in anaerobic systems has raised several new questions concerning the control of microbial hydrogen metabolism. It is suggested that competition among anaerobic bacteria hydrogen may involve the maintenance of partial pressures that are so low that those organisms performing the less energetically favorable reactions are excluded. This research examines physiological mechanisms whereby hydrogen-consuming microorganisms may control the steady-state, hydrogen partial pressures in anaerobic ecosystems. The hydrogen to be tested is that hydrogen partial pressures are determined by the physiological properties of the hydrogen-consuming microorganisms and that this directly determines the results of competition among hydrogen consumers. This hypothesis has been derived from the more general hypothesis that the steady-state concentration of any intermediary product is controlled by the physiology of the consuming organisms and independent of the producing organisms. The specific objectives are to 1) isolate hydrogen-consuming organisms at hydrogen partial pressures representative of those found in natural environments and compare their hydrogen uptake kinetics, 2) measures concentrations of products and reactants in natural environments to determine the energetics of hydrogen a production and consumption under in situ conditions, 3) monitor hydrogen metabolism in defined mixed cultures to determine the mechanisms controlling hydrogen. The research is designed to elucidate connections between thermodynamic, physiological hydrogen uptake properties of consuming organisms, and the hydrogen partial pressures in natural environments. The results will further test a model for competition between sulfate reducers and methanogens for hydrogen that is based on the concept of minimum substrate thresholds.
