Thermodynamic and kinetic analysis of the H2 threshold for Methanobacterium bryantii M.o.H.

H2 thresholds, concentrations below which H2 consumption by a microbial group stops, have been associated with microbial respiratory processes such as dechlorination, denitrification, sulfate reduction, and methanogenesis. Researchers have proposed that observed H2 thresholds occur when the available Gibbs free energy is minimal (DeltaG approximately 0) for a specific respiratory reaction. Others suggest that microbial kinetics also may play a role in controlling the thresholds. Here, we comprehensively evaluate H2 thresholds in light of microbial thermodynamic and kinetic principles. We show that a thermodynamic H2 threshold for Methanobacterium bryantii M.o.H. is not controlled by DeltaG for methane production from H2 + HCO3-. We repeatedly attain a H2 threshold near 0.4 nM, with a range of 0.2-1 nM, and DeltaG for methanogenesis from H2 + HCO3- is positive, +5 to +7 kJ/mol-H2, at the threshold in most cases. We postulate that the H2 threshold is controlled by a separate reaction other than methane production. The electrons from H2 oxidation are transferred to an electron sink that is a solid-phase component of the cells. We also show that a kinetic threshold (S(min)) occurs at a theoretically computed H2 concentration of about 2400 nM at which biomass growth shifts from positive to negative.