Abstract:
H-2 thresholds, concentrations below which H-2 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 H-2 thresholds occur when the available Gibbs free energy is minimal (Delta G approximate to 0) for a specific respiratory reaction. Others suggest that microbial kinetics also may play a role in controlling the thresholds. Here, we comprehensively evaluate H-2 thresholds in light of microbial thermodynamic and kinetic principles. We show that a thermodynamic H-2 threshold for Methanobacterium bryantii M.o.H. is not controlled by Delta G for methane production from H-2 + HCO (3) over bar. We repeatedly attain a H-2 threshold near 0.4 nM, with a range of 0.2-1 nM, and Delta G for methanogenesis from H-2 + HCO (3) over bar is positive, +5 to +7 kJ/mol-H-2,H- at the threshold in most cases. We postulate that the H-2 threshold is controlled by a separate reaction other than methane production. The electrons from H-2 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 H-2 concentration of about 2400 nM at which biomass growth shifts from positive to negative.
