The capacity of hydrogenotrophic anaerobic bacteria to compete for traces of hydrogen depends on the redox potential of the terminal electron acceptor

The effect of different electron acceptors on substrate degradation was studied in pure and mixed cultures of various hydrogenotrophic homoacetogenic, methanogenic, sulfate-reducing, fumarate-reducing and nitrate-ammonifying bacteria. Two different species of these bacteria which during organic substrate degradation produce and consume hydrogen, were cocultured on a substrate which was utilized only by one of them. Hydrogen, which was excreted as intermediate by the first strain (and reoxidized in pure culture), could, depending on the hydrogen acceptor present, also be used by the second organism, resulting in interspecies hydrogen transfer. The efficiency of H₂ transfer was similar when methanol, lactate or fructose were used as organic substrate, although the free energy changes of fermentative H₂ formation of these substrates are considerably different. In coculture experiments nitrate or fumarate>sulfate> CO₂/CH₄>sulfur or CO₂/acetate were the preferred electron acceptors, and an increasing percentage of H₂ was transferred to that bacterium which was able to utilize the preferred electron acceptor. In pure culture the threshold values for hydrogen oxidation decreased in the same order from 1,100 ppm for homoacetogenic bacteria to about 0.03 ppm for nitrate or fumarate reducing bacteria. The determined H₂-threshold values as well as the percentage of H₂ transfer in cocultures were related to the Gibbs free energy change of the respective hydrogen oxidizing reaction.Parts of this work (grant to R C-R) was supported by the European Community (ST2A-0022)