Energetics of syntrophie ethanol oxidation in defined ehemostat eocultures 1 . Energy requirement for H 2 production and H 2 oxidation

The ethanol-oxidizing, proton-reducing Pelobacter acetylenicus was grown in chemostat cocultures with either Acetobacterium woodii, Methanobacterium bryantii, or Desulfovibrio desulfuricans. Stable steady state conditions with tightly coupled growth were reached at various dilution rates between 0.02 and 0.14 h1. Both ethanol and H2 steady state concentrations increased with growth rate and were lower in cocultures with the sulfate reducer < methanogen < homoacetogen. Due to the higher affinity for H2, D. desulfuricans outcompeted M. bryantii, and this one A. woodii when inoculated in cocultures with P. acetylenicus. Cocultures with A. woodii had lower H2 steady state concentrations when bicarbonate reduction was replaced by the energetically more favourable caffeate reduction. Similarly, cocultures with D. desulfuricans had lower H2 concentrations with nitrate than with sulfate as electron acceptor. The Gibbs free energy (AG) available to the H2-producing P. acetylenicus was independent of growth rate and the H2utilizing partner, whereas the A G available to the latter increased with growth rate and the energy yielding potential of the H2 oxidation reaction. The "critical" Gibbs free energy (A Go), i.e. the minimum energy required for H2 production and H2 oxidation, was -5 .5 to -8 .0 kJ mo1-1 H2 for P. aeetylenicus, -5.1 to -6 .3 kJ mo1-1 H2 for A. woodii, -7 .5 to -9.1 kJ mo1-1 H2 for M. bryantii, and -10.3 to -12,3 kJ rno1-1 H2 for D. desulfuricans. Obviously, the potentially available energy was used more efficiently by homoacetogens > methanogens > sulfate reducers.