Energetics of syntrophic ethanol oxidation in defined chemostat cocultures

The ethanol-oxidizing, proton-reducing Pelobacter acetylenicus was grown in chemostat cocultures with either Acetobacterium woodii, Methanobacterium bryantii, or Desulfovibrio desulfuricans. Ymax and me were determined from the total molar growth yields determined at growth (dilution) rates between 0.02 and 0.14 h-1. The individual growth yields of the partner organisms were determined from their numbers and cellular mass in the chemostat cocultures. The Gibbs free energy (ΔG=-16.3 kJ/mol ethanol) available to P. acetylenicus as well as its Ymax (1.7–2.2 g/mol ethanol) were almost constant in the different cocultures. P. acetylenicus shared 44–67% of the total biomass produced, whereas it shared only 19, 23, and 37% of the total Gibbs free energy (ΔG) available from ethanol oxidation coupled to sulfate reduction, methanogenesis, and homoacetogenesis, respectively. The residual 63–81% of the total available ΔG were shared by the H2 oxidizers which exhibited Ymax values being highest for A. woodii (6.6 g/mol acetate) > D. desulfuricans (3.8 g/mol sulfide) > M. bryantii (2.2 g/mol CH4). The results are discussed with respect to ATP generation and coupling of catabolism with cell production.