Homoacetogenesis as the alternative pathway for H2 sink during thermophilic anaerobic degradation of butyrate under suppressed methanogenesis

Butyrate degradation for hydrogen production under conditions suppressing methanogenesis was evaluated in continuously fed-tank reactors operated at 55 °C and started up with digested manure as inoculum. This study shows that the reaction of butyrate degradation to acetate and hydrogen could happen when gas sparging was applied. Gas sparging was very important for reducing hydrogen partial pressure and made the reaction thermodynamically possible. Almost no hydrogen or methane (methane production was prevented by the addition of 2-bromoethane-sulfonic acid) was detected, indicating that the H2 produced from butyrate oxidation was consumed in a subsequent step. It was found by isotope experiments that hydrogen produced from butyrate degradation reacted immediately with CO2 to form acetate via homoacetogenesis. When CO2/HCO3− was not provided in the system, butyrate degradation was no longer possible and butyrate-degrading cultures were washed out. It was furthermore found that the microorganisms responsible for homoacetogenesis were likely present in normal anaerobic environments, such as biogas reactors.