Recirculation of H2, CO2, and Ethylene Improves Carbon Fixation and Carboxylate Yields in Anaerobic Fermentation

Anaerobic fermentation with mixed cultures has gained momentum as a bioprocess for its promise to produce platform carboxylates from low-value biomass feedstocks. Anaerobic fermenters are net carbon emitters, and their carboxylate yields are limited by electron donor availability. In a new approach to tackle these two disadvantages, we operated two bioreactors at pH 6.0 and 32 °C fed with acetate and lactate as a model feedstock while recirculating H2/CO2 to stimulate concomitant autotrophic activity. After 42 days of operation, hydrogenotrophic methanogenesis was predominant, and ethylene (≥1.3 kPa) was added to one of the reactors, inhibiting methanogenesis completely and recovering net carbon fixation (0.20 g CO2 L–1 d–1). When methanogenesis was inhibited, exogenous H2 accounted for 17% of the consumed electron donors. The lactate-to-butyrate selectivity was 101% (88% in the control without ethylene), and the lactate-to-caproate selectivity was 17% (2.3% in the control). Community analysis revealed that ethylene caused Methanobacterium to be washed out, giving room to acetogenic bacteria. In contrast to 2-bromoethanosulfonate, ethylene is a scalable methanogenesis inhibition strategy that did not collaterally block i-butyrate formation. By favoring the bacterial share of the community to become mixotrophic, the concept offers a way to simultaneously increase the selectivity to medium-chain carboxylates and develop a carbon-fixing chain elongation process.