Carbon fixation and chain elongation during mixotrophic fermentation of biomass
Anaerobic fermentation of biomass feedstocks using mixed cultures is a promising technology to produce platform carboxylates. Syngas, a mixture of H2, CO2, and CO, can be sourced sustainably and used to supplement biomass feedstocks as a source of acetyl-CoA, a necessary intermediate for carboxylate chain elongation. To test this, syngas and corn silage was provided to a 10-L semi-continuous fermenter for 209 days of operation in a first-of-a-kind study at this scale. After acclimation to syngas, a two-fold reduction in average CO2 production rate (0.097 vs. 0.21 g L-1d-1) was observed over a period of 42 days in comparison to a con-trol. Syngas co-feeding also increased n-butyrate and n-caproate average production rates by 74% and 27%, respectively, although these effects were observed at relatively low elongated carboxylate concentrations of ca. 4 g L-1. Surprisingly, members of the Veillonellaceae family, Megasphaera and Dialister, showed significant correlation (p<0.05) to mixotrophic metabolism by consum-ing H2 and CO while producing elongated carboxylates. A feasibility analysis showed that syngas recirculation could return addi-tional 1.54 USD m-3broth while costing 1.26 USD m-3broth and avoiding 1.81 kg CO2 eq. m-3broth in emissions in comparison to hetero-trophic fermentation. We propose mixotrophic fermentation as a 'low-tech' technology to turn fermenters into decentralized indus-trial carbon sinks.