Reduced Carbon Emissions and Chain Elongation during Mixotrophic Fermentation of a Biomass Feedstock

Abstract Anaerobic fermentation of biomass feedstocks using open 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, an intermediate for carboxylate chain elongation. To test this, syngas and corn silage were provided to a 10-L semicontinuous fermenter for 208 days of operation in a first-of-a-kind study at this scale. After acclimation to syngas, a 2-fold reduction in average CO2 production rate (0.097 vs 0.21 g L–1 d–1) was observed over a period of 42 days in comparison to a control. Syngas cofeeding also increased average production rates of n-butyrate (C4) and n-caproate (C6) by 74 and 27%, respectively, although these effects were observed at relatively low C6 concentrations up to 4 g L–1. Abundances of Megasphaera and Dialister showed significant correlation (p < 0.05) to consumption of H2, CO2, and CO as well as production of C3, C4, C5, or C7, suggesting involvement of these genera in mixotrophic metabolism. A feasibility analysis showed that syngas recirculation could return additional 1.54 USD mbroth–3 while costing 1.26 USD mbroth–3 and avoiding 1.81 kg CO2 eq. mbroth–3 in emissions compared to heterotrophic fermentation. With still much room to increase syngas consumption and carboxylate production, we propose mixotrophic fermentation as a “low-tech” technology to turn fermenters into decentralized industrial carbon sinks.