Reduced carbon emissions and chain elongation during mixotrophic fermentation of a biomass feedstock

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 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 control. Syngas co-feeding 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. Relative abundances of Megasphaera and Dialister showed significant correlation (p<0.05) to consumption of H2 and CO as well as production of C4 and C6, suggesting involvement of these genera in mixotrophic metabolism. A feasibility analysis showed that syngas recirculation could return additional 1.54 USD m-3broth while costing 1.26 USD m-3broth and avoiding 1.81 kg CO2 eq. m-3broth in emissions compared to heterotrophic fermentation. We propose mixotrophic fermentation as a low-tech technology to turn fermenters into decentralized industrial carbon sinks. SYNOPSISFermentation of biomass and syngas could be used for sustainable and circular chemical production. This study tested the feasibility of this technology and identified relevant process parameters and microbial genera.