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 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 m_broth ^-3 while costing 1.26 USD m_broth ^-3 and avoiding 1.81 kg CO_2 eq. m_broth ^-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.