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 H ₂ , CO ₂ , 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 CO ₂ 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 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 H ₂ 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 ^-3 _broth while costing 1.26 USD m ^-3 _broth and avoiding 1.81 kg CO _{2 eq.} m ^-3 _broth in emissions compared to heterotrophic fermentation. We propose mixotrophic fermentation as a ‘low-tech’ technology to turn fermenters into decentralized industrial carbon sinks.