Formate-induced CO tolerance and innovative methanogenesis inhibition in co-fermentation of syngas and plant biomass for carboxylate production

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Production of monocarboxylates using microbial communities is highly dependent on local and degradable biomass feedstocks. Syngas or different mixtures of H 2 , CO, and CO 2 can be co-fed to a fermenter to alleviate this dependence. To understand the effects of adding these gases during anaerobic fermentation of plant biomass, a series of batch experiments was carried out with different syngas compositions and corn silage (pH 6.0, 32°C). Co-fermentation of syngas with corn silage increased the overall carboxylate yield per gram of volatile solids (VS) by up to 44% (0.36 ± 0.07 g g VS -1 ; in comparison to 0.23 ± 0.04 g g VS -1 with a N 2 /CO 2 headspace), despite slowing down biomass degradation. Ethylene and CO exerted a synergistic effect in preventing methanogenesis, leading to net carbon fixation. Less than 12% of the electrons were misrouted to CH 4 when either 15 kPa CO or 5 kPa CO + 1.5 kPa ethylene was used. CO increased the selectivity to acetate and propionate, which accounted for 86% (electron equivalents) of all products at 49 kPa CO, by favoring lactic acid bacteria and actinobacteria over n -butyrate and n -caproate producers. This happened even when an inoculum pre-acclimatized to syngas and lactate was used. Intriguingly, the effect of CO on n -butyrate and n -caproate production was reversed when formate was present in the broth. The concept of co-fermenting syngas and plant biomass shows promise in two aspects: by making anaerobic fermentation a carbon-fixing process and by increasing the production of propionate and acetate. Testing the concept in a continuous process could improve selectivity to n -butyrate and n -caproate by enriching chain-elongating bacteria adapted to CO and complex biomass.