Impact of carbon monoxide partial pressures on methanogenesis and medium chain fatty acids production during ethanol fermentation

Medium-chain fatty acids (MCFA) are important biofuel precursors. Carbon monoxide (CO) is a sustainable electron and carbon donor for fatty acid elongation, since it is metabolized to MCFA precursors, it is toxic to most methanogens, and it is a waste product generated in the gasification of waste biomass. The main objective of this work was to determine if the inhibition of methanogenesis through the continuous addition of CO would lead to increased acetate or MCFA production during fermentation of ethanol. The effects of CO partial pressures (P_CO; 0.08–0.3 atm) on methanogenesis, fatty acids production, and the associated microbial communities were studied in batch cultures fed with CO and ethanol. Methanogenesis was partially inhibited at P_CO ≥ 0.11 atm. This inhibition led to increased acetate production during the first phase of fermentation (0–19 days). However, a second addition of ethanol (day 19) triggered MCFA production only at P_CO ≥ 0.11 atm, which probably occurred through the elongation of acetate with CO-derived ethanol and H₂:CO₂. Accordingly, during the second phase of fermentation (days 20–36), the distribution of electrons to acetate decreased at higher P_CO, while electrons channeled to MCFA increased. Most probably, Acetobacterium, Clostridium, Pleomorphomonas, Oscillospira, and Blautia metabolized CO to H₂:CO₂, ethanol and/or fatty acids, while Peptostreptococcaceae, Lachnospiraceae, and other Clostridiales utilized these metabolites, along with the provided ethanol, for MCFA production. These results are important for biotechnological systems where fatty acids production are preferred over methanogenesis, such as in chain elongation systems and microbial fuel cells.