Dual role of carbon monoxide in medium-chain fatty acids production from food waste

Upcycling biowaste into useful biochemicals, including medium-chain fatty acids (MCFAs) represents a crucial node in the transition toward a circular economy. However, the output and stability of anaerobic microbiomes for MCFAs production are strongly anchored on the effective inhibition of competing pathways, including methanogenesis, while stimulating the growth of bacteria producing MCFAs. Here, we proposed a mixotrophic chain elongation (CE) concept for producing MCFAs from food waste by exploring carbon monoxide (CO) as a methanogenic inhibitor. Our findings suggest that CO supplementation at an optimum partial pressure (P_CO) of 0.25 atm enhances carbon flux toward MCFAs production instead of methanogenesis, resulting in the highest MCFAs concentration observed in this study (10.4 ± 0.4 g-COD/L), with n-caproate as the predominant MCFA. Experiments with CO as the sole substrate demonstrated that the supplied CO could be converted into intermediates such as acetate and ethanol that potentially augmented MCFAs synthesis. Homoacetogens and chain elongators, notably Megasphaera spp. and members of the Lachnospiraceae family were enriched in the P_CO of 0.25 atm system, forming a putative metabolic network to promote CE. These findings provide insights into valuable MCFAs biosynthesis from biowaste.