Enhancing Chain Elongation via Bacteria-Yeast Cooperation Using an Endogenous Electron Donor for Medium-Chain Fatty Acids Production

Abstract Recovering high-value medium-chain fatty acids (MCFAs) from food waste (FW) through chain elongation (CE) represents a promising strategy for sustainable resource recovery. However, its performance is often constrained by inefficient electron donors and unstable microbial cooperation. In this study, the bacteria-yeast cooperation during caproate production was systematically explored in batch anaerobic fermentation mode by inoculating waste activated sludge (WAS), distiller yeast (DY), and their coinoculation. The results showed that DY inoculation significantly promoted endogenous ethanol accumulation (7.20 ± 0.28 g COD/L), enabling earlier initiation and enhanced caproate production compared with WAS. Coinoculation further improved caproate production, achieving a maximum concentration of 4.33 ± 0.67 g COD/L on the 18th day. Microbial community analysis revealed that the relative abundance of chain-elongating bacteria (Caproiciproducens and Clostridium_sensu_stricto_12) reached 2.2%. Notably, under the same buffering conditions, the yeast-inoculated reactor exhibited the highest caproate production (13.51 ± 1.96 g COD/L), characterized by a distinct sequential pattern in which ethanol accumulated at the early stage and was consumed during caproate formation. Yeasts acted as key providers of electron donors, while bacterial communities fulfilled dual roles by generating short-chain fatty acids (SCFAs) and converting them into caproate. This study elucidates the complementary roles of bacteria and yeast during ethanol-driven CE and provides a useful basis for the future assembly of functionally complementary microbial communities in sustainable MCFA production.