Pivotal role of lactate-to-ethanol ratio in medium-chain fatty acids production without operational pH adjustment

Medium-chain fatty acids (MCFAs) production via chain elongation (CE) with endogenous ethanol or lactate as electron donors is a promising biotechnology for resource recovery from biowaste. However, a key bottleneck of the sole ethanol- or lactate-mediated CE is the requirement for exogenous buffering agents for pH control. In this study, we explored the possibility of system self-buffering by coupling ethanol and lactate as co-electron donors to capitalize on their respective net proton (H+) releasing and consuming behaviors during CE. Favorable pH conditions could be attained with a lactate-to-ethanol ratio of 1.5 (mM/mM). Combining lactate and ethanol as co-electron donors at a molar ratio of 1.5 facilitated MCFAs production (13.5 ± 1.7 g COD/L) and selectivity (43.4 ± 5.6 %, based on chemical oxygen demand) due to their complementary effects compared to when used in solitary. Ruminococcaceae bacterium CPB6 and Clostridium spp. were enriched in the co-electron donor-driven systems and could be associated with MCFAs production. The occurrence of genes encoding the fatty acid biosynthesis (FAB) pathway exceeded those assigned to the reverse β-oxidation pathway. The core FAB-related genes originated mainly from the putative MCFAs-producing species from class Clostridia. The results provide useful insights into high-value carboxylates production without operational pH adjustment and exogenous inorganic carbon supplementation.