Enhanced Medium-Chain Fatty Acids Production by Electro-fermentation: Insights into the Effect of Biocathode and Ethanol Supply

This study explored the enhanced medium-chain fatty acids (MCFA) production by introducing a biocathode as a coelectron donor (ED) in the presence of different ethanol/acetate ratios (R_E/A). Results showed that the introduction of a biocathode effectively promoted MCFA production by 165%–749%. The highest promotion rate was achieved at R_E/A = 0:3, and the highest MCFA concentration of 305.1 mmol C/L was obtained at R_E/A = 2:1. Besides, the introduction of a biocathode also triggered the formation of longer-chain MCFA (i.e., caprylate), and caprylate production was increased with the increase of R_E/A. Electrochemical analyses exhibited a positive correlation between the electrochemical activity and R_E/A. Microbiological analyses showed that the introduction of a biocathode promoted MCFA production by enriching chain elongation functional microorganisms (unclassified_f_Neisseriaceae sp. and Clostridium_sensu_stricto_12 sp.) and electrochemically active bacteria (Alcaligenes sp.). Enzyme analyses indicated that promoted MCFA production was achieved by strengthening the acetyl Co-A formation and fatty acid biosynthesis pathway.