Optimizing Caproic Acid Biosynthesis in Anaerobic Fermentation of Ethanol and Butanoic Acid: The Effects of C/N Ratio

The carbon-to-nitrogen (C/N) ratio is a critical player in microbial growth and metabolism. This study explored the effects of this ratio on caproic acid yield, electron efficiency, and microbial community composition in an anaerobic fermentation system wherein ethanol and butanoic acid were used as electron donors and acceptors, respectively. With a C/N ratio of 3–25, the system maintained a reducing environment conducive to carbon chain elongation, which led to a high caproic acid yield. The highest caproic acid concentration of 6175.9 mg/L was attained at a C/N ratio of 3, with an electron efficiency of 72.9% and a selectivity of 60.8%. At C/N ratios of 58, 75, and 100, the highest concentration of caproic acid decreased by 26.2%, 35.4%, and 39.4%, respectively, compared to that at a C/N ratio of 3. At a C/N ratio of 1, acetic acid-producing bacteria were enriched, severe excessive ethanol oxidation occurred, and the caproic acid concentration was only 31% of that at a C/N ratio of 3. Caproic acid biosynthesis was attributed to the cooperative activity of Clostridium_sensu_stricto_12, DMER64, Para clostridium, Thermovirga, and Sporanaerobacter.