Effect of Mn²⁺ levels on caproic acid production during anaerobic fermentation using ethanol and butyric acid as co-substrates.

As an enzyme cofactor, antioxidant, and stabilizer of cellular structures, Mn²⁺is essential for microbial growth, metabolism, and environmental adaptation. In this work, the impact of varying Mn²⁺ levels on caproic acid formation and microbial community shifts was examined under anaerobic fermentation, where ethanol functioned as the electron donor and butyric acid acted as the electron acceptor. It was observed that 0.5 mg/L Mn²⁺ did not significantly affect caproic acid formation, whereas 1.0 mg/L Mn²⁺ improved the reducing capacity of the fermentation system and promoted interspecies electron transfer. Consequently, caproic acid yield and electron efficiency were enhanced by 31.7% and 19.0%, respectively. Furthermore, the addition of 1.0 mg/L Mn²⁺effectively inhibited ethanol peroxidation and enriched Desulfovibrio, whose relative abundance showed a positive correlation with caproic acid yield. In contrast, 20.0 mg/LMn²⁺enriched Bacteroidota and promoted excessive ethanol oxidation (EEO), leading to a 105.3% increase in acetic acid yield, while caproic acid yield decreased by 34.2%.