Mechanisms for the Enhancement of Caproic Acid and H₂ Production in Ruminococcaceae Bacterium CPB6 by Fe(II) and...

The production of caproic acid (CA) and hydrogen gas (H₂) from organic wastewater is economically attractive. The Ruminococcaceae bacterium CPB6 has demonstrated potential for CA production from lactate-containing wastewater. However, our understanding of the effects of Fe²⁺ and Mg²⁺ on the growth and metabolism of strain CPB6 remains limited. Therefore, this study aims to investigate the impact of Fe²⁺ and Mg²⁺ on CA and H₂ production, as well as on the expression of key genes involved in CA and H₂ biosynthesis pathway. The results indicate that Fe²⁺ positively affects cell proliferation and H₂ production while minimally impacting CA production. The highest levels of H₂ production were achieved with the addition of 200 mg/L Fe²⁺. Conversely, Mg²⁺ significantly enhances CA and H₂ production, with the optimal yield observed in a medium enriched with 300 mg/L Mg²⁺. Reverse transcription quantitative PCR (RT-qPCR) analysis reveals that Fe²⁺ promotes the expression of the hydrogenase gene, whereas Mg²⁺ has a negligible effect on hydrogenase expression. Notably, Fe²⁺ and Mg²⁺ inhibit the expression of key genes involved in CA synthesis. These findings suggest that Fe²⁺ enhances H₂ production by boosting cell biomass and the expression of the hydrogenase gene, whereas Mg²⁺ improves CA and H₂ production primarily by increasing cell biomass rather than influencing the expression of functional genes involved in CA biosynthesis.