Mechanisms for the Enhancement of Caproic Acid and H 2 Production in Ruminococcaceae Bacterium CPB6 by Fe(II) and Mg(II): Growth and Gene Transcription Analyses.

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.