Mechanisms for the Enhancement of Caproic Acid and H 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 (H) 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 Fe and Mg on the growth and metabolism of strain CPB6 remains limited. Therefore, this study aims to investigate the impact of Fe and Mg on CA and H production, as well as on the expression of key genes involved in CA and H biosynthesis pathway. The results indicate that Fe positively affects cell proliferation and H production while minimally impacting CA production. The highest levels of H production were achieved with the addition of 200 mg/L Fe. Conversely, Mg significantly enhances CA and H production, with the optimal yield observed in a medium enriched with 300 mg/L Mg. Reverse transcription quantitative PCR (RT-qPCR) analysis reveals that Fe promotes the expression of the hydrogenase gene, whereas Mg has a negligible effect on hydrogenase expression. Notably, Fe and Mg inhibit the expression of key genes involved in CA synthesis. These findings suggest that Fe enhances H production by boosting cell biomass and the expression of the hydrogenase gene, whereas Mg improves CA and H production primarily by increasing cell biomass rather than influencing the expression of functional genes involved in CA biosynthesis.