Hydrogen supplementation improves glucose-based n-caproate production in Caproiciproducens galactitolivorans with reverse β-oxidation-associated redox remodeling

Abstract

Medium-chain carboxylic acids, such as n-caproate, are attractive sustainable platform chemicals. However, their microbial production is often limited by electron availability and low product selectivity. This study investigated the effect of hydrogen supplementation on glucose-based n-caproate production by Caproiciproducens galactitolivorans. Batch fermentation was performed under various hydrogen pressures (0 to 600 kPa). At 600 kPa hydrogen, n-caproate reached 8.0 g L−1, accompanied by a 66% increase in n-caproate selectivity and a tenfold increase in the intracellular redox-cofactor ratio, consistent with enhanced redox-cofactor turnover. Multi-omics analysis indicated metabolic remodeling under hydrogen-enriched conditions, including the increased abundance of key enzymes in the reverse β-oxidation pathway and redox-associated functions. In contrast, although fatty acid biosynthesis genes were transcriptionally induced, this transcriptional increase was not reflected at the protein level, suggesting a metabolic response more consistent with reverse β-oxidation-supported n-caproate synthesis than with fatty acid biosynthesis. These results support the use of hydrogen as a clean external reducing agent for improving n-caproate yield and redox efficiency in defined microbial systems. This study provides mechanistic insights into redox-driven metabolic control and selective n-caproate production in a defined microbial chain elongation system.