Deciphering the role and mechanism of zero-valent iron on medium-chain fatty acid production from acetate via chain elongation in microbial electro-fermentation

Medium-chain fatty acids represents one of the promising high-value products through chain elongation (CE) via microbial electro-fermentation (EF), while its biosynthesis is constrained by the low electron transfer efficiency. This study introduced zero-valent iron as an innovative anode material to enhance CE performance and elucidated its underlying mechanism. The EF system mediated by iron anode achieved a peak caproate concentration of 2333.9 ± 102.1 mg COD/L, manifesting a 2.8-fold increase compared with that obtained in the conventional EF system, and demonstrated the superior electrochemical performance. Microbial community analysis revealed the symbiotic interactions among caproate-synthesizing bacteria, anaerobic fermentative bacteria, electroactive bacteria, and homo-acetogens attached on the bio-cathodes. Moreover, the iron anode upregulated the functional genes encoding key enzymes in the critical metabolic pathways, including reverse β-oxidation, fatty acid biosynthesis, and electron transport. These findings may establish a theoretical basis for accelerating the production of high-value added chemicals in EF system.