Ethanol utilization strategies in chain elongation: insights from bio-iron systems

Carbon chain elongation plays a pivotal role in biomass valorization by enabling the efficient conversion of organic substrates into higher-value biochemicals. Ethanol is a key substrate in chain elongation, yet how its oxidation reshapes carbon flux under complex electron transfer networks remains unclear. Based on an open-culture system, this study elucidates how iron species regulate ethanol oxidation. Metagenomic analysis revealed that iron acting as an electron acceptor (e.g., hematite, magnetite) enhanced electron transfer and iron respiration, while serving as an electron donor (e.g., iron powder) activated ethanol metabolism and promoted ATP generation to couple it with chain elongation pathways. These processes improved ethanol oxidation efficiency, redirected carbon flux toward acetate accumulation, and facilitated even-chain elongation of acetate. This study demonstrates an iron-regulated ethanol self-upgrading pathway with significant potential. However, in the presence of fermentation substrates, this phenomenon may reduce substrate conversion efficiency and alter product selectivity. To quantitatively characterize these processes, an integrated evaluation method for ethanol oxidation and self-upgrading was established, providing a robust analytical framework for related phenomena. Overall, this work uncovers the mechanism of bio-iron-mediated carbon flux reorganization and offers both theoretical and practical guidance for the development of advanced ethanol valorization systems and resource-efficient solid waste utilization strategies.