Enhancing ethanol-driven chain elongation via iron speciation: impacts on metabolic flux and dual FAB/RBO pathway activation.

Anaerobic chain elongation (CE) has emerged as a promising technology for upgrading low-value organic substrates into high-value medium-chain fatty acids (MCFAs); however, achieving targeted metabolic flux and efficient electron transfer remains challenging. To address this, this study explores the role of iron speciation in enhancing chain elongation (CE) driven by ethanol. Two iron-modified activated carbons, Fe3O4@AC and ZVI@AC, were evaluated to assess their impact on microbial metabolic networks. Results revealed that Fe3O4@AC significantly enhanced caproate production (4600.0 mg/L) and electron transfer efficiency (87.0 %), while ZVI@AC triggered a diversion towards alcohol production (940.61 mg/L n-butanol). The superior performance of Fe3O4@AC was attributed to its semiconductive properties, which facilitated interspecies electron transfer (potentially via DIET-like mechanisms) and balanced electron flow, promoting the activation of both fatty acid biosynthesis (FAB) and reverse β-oxidation (RBO) pathways. Metagenomic analysis revealed a shift in microbial community composition, with Massilibacterium enrichment under Fe3O4@AC, highlighting the importance of tailored material design for targeted MCFA production. These findings provide insights into optimizing microbial metabolism for enhanced CE efficiency.