Bio-driven Fe bond restructuring in bio-iron systems enhances electron transfer for chain elongation

The bio-iron system effectively overcomes the bottleneck in the biosynthesis of medium-chain fatty acids (MCFAs) by enhancing extracellular electron transfer capability. However, the key structural characteristics and dynamic evolution of iron-based electron shuttles remain unclear, limiting system optimization and mechanistic understanding. Among the tested iron species in this study, Fe 3 O 4 exhibited the most favorable extracellular electron transfer performance and MCFAs production, highlighting its potential as an efficient electron shuttle. In addition, this study revealed that microorganisms can induce partial transformation of iron powder and Fe 2 O 3 into Fe 3 O 4 through redox cycling. In contrast, FeCO 3 lacked both structural reconfiguration ability and electron transfer activity, thus limiting MCFAs biosynthesis. Partial least squares regression model, combined with characterization analysis, identified the Fe(III)-O-Fe(II) structure as the key structural characteristics of the electron shuttle. Moreover, X-ray absorption fine structure analysis, interpreted through Marcus theory, demonstrated that bio-driven Fe O bond shortening reduces reorganization energy, while Fe Fe bond elongation expands the electron transfer network. This study elucidates, at the atomic level, the microbial mechanisms underlying the construction and regulation of iron-based electron shuttles, providing a theoretical foundation for the design of bioinspired materials and the optimization of bio-iron systems.