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, Fe3O4 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 Fe2O3 into Fe3O4 through redox cycling. In contrast, FeCO3 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 FeO bond shortening reduces reorganization energy, while FeFe 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.