Visualizing Microbial Indirect Extracellular Electron Transfer

Indirect extracellular electron transfer (IEET) mediated by soluble electron shuttles is a critical pathway for anaerobic microbial respiration, influencing redox transformations and element cycling in natural environments. However, direct spatial visualization of the electron transfer extent has remained limited. Here, employing silver ions (Ag + ) as electron traps and photothermal imaging of as-formed Ag nanoparticles, we visually demonstrated that microbes can effectively transfer electrons over centimeter distances. For instance, Shewanella oneidensis MR-1 transferred electrons across 11.5 ± 1.0 mm within 24 h, reaching 12.4 ± 0.2 mm after 48 h. Both endogenous molecules (e.g., phenazine-1-carboxylic acid, riboflavin) and exogenous compounds (e.g., natural organic matter) could function as electron shuttles, mediating long electron transfer (12.0 ± 0.7 mm to 19.2 ± 0.8 mm for endogenous molecules, and 1.3 ± 0.2 mm to 2.5 ± 0.4 mm for exogenous molecules within 24 h, respectively). Moreover, long-distance IEET was observed in taxonomically and ecologically diverse microbes that are abundant in aquatic and terrestrial environments, confirming its ubiquity. Such long-distance IEET profoundly impacts elemental cycles, as exemplified by enhanced remote methanogenesis and reductive iron mineral dissolution, suggesting that centimeter-scale IEET enables microbial access to distant electron acceptors and promotes interspecies electron flow. Our study provides visualized evidence for the pivotal IEET processes and offers a robust in situ imaging approach for studying IEET-triggered biogeochemical processes.