Sulfate-reducing bacteria as natural bioelectrochemical catalysts: unveiling their role in extracellular electron transfer

Sulfate-reducing bacteria (SRB) are the focus of traditional studies of anaerobic sulfate respiration; however, gradually, SRB are also found to achie + ve extracellular electron transfer (EET) (a capability that remains poorly integrated into mainstream SRB physiology). This review is the first to bring together molecular, electrochemical, and ecological data in a systematic way to show that EET is not a side effect but a crucial, overlooked, and hence, silent, extension of the SRB metabolism. We integrate knowledge on the outer-membrane multiheme cytochromes, the putative conductive pili, the redox-active shuttles, and the hybrid DET–MET pathways that in concert allow SRB to interchange electrons with minerals, electrodes, and syntrophic partners to a new level. This paper’s key new aspect is in bringing the scattered results together into one comprehensive mechanistic model that positions SRB as new natural bioelectrochemical catalysts, instead of just classical sulfate reducers. Major research gaps, such as the electron partitioning between DSR and EET pathways that is still unresolved, the absence of structural confirmation of nanowires, and the low electrochemical performance in real wastewater, are openly discussed. We forecast a future plan combining multi-omics, nanoscale imaging, and in situ electrochemical profiling to rationally design SRB-based biotechnologies. This review conveys a conceptual change by depicting SRB as electrochemically active microorganisms whose capabilities are hardly utilized and who have a revolutionary potential in bioremediation, resource recovery, and renewable energy systems.