Electron shuttling at bio-geological interfaces: Towards sustainable reductive dechlorination in groundwater under multi-stressor constraints.

Groundwater contamination by chlorinated organic compounds threatens drinking water security and ecosystem integrity, yet outcomes of microbial reductive dechlorination vary widely among aquifers with apparently similar contaminant burdens. This variability cannot be explained solely by microbial community composition or individual geochemical factors because electron transfer underlying dechlorination spans multiple biological and abiotic processes that are frequently conflated. Here, we critically synthesize microbiological, electrochemical, geochemical and hydrogeological evidence to distinguish four coupled but mechanistically distinct layers of electron transfer: membrane-associated organohalide respiration, community cross-feeding, reactions at mineral, organic-matter and engineered-material interfaces, and formation of reactive dechlorination niches. Integrating evidence across these layers indicates that multi-stressor groundwater constraints, including cryptic subsurface conditions, hydrogeological heterogeneity and physicochemical variability, govern dechlorination by reorganizing electron transfer networks across cellular, community and interfacial scales. We argue that contrasting remediation outcomes arise not solely from isolated differences in microbial communities or geochemical conditions, but from differences in how electron transfer networks are organized and constrained. We therefore frame electron transfer networks as a testable accounting framework that links electron equivalents and transport limitations to measurable field diagnostics. This framework clarifies where evidence is established, inferred or hypothetical, and provides a bounded basis for diagnosing electron transfer bottlenecks and designing site-specific dechlorination strategies.