Redox reactions -- the give-and-take of electrons between chemical species -- are fundamental to life and environmental stability. They govern how nutrients cycle, how contaminants move, and how microbes harvest energy. Traditionally, scientists believed these reactions were confined to microscopic "hotspots" at mineral or microbial surfaces. But the new study, led by researchers from the China University of Geosciences, shows that electron transfer (ET) in the subsurface can extend far beyond the nanoscale, linking distant chemical zones into vast underground electron networks.

At the smallest scales, ET occurs directly at mineral-water or microbe-mineral interfaces, where single molecules or cells exchange electrons over nanometers. But recent discoveries reveal more dramatic processes: conductive minerals, natural organic molecules, and even specialized bacteria known as "cable bacteria" can act as electron bridges, transmitting charges across centimeters. In some cases, stepwise connections form "long-distance ET chains" that span tens of centimeters or more, effectively creating underground electron highways.

"These findings challenge the old view that electron transfer is strictly local," said corresponding author Prof. Songhu Yuan. "We now know that redox processes can connect across surprisingly large distances, coupling reactions in one zone with those in another. This has profound implications for contaminant remediation and environmental sustainability."

The review highlights how these multiscale ET processes influence both natural cycles and human-driven pollution management. For example, long-distance ET can enable "remote remediation," in which contaminants are degraded in hard-to-reach zones without direct chemical injection. Conductive minerals or added biochar can expand microbial activity, while cable bacteria help couple oxygen at the sediment surface with sulfide deep below, reducing harmful emissions.

The authors also outline the next frontiers in ET research: developing better tools to measure electron flows across scales, creating models that integrate nanoscale reactions with field-scale processes, and designing remediation technologies that harness these natural electron pathways.

"Our work provides a conceptual framework for thinking about the subsurface as an interconnected redox system," said co-author Dr. Yanting Zhang. "By understanding how electrons move underground, we can better predict the fate of nutrients and pollutants and design more effective strategies to protect groundwater and ecosystems."

This synthesis bridges fundamental science with practical applications, offering hope that tomorrow's environmental engineers may one day plug into Earth's own "electron grid" to restore contaminated soils and aquifers.