Crustal faulting drives biological redox cycling in the deep subsurface

In the deep biosphere, where surface-derived substrates are depleted, microbial communities rely on redox pairs generated through water-rock reactions to sustain metabolism. A notable example of this is the production of hydrogen gas (H₂) and oxidants from rock fracturing. However, the potential interactions between these initial redox pairs and a key subsurface element-iron (Fe)-remain underexplored. Here, we simulated radical-induced water splitting to investigate the formation and evolution of redox gradients. Our results show that in the presence of Fe, ferrous iron (Fe²⁺) was marginally oxidized to ferric iron (Fe³⁺) by low concentrations of oxidants, whereas Fe³⁺ was efficiently reduced back to Fe²⁺ by reactive hydrogen atoms (•H). We propose that crustal faulting can generate various redox pairs and drive Fe redox cycling, thereby providing a sustained energy source for subsurface life on Earth and potentially on other planetary bodies.