Crustal faulting drives biological redox cycling in the deep subsurface

In the deep biosphere, where substrates from the surface are depleted, microbial communities metabolize using redox pairs from water-rock reactions. A notable example of this is the production of hydrogen gas (H2) and oxidants from rock fracturing. However, the potential interactions between such initial redox pairs and the key iron (Fe) element are underexplored. Here, we simulated radical-induced water splitting to investigate how redox gradients form and evolve. In the presence of Fe, ferrous iron (Fe2+) was marginally oxidized to ferric iron (Fe3+) by low levels of oxidants, while Fe3+ was significantly reduced by reactive hydrogen (•H) back to Fe2+. We propose that crustal faulting could generate various redox pairs and drive Fe redox cycling, thereby sustaining subsurface life on Earth and other planets.