Cycling of Fe(II)/Fe(III) in the euphotic zone mediated by mineral photocatalysis

Abstract

The marine iron cycle is now recognized as fast and dynamic, partly fueled by vast particulate iron inputs. Yet the mechanisms that activate this reservoir to sustain productivity remain unresolved. Here we show that mineral photocatalysis, an overlooked abiotic process, provides a key activation pathway for Fe(II) production. Laboratory experiments using synthetic iron oxides and anatase in artificial seawater demonstrate that anatase (TiO₂) drives rapid Fe(III)-to-Fe(II) cycling at rates up to fourfold higher than current model assumptions. We propose this process explains the paradox of persistent Fe(II) in sunlit waters as a high-flux steady state of photochemical production coupled with biological uptake. Our findings establish mineral photocatalysis as a key driver in the ocean iron cycle, indicating that bioavailable iron fluxes from particles have been underestimated. Integrating photo-geochemical pathways into ocean models will improve predictions of iron recycling and ecosystem responses.