Electron flux is a key determinant of uranium isotope fractionation during bacterial reduction
A. Brown, Margaux Molinas, Yvonne Roebbert, Ataru Sato, Minori Abe, Stefan Weyer, Rizlan Bernier‐Latmani
Abstract
Abstract Uranium isotopic signatures in the rock record are utilized as a proxy for past redox conditions on Earth. However, these signatures display significant variability that complicates the interpretation of specific redox conditions. Using the model uranium-reducing bacterium, Shewanella oneidensis MR-1, we show that the abundance of electron donors (e.g., labile organic carbon) controls uranium isotope fractionation, such that high electron fluxes suppress fractionation. Further, by purifying a key uranium-reducing enzyme, MtrC, we show that the magnitude of fractionation is explicitly controlled by the protein redox state. Finally, using a mathematical framework, we demonstrate that these differences in fractionation arise from the propensity for back-reaction throughout the multi-step reduction of hexavalent uranium. To improve interpretations of observed fractionations in natural environments, these findings suggest that a variable intrinsic fractionation factor should be incorporated into models of uranium isotope systematics to account for differences in electron flux caused by organic carbon availability.