Electron flux is a key determinant of uranium isotope fractionation during bacterial reduction

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