The isotopic signature of UV during bacterial reduction

The two step electron transfer during bacterial reduction of U VI to U IV is typically accompanied by mass-independent fractionation of the 238 U and 235 U isotopes, whereby the heavy isotope accumulates in the reduced product.However, the role of the U V intermediate in the fractionation mechanism is unresolved due to the challenges associated with its chemical stability.Here, we employed the U V stabilising ligand, dpaea 2-, to trap aqueous U V during U VI reduction by Shewanella oneidensis.Whilst the first reduction step from U VI to U V displayed negligible fractionation, reduction of U V to U IV revealed mass-dependent isotope fractionation (preferential reduction of the 235 U), contrary to most previous observations.This surprising behaviour highlights the control that the U-coordinating ligand exerts over the balance between reactant U supply, electron transfer rate, and U IV product sequestration, suggesting that U V speciation should be considered when using U isotope ratios to reconstruct environmental redox conditions.