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 ²³⁸U and ²³⁵U 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 ²³⁵U), 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.