Uranium isotopic fractionation factors during U(VI) reduction by bacterial isolates

Abstract We experimentally determined the magnitude of uranium isotopic fractionation induced by U(VI) reduction by metal reducing bacterial isolates. Our results indicate that microbial U(VI) reduction induces isotopic fractionation; heavier isotopes (i.e., 238 U) partition into the solid U(IV) products. The magnitudes of isotopic fractionation (expressed as e  = 1000‰ * ( α −1)) for 238 U/ 235 U were 0.68‰ ± 0.05‰ and 0.99‰ ± 0.12‰ for Geobacter sulfurreducens strain PCA and strain IFRC-N, respectively. The e values for Anaeromyxobacter dehalogenans strain FRC-W, strain FRC-R5, a novel Shewanella isolate, and Desulfitobacterium sp. strain Viet1 were 0.72‰ ± 0.15‰, 0.99‰ ± 0.12‰, 0.96‰ ± 0.16‰ and 0.86‰ ± 0.06‰, respectively. Our results show that the maximum e values of ∼1.0‰ were obtained with low biomass (∼10 7  cells/mL) and low electron donor concentrations (∼500 μM). These results provide an initial assessment of 238 U/ 235 U shifts induced by microbially-mediated U(VI) reduction, which is needed as 238 U/ 235 U data are increasingly applied as redox indicators in various geochemical settings.