Influence of riboflavin on the reduction of radionuclides by Shewanella oneidenis MR-1

Uranium (as UO22+), technetium (as TcO4-) and neptunium (as NpO2+) are highly mobile radionuclides that can be reduced enzymatically by a range of anaerobic and facultatively anaerobic microorganisms, including Shewanella oneidensis MR-1, to poorly soluble analogues. The redox chemistry of Pu is more complicated, but the dominant oxidation state in most environments is poorly soluble Pu(IV), which can be reduced to the potentially more soluble Pu(III), which could enhance migration of Pu in the environment. Recently it was shown that flavins (riboflavin and flavin mononucleotide (FMN)) secreted by Shewanella oneidensis MR-1 can act as electron shuttles, promoting anoxic growth coupled to the accelerated reduction of poorly-crystalline Fe(III) oxides. Here we studied the role of riboflavin in mediating the reduction of radionuclides in cultures of Shewanella oneidensis MR-1. Our results demonstrate that the addition of 10 µM riboflavin enhances the reduction rate of Tc(VII) to Tc(IV) and Np(V) to Np(IV), but has no significant influence on the reduction rate of U(VI) by Shewanella oneidensis MR-1. The presence of riboflavin also accelerated Pu(IV) reduction, demonstrated by an increase in the percentage of Pu(IV) reduced to Pu(III), with and without riboflavin present (17 and 3%, respectively). Thus riboflavin can act as an extracellular electron shuttle to enhance rates of Tc(VII), Np(V) and Pu(IV) reduction, and may therefore play a role in controlling the oxidation state of key redox active actinides and fission products in natural and engineered environments. These results also suggest that the addition of riboflavin could be used to accelerate the bioremediation of radionuclide-contaminated environments