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

Abstract

Uranium (as UO2(2+)), 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 species. The redox chemistry of Pu is more complicated, but the dominant oxidation state in most environments is highly insoluble Pu(IV), which can be reduced to Pu(III) which has a potentially increased solubility 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), Pu(IV) to Pu(III) and to a lesser extent, Np(V) to Np(IV), but has no significant influence on the reduction rate of U(VI) by Shewanella oneidensis MR-1. 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.

Reduction of platinum (IV) ions to elemental platinum nanoparticles by anaerobic sludge

“…In addition, there is one report which studied the effect of the addition of riboflavin in the microbial reduction of uranium (U), technetium (Tc), neptunium (Np) and plutonium (Pu) with S. oneidenis MR-1. 28 As was reported, the addition of 10 mol L -1 of riboflavin, enhanced the rate of microbial reduction of Tc(VII) to Tc(IV), Pu(VI) to Pu(III) and Np(V) to Np(IV) to a lesser extent, but riboflavin did not have any impact on the reduction of U(VI) to U(IV). Interestingly, the range of redox mediator concentrations in all three cases is very similar 160-16 mol L -1 in the case of Te(VI) and Te(IV), 130-13 mol L -1 in the case of Pt(IV) and 10 mol L -1 in the cases of Tc(VII), Pu(VI) and Np(V).…”

“…In that research, lawsone and riboflavin were found capable of increasing the rate of the microbial reduction of Te(VI) and Te(IV), being lawsone the most effective for the reduction of Te(VI) and riboflavin the most effective for the reduction of Te(IV). In addition, there is one report which studied the effect of the addition of riboflavin in the microbial reduction of uranium (U), technetium (Tc), neptunium (Np) and plutonium (Pu) with S. oneidenis MR-1 28 . As it was reported, the addition of 10 µM of riboflavin, enhanced the rate of the microbial reduction of Tc(VII) to Tc(IV), Pu(VI) to Pu(III) and Np(V) to Np(IV) to a lesser extent, but riboflavin did not have any impact on the reduction of U(VI) to U(IV).…”

Interactions of Plutonium with Pseudomonas sp. Strain EPS-1W and Its Extracellular Polymeric Substances

“…Microorganisms are associated with a variety of radioactive materials at U.S. Department of Energy contaminated sites (14,15), and it is apparent that Pu speciation and solubility can be affected by microbial activity (16)(17)(18)(19)(20)(21). Previous studies of Pseudomonas stutzeri and Bacillus sphaericus reported a sorption affinity of nearly 100 g Pu/g dry cell weight for Pu(VI) to the surface of vegetative cells and spores (20).…”

Biogeochemical Cycling of 99Tc in Alkaline Sediments

“…At circumneutral pH in pure culture experiments, microbial reduction to Tc­(IV) can be coupled to electron donor (hydrogen and in some cases organic matter) oxidation via enzymatic pathways. , More widely, TcO 4 – reduction in many environmental systems seems to be dominated by redox reactions with Fe­(II)-bearing minerals, such as magnetite (Fe 2 O 3 ), , sorbed Fe­(II), − which can be formed as a result of microbial Fe­(III) reduction, or pyrite (FeS 2 ) formed after reaction of Fe­(II) with sulfide, and again linked to microbial metabolism. At circumneutral pH, the rate of TcO 4 – reduction is influenced by Fe­(II) speciation and concentration, and potentially, the presence of redox shuttles. ,− When TcO 4 – is reduced in systems containing Fe­(II), X-ray absorption spectroscopy has shown that Tc forms poorly soluble hydrous TcO 2 -like phases (monomers–polymers), potentially with Fe associations. ,,,− Incorporation of Tc­(IV) into Fe­(II)-bearing mineral phases has also been reported. − Additionally, at ultradilute concentrations, where direct XAS speciation analysis is not possible (<10 –11 mol L –1 , below the solubility threshold for TcO 2 precipitation), Tc (as 99m Tc) has been shown to be retained on solids under reducing conditions, presumably as Tc­(IV) sorbed to geomaterials. − Interestingly, Tc­(IV) solubility may increase at elevated salinity as well as when organics and carbonates are present at enhanced concentrations, and Tc has also been shown to be associated with Fe-bearing colloids. ,,,− However, the effect of changing biogeochemistry under alkaline conditions, on the reduction of Tc­(VII) remains poorly constrained.…”