Addition of U(VI) to a Uranium Mining Waste Sample and Resulting Changes in the Indigenous Bacterial Community

Changes of natural bacterial community structure induced by addition of uranyl or sodium nitrate to weakly contaminated soil samples from a uranium mining waste pile were studied using the 16S rDNA retrieval. Our results demonstrated that both treatments caused drastic changes in bacterial composition of the studied samples resulting in strongly reducing the originally predominant Acidobacteria and Alphaproteobacteria. The addition of sodium nitrate induced a strong propagation of denitrifying and nitrate reducing populations of Actinobacteria and of Bacteroidetes. The treatment of the samples with uranyl nitrate demonstrated that most part of the mentioned Bacteroidetes and some of the actinobacterial populations do not tolerate high U(VI) concentrations. Most interesting is the strong propagation of Gamma-Pseudomonas spp. capable to accumulate significant amounts of U(VI) and to build biofilms, hence being able to protect the community from the added toxic metal. At the initial stages of incubation (four weeks after the addition of uranyl nitrate) also U(VI) reducing Geobacter spp. appeared. However, at the later stages of incubation (fourteen weeks after the supplementation) no Geobacter populations were detected anymore. The latter indicated that no biological reduction of U(VI) occurred in these samples most probably because of the lack of electron donors to support this reaction. Interestingly, different U-sensitive Bacteroidetes and alphaproteobacterial populations propagated in the U(VI) treated samples at these late stages of incubation. That indicated that the added U(VI) was immobilized and did not disturb anymore the studied bacterial community.
The drastic changes induced by the addition of U(VI) indicate that bacterial communities are able to protect their members from the toxicity of this radionuclide by propagation of resistant and capable to interact with U(VI) populations. On the other hand, the large number of dead bacteria liberates phosphate-rich and other biopolymers capable to bind U(VI). Hence, bacteria together with the abiotic soil components such as minerals and humic acids play a key role in the immobilization of U(VI) in nature.