Based on 16S rRNA gene sequence retrieval, changes in natural bacterial community structure induced by addition of uranyl or sodium nitrate to soil samples from a uranium mining waste pile were investigated. Our results demonstrate that both treatments cause drastic changes in the 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 particular denitrifying and nitrate-reducing populations belonging to Actinobacteria and 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. Instead, a strong propagation of Pseudomonas spp. from the Gammaproteobacteria occurred. At the initial stages of incubation (4 weeks after the addition of uranyl nitrate) U(VI)-reducing Geobacter spp. appeared. However, at the later stages of incubation (14 weeks after the beginning of supplementation) no Geobacter populations were detected anymore. 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 no longer bioavailable.
The drastic changes in bacterial community structure of the soil samples from the depleted uranium mining waste caused by the addition of uranyl nitrate indicate that most of the established indigenous bacterial populations do not tolerate U(VI). By the treatment with uranyl nitrate they are replaced by particular uranium resistant nitrate-reducing and denitrifying populations that potentially interact with the added radionuclide. On the other hand, the large number of dead uranium-sensitive bacteria likely liberates phosphate-rich and other biopolymers capable of binding U(VI). On the basis of our results, we propose that bacteria along with the abiotic soil components such as minerals and humic acids may influence the behaviour of U(VI) in nature.