Monitoring of the biogeochemical changes induced by increased U(VI) concentrations in natural environments

Natural bacterial community structures in the wetlands of uranium mill-tailings with different geographic origin (Germany and USA) and possessing various levels of uranium contamination were studied. Comparative analyses of bacterial communities via direct molecular approaches based on 16S rRNA gene sequences revealed strong predominance of Gamma-, Beta-, or Deltaproteobacteria in dependence on the geographic origin and on the anthropologic history of the studied wastes. In all studied samples novel, yet to be cultured, Betaproteobacterial populations were identified. It was possible to culture representatives of Gamma-Pseudomonas as well as some Alphaproteobacteria (mainly representatives of Sphingomonas sp.) and also different groups of Firmicutes.
Treatments of the samples from one of the studied uranium wastes with uranyl nitrate resulted in a strong shifting in the natural bacterial community and proliferation of representatives of Gamma-, Betaproteobacteria, and/or Firmicutes, depending on the aeration conditions of the experiments. The increased size of the mentioned bacterial populations was confirmed by using a combination of direct monitoring methods based on 16S rRNA and narG genes.
The fate of the added U(VI) in a form of uranyl nitrate was monitored via Time Resolved Laser-induced Fluorescence Spectroscopic (TRLFS) analysis. The latter demonstrated that almost all added uranium was complexed in uranyl phosphate compounds.
Transmission Electron Microscopic (TEM) and X-ray Absorption Spectroscopic (XAS) analyses of the interactions of particular uranium waste bacterial isolates with U(VI) under conditions, corresponding to the natural ones, revealed that the added uranium is immobilized mainly on their cell walls via sorption by organic phosphate groups and/or via precipitation in inorganic uranyl phosphate mineral phases (biomineralization). The observed biomineralization process was connected to release of orthophosphate by the stressed by the treatments bacterial cells. No reduction of U(VI) to U(IV) was observed at the studied oligotrophic conditions which are relevant to the natural ones in the studied wastes.