Interaction mechanisms of bacterial strains isolated from uranium mining waste piles with U and Pb

Bacterial isolates belonging to two different groups of Gram-positive bacteria, namely to Arthrobacter sp. of Actinobacteria and to Bacillus sphaericus of Firmicutes were cultivated from a uranium mining waste pile near the town of Johanngeorgenstadt in Germany. We demonstrate that these two kinds of isolates possess different strategies to protect their populations from the toxicity of uranium. Transmission electron microscopy (TEM) analyses showed that the Arthrobacter sp. strains accumulated this radionuclide intracellularly, although a significant part of the uranium treated Arthrobacter sp. population remained radionuclide-free. X-ray absorption spectroscopy (XAS) studies demonstrated that uranium was mainly coordinated to bacterial organic phosphate groups in a monodentate binding mode.
In the case of the Bacillus sphaericus isolate JG-A12 uranium was bound on the cell surface by the bacterial surface layer (S-layer) protein. Extended x-ray absorption fine structure (EXAFS) spectroscopic analyses demonstrated that the radionuclide was complexed by carboxylic groups in a bidentate fashion and by phosphate groups in a monodentate fashion.
X-ray diffraction (XRD) analysis indicated that the growing cells of the arthrobacterial strains precipitated Pb as galena (PbS), whereas in non-growing conditions pyromorphite (Pb5(PO4)3Cl) phase was produced by the cells, alleviating probably the toxicity of Pb. TEM analyses showed that the pyromorphite precipitates were mainly located at the cell surface.
Because representatives of Arthrobacter and Bacillus were induced in uranium supplemented samples of the same uranium mining waste we suggest that these organisms play an important protective role for the natural bacterial community of the studied habitat.