X-ray absorption spectroscopy characterization of uranium (VI) complexes formed by Bacillus sphaericus JG-A12 and NCTC 9602 and their recrystalized S-layers

Mohamed Merroun1, Johannes Raff1, Christoph Hennig1, Andre Rossberg1, Tobias Reich2 and Sonja Selenska-Pobell1
1Institute of Radiochemistry, Forschungszentrum Rossendorf,D-01314 Dresden, Germany, 2Johannes Gutenberg Universität Mainz, Institut für Kernchemie, Mainz, Germany
Microorganisms have a potential to affect mobility and overall environmental behaviour of heavy metals and radionuclides via bio-transformations such as oxidation and reduction which can change metal's speciation and solubility and also via different biosorption, bioaccumulation, and biomineralisation processes. In this study Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy was used to determine the structural parameters of the uranium complexes formed by vegetative cells of B. sphaericus JG-A12 which was recovered from a uranium mining waste pile and it's genomic analogue, B. sphaericus NTCC 9602. The complexation of uranium (VI) by the purified and recrystalized S-layers of these two strains was studied as well. The EXAFS analysis demonstrated that in all cases studied the U(VI) was coordinated to carboxyl groups in a bidentate fashion with an average distance between the U atom and the C atom of 2.91 ± 0.02 Å and to phosphate groups in a monodentate fashion with an average distance between the U atom and the P atom of 3.59 ± 0.02 Å. By using ICP-MS, "Stain-all" analysis and colourimetric methods it was demonstrated that the S-layers of B. sphaericus NCTC 9602 and B. sphaericus JG-A12 are phosphorylated. The latter explains the atomic structures of the U-complexes formed at the surfaces of the strains.
Interestingly, the uranium complexes formed by both the bacterial cells and the purified S-layer proteins possess the same structural parameters. These results indicate that the S-layers play the main role in the binding of this radionuclide and that they are acting as a protective barrier against it in the polluted environments.