Spectroscopic and microscopic characterization of uranium biomineralization in Myxococcus xanthus

In this work, synchrotron-based X-ray absorption spectroscopy (XAS) and transmission electron microscopy (TEM) studies were carried out to elucidate at molecular scale the interaction mechanisms of Myxococcus xanthus with uranium at different pH values. Extended X-ray absorption fine structure (EXAFS) experiments show that there are significant differences in the structural parameters of the U complexes formed by this bacterium at pH 2 and 4.5. At very low acidic pH of 2, the cells accumulated U(VI) as organic phosphate-metal complexes. However, at pH 4.5, the cells of this bacterium precipitated U(VI) as meta-autunite-like phase. TEM indicated that at pH 2 the uranium accumulates are located mainly at the cell surface. Whereas, at pH 4.5 the uranium precipitation takes place on the cell wall and within the extracellular polysaccharides (EPS) characteristic of this bacterium. Dead/live staining studies showed that 30% and 50% of the uranium-cell treated cell populations are alive at pH 2 and 4.5, respectively. The precipitation of uranium as mineral phase is possibly due to the acid phosphatase activity which was detected at both pH values. Precipitation of U as mineral phase may lead to more stable U(VI) sequestration that may be suitable for remediation uses. These observations, combined with the very high uptake capacity of this bacterium, imply that bacterial cells may significantly influence the fate of uranium in soils where these bacterial groups are mainly found.