Description:
Uranium (U) is the most common radionuclide contaminant at US Department of Energy facilities (Riley et al., DOE/ER-0547T, 1992). More information on this element can be found at the Web Elements U page. The uranyl ion [U(VI)O2]2+ is a common, soluble form of this element in the environment. Microbes can immobilize uranyl ion in several ways, three of which are shown here. The mineral uraninite, U (IV)O2, is highly insoluble. Microbes can reduce uranyl ion to hydrated uraninite. The reduction can be carried out by a cytochrome-c3 hydrogenase from Desulfovibrio vulgaris ([http://www.ncbi.nlm.nih.gov/pubmed/8285665|Lovley et al., 1993]) and other organisms, as shown in the left-most pathway branch. Reaction A, in the middle branch, can be carried out by Deinococcus radiodurans R1 in the laboratory with concomitant oxidation of the humic acid analog anthranhydroquinone-2,6-disulfonate (AQDSH2) to its quinone form ([http://www.ncbi.nlm.nih.gov/pubmed/10788374|Fredrickson et al., 2000]). Humic acid, a brown-colored mixture of polymers, found in lignite, peat, and soils, is assumed to be the environmental cofactor. Uranyl ion can be precipitated as cell-bound hydrogen uranyl phosphate without change in oxidation state of the uranium, as shown in the right-most pathway branch. This reaction is facilitated by acid phosphatase N from Citrobacter sp. N14 ([http://www.ncbi.nlm.nih.gov/pubmed/9763695|Basnakova et al., 1998]).