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 (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 (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 (Basnakova et al., 1998).
The following is a text-format uranium pathway map. Organisms which are known to carry out these reactions are given, however other organisms may also carry out these steps. Follow the links for more information on compounds or reactions. This map is also available in graphic (5k) format.
U(VI)O22+ U(VI)O22+ U(VI)O22+ Desulfovibrio vulgaris Deinococcus Citrobacter sp. N14 radiodurans R1 | | | phosphate | | | / | | |/ | cytochrome-c3 | A | acid phosphatase N | hydrogenase | | | | | | | | | | | v v v U(IV)O2 U(IV)O2 U(VI)O2HPO4
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