Uranium is a naturally occuring radioactive element. Uranium isotopes have long half-lives (>105 years) and the use of uranium in nuclear power plants and weapons can concentrate uranium to harmful levels in localized environments (Tox FAQs: Uranium). Research into microbial interactions with uranium has focused on bioreduction of soluble uranium compounds to less mobile oxidation states, bioaccumulation and bioprecipitation of soluble uranium species, and indirect oxidative bioleaching of uranium from ores.

Prokaryotic reduction of soluble uranium(VI) to insoluble uranium(IV) has been proposed as a method for bioremediation of uranium-contaminated groundwater and hazardous waste sites (reviewed by Lovley, 1995). Desulfovibrio vulgaris was shown to use cytochrome c3 as a uranium reductase (Lovely et al, 1993). Geobacter metallireducens and Shewanella putrefaciens were found to use U(VI) as a terminal electron acceptor during acetate oxidation, but the uranium reductases were not identified (Lovely et al, 1991). Deinococcus radiodurans cannot reduce U(VI) directly, but can reduce U(VI) indirectly using the humic acid analog anthraquinone-2,6-disulfonate (AQDS) as an electron shuttle (Fredrickson et al, 2000).

Citrobacter sp. N14 was shown to precipitate soluble uranyl ions as cell-bound hydrogen uranyl phosphate in a process mediated by an acid-type phosphatase (PhoN) (Macaskie et al, 2000). Expression of Salmonella typhimurium PhoN in Eschericia coli DH5α conferred the ability to precipitate uranyl ion as hydrogen uranyl phosphate to the transformed cells (Basnakova et al, 1998). Efficient biosorption of uranium by Saccharomyces cerevisiae, Rhizopus arrhizus, and Streptomyces longwoodensis biomass has been demonstrated (Volesky and Holan, 1995).

Insoluble uranium can be mobilized by bacterial activity. Under aerobic conditions, the iron-sulfur chemolithoautotrophic bacteria Thiobacillus ferrooxidans and Leptospirillum ferrooxidans can oxidize pyrite (Fe2S) to ferrous sulfate (Fe2O4) and then to ferric sulfate (Fe2(O4)3). Ferric ions can then oxidize insoluble uranium(IV) to soluble uranium(VI) (reviewed by Tuovinen and Bhatti, 1999).

For more information:

Search Medline for uranium metabolism AND bacteria

Lovley DR, Phillips EJP, Gorby YA, Landa ER. Microbial reduction of uranium. Nature. 1991;350:423-417.

Tuovinen OH, Bhatti TM. Microbiological leaching of uranium ores. Minerals Metallurg Process. 1999;16:51-60.

 


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