Article

Role of fungi in the biogeochemical fate of depleted uranium

Division of Molecular and Environmental Microbiology, College of Life Sciences, University of Dundee, Dundee, Scotland, UK.
Current Biology (Impact Factor: 9.57). 06/2008; 18(9):R375-7. DOI: 10.1016/j.cub.2008.03.011
Source: PubMed

ABSTRACT

The testing of depleted uranium (DU; a 97.25% U:0.75% Ti alloy) ammunition and its use in recent war campaigns in Iraq (1991 and 2003) and the Balkans (1995 and 1999) has led to dispersion of thermodynamically unstable DU metal into the environment [1-3]. Although less radioactive, DU has the same chemotoxicity as natural uranium and poses a threat to human populations [1]. Uranium tends to form stable aqueous complexes and precipitates with organic ligands [4], suggesting that living organisms could play an important role in geochemical transformations and cycling. Fungi are one of the most biogeochemically active components of the soil microbiota [5], particularly in the aerobic plant-root zone. Although the mutualistic symbiotic associations (mycorrhizas) of fungi with plants are particularly important in mineral transformations [5], fungal effects on metallic DU have not been studied. Here, we report that free-living and plant symbiotic (mycorrhizal) fungi can colonize DU surfaces and transform metallic DU into uranyl phosphate minerals.

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    • "The crystalline form of the bacterial uranium phosphate produced showed very close similarity to the mycogenic uranium phosphate crystals observed in this study (Fig. 1C). This bioprecipitation of uranium is likely to be preceded by physico-chemical processes such as adsorption, surfaceVanHaverbeke et al., 1996) Uranyl phosphate hydrateMarkovic and Pavkovic, 1983) Uramphite NH4UO2PO4·3H2O NH4UO2PO4·3H2O ⇋ NH4 + + UO2 2+ + PO4 3− + 3H2O −26.50 (± 0.09) (Markovic et al., 1988)complexation and ion exchange at hyphal surfaces (Fomina et al., 2008). Fungal and bacterial cell walls and outer surfaces contain many functional groups, and the ability to strongly biosorb soluble uranium species has been documented widely (Krueger et al., 1993;Guibal et al., 1995;Fowle et al., 2000;Macaskie et al., 2000;Francis et al., 2004;Gorman-Lewis et al., 2005). "
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    ABSTRACT: Geoactive soil fungi were investigated for phosphatase-mediated uranium precipitation during growth on an organic phosphorus source. Aspergillus niger and Paecilomyces javanicus were grown on modified Czapek-Dox medium amended with glycerol 2-phosphate (G2P) as sole P source and uranium nitrate. Both organisms showed reduced growth on uranium-containing media but were able to extensively precipitate uranium and phosphorus-containing minerals on hyphal surfaces, and these were identified by X-ray powder diffraction as uranyl phosphate species, including potassium uranyl phosphate hydrate (KPUO6.3H2O), metaankoleite ((K1.7Ba0.2)(UO2)2(PO4)2.6H2O), uranyl phosphate hydrate ((UO2)3(PO4)2.4H2O), metaankoleite (K(UO2)(PO4).3H2O), uramphite (NH4UO2PO4.3H2O), and chernikovite ((H3O)2(UO2)2(PO4)2.6H2O). Some minerals with a morphology similar to bacterial hydrogen uranyl phosphate were detected on A. niger biomass. Geochemical modelling confirmed the complexity of uranium speciation, and the presence of metaankoleite, uramphite and (UO2)3PO4.4H2O between pH 3 and 8 closely matched the experimental data, with potassium as the dominant cation. We have therefore demonstrated that fungi can precipitate U-containing phosphate biominerals when grown with an organic source of P, with the hyphal matrix serving to localize the resultant uranium minerals. The findings throw further light on potential fungal roles in U and P biogeochemistry as well as the application of these mechanisms for element recovery or bioremediation.
    No preview · Article · Jan 2015 · Environmental Microbiology
    • "We hypothesized earlier about the role of fungal phosphatases in releasing phosphate from organic sources and this may contribute to so-called biologically induced mineralization where an organism alters the local microenvironment so that there is extracellular chemical precipitation of mineral phases (Bazylinski and Schübbe 2007;Hamilton, 2003;Dupraz et al., 2009;Gadd, 2010;Gadd et al., 2012). Other contributory factors include acidolysis and complexolysis by excreted organic acids with secondary mineral formation occurring through precipitation reactions depending on the local chemical environment (Sayer et al., 1999;Fomina et al., 2008;Gadd et al., 2012;Rhee et al., 2012). "
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    • "The hyphae were found to be encrusted with uranium precipitates associated with phosphorus and some fungal species caused the biomineralization of uranyl phosphate minerals of the meta-autunite group. A similar ability of fungi has been demonstrated also in metallic depleted uranium (Fomina et al. 2008). As suggested by Fomina et al. (2007, 2008), the fact that fungi are able to solubilize uranium solids indicates their possible role in biogeochemical cycling of U in the environment. "
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