Recent studies have shown that levels of uranium (U) in lichens located near a U mine decreased substantially within three years after the mine ceased operations. To determine whether aqueous washing could displace U, podetia of Cladina rangiferina (L.) Harm. and Cladina mitis (Sandst.) Hale & Culb. that were impregnated with soluble uranyl nitrate hexahydrate (UO2(NO3)2.6H20) were subjected to one hour washes with either double distilled water (ddH20) or solutions of NiSO4.6H20, CH3COOH, or H2S04. Scanning electron microscopy and energy dispersive X-ray analysis were used to locate and measure weight percent U before and after washing. Uranium was incorporated in the medulla, inner conglutinate zone, and photobiont clusters, but mean weight % U was higher in thallus regions involving the mycobiont. After washing of C. rangiferina with ddH20 and Ni solutions, significant reductions (p < 0.01) in weight % U were observed only in the conglutinate region. After immersion of C. mitis in Ni solutions (p < 0.01) and H2SO4 (p < 0.01), U levels were also significantly lower in the conglutinate zone. Three years following the cessation of U bearing emissions from a mine near Elliot Lake, Ontario, Canada, C. rangiferina growing nearby exhibited no U in any of the three main regions of the podetium, but only in association with mineral particulates on the inner surface. Heavy metals may accumulate in lichen thalli by trapping of airborne particulate matter (e.g., Beckett et al. 1982; Garty et al. 1979; Looney et al. 1985; Nieboer et al. 1982) or by absorption of dissolved metals contained in rain, natural run-off waters, streams, or substrates (e.g., Beck & Ramelow 1990; Boileau et al. 1985b; Nieboer et al. 1977; Nieboer et al. 1978). However, metals may be dissolved from particulates after deposition on lichens, in the same way as mineral particles are solubilized in soil. In thalli, particulates may be degraded due to interaction with protons from metabolically active cells or acidic organic substances (Brown & Brown 1991) and, as we recently determined, radionuclides can be released from mine dust into distilled water even without acidity. Metal ions thus released may presumably be taken up in the dissolved form. Dissolved metals in a lichen are retained in an unbound condition, become bound to ionic functional sites in cell walls and plasma membranes, or are taken up intracellularly (Nieboer et al. 1978). Because a lichen thallus generally has no covering such as a cuticle to prevent direct contact with the environment, atmospheric depositions may impinge directly on the loosely arranged hyphae of the outer medulla (Fig. 1) or on a more or less continuous cortical layer comprised of conglutinate hyphae in a matrix of hydrophilic mucilaginous material. Because lichens are long-lived, and older tissues may be retained within the thallus, extended exposure to atmospheric contaminants may predispose lichens to incorporate elevated levels of atmospheric contaminants compared to higher plants, which may shed older tissues or deciduous parts. The hyphal network exposed in ecorticate lichens (Fig. 2) offers a particularly large surface area, as does a highly branched podetium (Fig. 3) or a multiple-lobed thallus of a foliose lichen. When lichens are immersed in a solution of metal ions under controlled conditions uptake is nearly complete within one hour (Nieboer et al. 1976). Release is thought to occur much more slowly (Walther et al. 1990), but the question of release rates has not been fully resolved (Richardson 1995). However, it is apparent that concentrations of some metals decrease markedly within a relatively short period of time, lead within a few months (Deruelle 1984) and several other metals within two or three years (Walther et al. 1990). Few investigations, however, have addressed the rate of decline in heavy metal content under controlled conditions, especially in relation to water or aqueous solutions. Nevertheless, lichens are naturally exposed to moisture in many forms. In terms of the sites where metals reside within lichen thalli, information is also fragmentary, although many investigators have used ion-exchange techniques to assess whether ions are bound or free (e.g., Boileau et al. 1985a, b; Brown & Beckett 1984). Thallus dissection has been used to assess concentrations of heavy metals in rhizines, as opposed to the remaining parts of the thallus, by 0007-2745/97/368-376$1.05/0 This content downloaded from 207.46.13.124 on Wed, 22 Jun 2016 05:33:51 UTC All use subject to http://about.jstor.org/terms 1997] TREMBLEY ET AL.: URANIUM IN CLADINA 369