Effects of Lichens on Uranium Migration
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Effects of Lichens on Uranium Migration Takeshi Kasama1, Takashi Murakami1, Toshihiko Ohnuki2 and O. William Purvis3 1
Department of Earth and Planetary Science, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan 2 Department of Environmental Science, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1106, Japan 3 Department of Botany, The Natural History Museum, Cromwell Road, London SW7 5BD, England ABSTRACT The lichen Trapelia involuta from uraniferous spoil heaps in Cornwall, England, growing directly on the secondary uranium minerals, metazeunerite and metatorbernite, was examined by field-emission scanning electron microscopy and transmission electron microscopy to assess the effect of Trapelia on uranium migration. We observed metazeunerite, sericite and scorodite as well as unidentified Fe-, Pb/As-, Fe/As-, Al/P-, Pb-bearing minerals concentrated in the lichen exciple and medulla. In addition, metazeunerite also occurred in the epithecium. The chemistries, sizes, and occurrences of the above minerals in the lichen suggest that fixation of U as well as Pb, As, Fe, and Al is dependent on lichen physiological processes. We suggest Trapelia accumulates these elements from groundwater and precipitates the above minerals within specific tissues. Our results indicate that some lichens retard uranium migration by accumulating uranium from groundwater and forming uranium-bearing minerals within their tissues.
INTRODUCTION The ability of microorganisms such as bacteria, algae and fungi to accumulate uranium and other poisonous metals from solution has been long recognized [1,2,3]. Thus microorganisms have been previously used for the economic recovery of metals from natural and industrial waste waters [4]. Such microbiological activity can also have an influence on radionuclide migration [5,6]. Fungi can exist either by themselves or by forming intimate associations termed ‘symbioses’ with green algae or cyanobacteria (ie as lichens) or else associated with the roots of vascular plants (as ‘mycorrhizae’). The lichen body or ‘thallus’ is often organized as layered tissues comprising an outer, protective cortex and internal medulla usually consisting of long-celled, loosely interwoven hyphae forming a cottony layer with a very high internal air space [7]. In many crustose lichens, as in Trapelia, the medulla may be in direct contact with the mineral (or soil) surface. Reproductive bodies ‘ascomata’ typically include an outer, protective wall, ‘exciple’ consisting of melanised, conglutinate hyphae and the upper, often pigmented part, or ‘epithecium’ A layer of algae or cyanobacteria is typically sandwiched between the medulla and cortex and may extend into the walls of the ascomata. The schematic diagram of a lichen ascoma was shown in Figure 1. Lichens are highly diverse and successful organisms, occurring in all major ecosystems including extreme environments. Lichen-dominated vegetation covers approximately 8% of the earth surface, giving them a globally important role in plant ecology, and C, N, and
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