Figure 1

Back-scattered electron image of Acarospora sampled near Zlatna smelter, Romania (Figure3b) showing extensive particulate matter (bright phases) on upper surface and within medulla. Possible metal uptake mechanisms and metal localization in Acarospora include: (1). Upper surface. Fixation of particles (mineral and combustion-derived) in dry and wet deposition, metal ion sorption via soluble phases, particulate entrapment in intercellular spaces. (2). Cortex. ca 30 μm thick. Particulates are not necessarily inert and may be solubilized by acid precipitation and/or lichen-derived organic acids leading to metal sorption to e.g. extracellular hydrophilic β-glucans secreted by mycobiont cortical cells with negatively charged anionic sites [e.g. (Sarret et al., 1998)]; extracellular oxalate and metal lichen acid-complex formation (Cu-Norstictic acid shown) (Purvis et al., 1987; Purvis et al., 1990; Takani et al., 2002); melanins are often present which may sorb metals (U, Cu and Fe) in melanized tissues (McLean et al., 1998; Purvis et al., 2004) (3). Photobiont Layer. Intracellular phytochelatin, thiol peptides containing metal-chelating sulphydryl groups of cysteine help protect photobionts from metal toxicity (Pawlik-Skowrońska et al., 2006). (4) . Medulla. Particles (mineral, combustion-derived and biogenic oxalates) trapped in intercellular spaces of fungal hyphae and/or attached to medullary hyphae coated with hydrophobic mycobiont-derived secondary metabolites which may further act as sites for metal complexation (Purvis et al., 1987; Purvis et al., 1990). (5). Lower surface. Rhizines occupy by far the bulk of the thallus in section and may extend to several millimetres, hyphae to several centimetres in the substrate. Particles and metals may also be removed from thalli by a variety of processes.