The localization of inorganic contaminants in lichens is an important step toward a better understanding of the accumulating capability and toxitolerance of species (Hauck et al. 2002). Different methods, such as sequential elution technique (Brown 1995; Tretiach et al. 1999), X-ray microanalysis (Hauck et al. 2002) and microscopic histochemistry (Garty & Theiss 1989), have been used.

On the thalline lobes of Pyxine subcinerea there are delimited parches of pruina consisting of bipyramidal prisms, bipyramids and druses of weddellite, i.e. calcium oxalate dihydrate (COD), as revealed by X-ray analysis and SEM observations. The disappearance of the crystals from older thalline areas is preceded by an increase in the frequency of corrosion figures, in the form of etch square pits, and a decrease in the number of crystals occurring on the thalline surface. These features follow a dissolution gradient from the lobe apex rewards the proximal edges of the patches. The corrosion figures are linked to the thermodynamic metastability of GOD. Partial dissolution of the crystals, performed in water and in EDTA, has confirmed this latter assumption. Ir is thought that the disappearance of pruina, which interferes with the transmission of light to the algal layer, may allow the formation of soredia in mature thalline portions.(C) 2001 The British Lichen Society.