In this work, we tried to go deeper inside distribution and toxicity of cadmium (Cd) in the macrolichen Xanthoria parietina (L.) Th. Fr. Thalli of this species were treated with 0 (control), 4.5, 9, 18, or 36 muM Cd for 24 or 48 hours. Transmission electron microscopy, X-ray microanalysis, and electron energy loss spectroscopy were exploited to study distribution and ultrastructural effects of Cd in thalli; spectrophotometric techniques were utilized for measuring Cd effects on chlorophyll (Chl) content; light fluorescence microscopy was used to evaluate Chl autofluorescence. The highest Cd concentration caused ultrastructural alterations both in the mycobiont and in the photobiont, more severe in the latter, decreased total Chl content and progressively quenched Chl autofluorescence. Cell wall immobilization was observed in both bionts, and evidence pointing to a Cd-binding ability by the concentric bodies in the mycobiont was also obtained. Lower Cd concentrations led to slight or even no effects on thallus structures and on Chl content and autofluorescence. The results obtained suggest that: (1) among the two bionts, the algal partner appears to be more susceptible to Cd stress, probably because of the presence of delicate and sensitive components such as the chloroplast and photosynthetic pigments; (2) a concentration threshold exists for the occurrence of evident structural and functional damage in X. parietina thalli exposed to Cd.
Thalli of the lichen Xanthoria parietina (L.) Th. Fr. were soaked for either 24 or 48 h in a buffered medium in the presence of environmentally relevant concentrations (4.8 and 9.6 μM) of hexavalent chromium [Cr(VI)]. Treatment effects on the antioxidant status, differential distribution and fate of Cr(VI) among the mycobiont and the photobiont cells, and potential damage to cell ultrastructure in the two bionts, were evaluated. The adopted conditions of low Cr(VI) stress caused: (i) an increase in the level of ascorbic acid and a decrease in that of reduced glutathione, as well as a moderate increase in guaiacol peroxidase activity, only observed after treatment with 9.6 μM Cr(VI); (ii) no changes in malondialdehyde content; (iii) a remarkable Cr accumulation in the mycobiont cytosol and compartmentalisation in the mycobiont vacuoles;(iv) a modest apoplastic Cr immobilisation by the outer part of the cell walls, of both the mycobiont and the photobiont. The response of X. parietina to low concentrations of Cr(VI) appears to be a complex phenomenon, which might reflect maintenance of cellular homeostatic equilibria, rather than specific response pathways
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