Cellular mechanisms of Cu-tolerance in the epilithic lichen Lecanora polytropa growing at a copper mine

Abstract

Cellular responses to copper stress were investigated for the first time in a saxicolous lichen species, Lecanora polytropa (Hoffm.) Rabenh. Bright blue-green apothecia accumulated up to 1·3% Cu on a dry weight basis (205 μ mol Cu g⁻¹), c. 50% in an exchangeable form. A bright turquoise-blue layer extended beneath the hymenium into the medulla, above and between a dentate photobiont layer. Oxalic (1·88 μ mol g⁻¹), citric (0·83 μ mol g⁻¹) and lower concentrations of malic (0·45 μ mol g⁻¹) acids were determined by GC/MS analysis. Short-term exposure to high Cu²⁺ concentrations (40 and 400 μ mol g⁻¹) under non-complexing conditions caused a dose-dependent decrease in chlorophyll a content; chlorophyll b and total carotenoid contents remained constant. The phaeophytinization quotient remained unchanged during Cu²⁺ exposure. Analysis of thiol peptides confirmed glutathione was reduced (GSH) in native L. polytropa (0·538 μ mol g⁻¹), and phytochelatins (PC₂ and PC₃) oxidised. Short-term exposure to 40 μ mol g⁻¹ Cu²⁺ oxidised c. 28% of the glutathione pool; oxidised phytochelatin concentrations remained unchanged. This is the first report of phytochelatin production and thiol peptide status in a crustose lichen. These represent two possible detoxification mechanisms in this Cu-tolerant species. Copper complexation by low molecular mass organic acids and non-protein thiols do not entirely account for its tolerance.