Differential sensitivity of four Lobaria lichens to copper in vitro

Abstract

Treatment of Lobaria amplissima, Lobaria pulmonaria, Lobaria scrobiculata, and Lobaria virens with 0 to 500 microM CuSO4 for 0 to 120 min at 25.0 degrees C resulted in a time- and copper-concentration-dependent decrease in the thallus total potassium content and a release of K- from the thallus, indicating that copper damaged the cytoplasmic membrane of the fungal hyphae. Lobaria pulmonaria was the most sensitive species, L virens the most resistant to copper treatment. Lobaria amplissima and L. scrobiculata displayed intermediate sensitivity. Treatment with copper also resulted in a time-dependent increase in the thallus copper content. For all four Lobaria species, the thallus potassium content decreased linearly with increasing thallus copper content, suggesting that, as cupric ions bound to the thallus, they reach target sites in the membrane. Acetone extracts contained low concentrations of chlorophyll a, amino acids, polyols, and sugars, but detectable or considerable amounts of lichen substances, i.e.. constictic, norstictic, stictic, and usnic acids and scrobiculin. Titration of the extracts with copper and assay of free Cu2+ revealed the presence of copper-binding ligands, most probably corresponding to the lichen substances detected in the extracts. Acetone extracts of L. amplissima and L. virens bound small or very small amounts of Cu2+; binding was monophasic and probably involved scrobiculin. Acetone extracts of L. pulmonaria and L. scrobiculata bound high or intermediate amounts of copper; binding was polyphasic and probably involved the lichen acids detected in the extracts. It was concluded that, in the studied Lobaria species, acetone extracts containing lichen substances could bind, in vitro, considerable amounts of Cu2+, but this binding did not prevent membrane damage in the thallus. Several hypotheses are presented to explain these results.