Impact of wet deposited nickel on the cation content of a mat-forming lichen Cladina stellaris.

The impact of experimentally sprayed aqueous nickel solution on the concentrations of potassium, calcium, magnesium and nickel in three horizontal strata (top, 0-20 mm; middle, 20-40 mm; and base, 40-60 mm) of the cushion-forming lichen Cladina stellaris was investigated. The experimental nickel deposition range used corresponded with that from the pristine forests of the Finnish border to polluted industrial sites of Russian Kola Peninsula (0-1000 mg Ni(2+) m(-2) year(-1)). The lichen mat retained ca. 31-66% of the nickel deposited during two growing seasons and the relative retention efficiency was highest at the low deposition end. The concentrations of cations in lichen thalli were significantly reduced only after the highest nickel deposition. Furthermore, the separate horizontal strata responded differently to nickel exposure indicating that the cation exchange sites of the top stratum were not completely saturated by nickel even after the most severe treatment. However, nickel deposited in high doses caused considerable reduction in potassium concentration indicating damage to cell membranes. Episodically deposited high concentrations of nickel can probably affect membrane integrity before detectable changes in total concentrations of cations in the lichen thallus take place. Thus, ratios of total concentrations of cations in the lichen thallus are fairly insensitive to nickel deposition, which reduces the risk of compounding effects when the ratios are used to indicate long-term acid deposition in areas with multiple pollution problems such as Kola Peninsula.