Chlorophyll a, chlorophyll b and chlorophyll a+b concentrations, as well as chlorophyll a/b ratios and assessment of chlorophyll a integrity (OD 435/OD 415, OD 665/OD 665 after acidification) were determined in order to compare the effects of copper, cobalt and mercury on five populations of the lichen Cetraria islandica and one population of the lichen Flavocetraria cucullata. Treatment with copper decreased chlorophyll a concentration. The effect of cobalt on chlorophyll a concentrations was not clear. Of all metals compared, mercury caused the greatest reduction in chlorophyll a concentration in both species. Copper caused statistically significant concentration dependent increase in chlorophyll b concentration. Mercury exposure consistently decreased chlorophyll b concentrations. In general, chlorophyll a+b content was not affected so strongly in our short-term experiments. Copper caused the highest reduction of chlorophyll a/b ratios in all lichen samples tested. Of all applied metals, mercury had the greatest effect on chlorophyll a integrity (measured as OD 435/OD 415). The ratio OD 665/OD 665 after acidification was sensitive to the presence of metals and decreased in response to mercury as well as cobalt. It was interesting that copper did not cause a significant decrease in OD 665/OD 665* in lichens with a history of copper exposure. In lichens without a copper history, experimentally introduced copper caused a significant decrease in the ratio OD 665/OD 665* at 10 mM, highest applied concentration.

  The concentration of chlorophyll a in Cetraria islandica was strongly dependent on age and physiological activity of different parts of individual thalli. The younger, apical and lateral peripheral parts, which are considered metabolically more active, had a significantly higher amount of chlorophyll a (about 2.3-6 fold) than older basal parts of the thallus.