Responses of antioxidants in the lichen Ramalina lacera may serve as an early-warning bioindicator system for the detection of air pollution stress

The aim of this study was to identify, in the lichen Ramalina lacera, antioxidants that could provide indications of air pollution stress, and respond earlier than traditionally used structural/physiological parameters. The pollution-sensitive lichen R. lacera was transplanted from its relatively unpolluted natural habitat to two air-polluted sites for a period of up to 6 months. The superoxide dismutase and catalase activities, total water- and lipid-soluble low-molecular-weight antioxidant capacities and chlorophyll b/chlorophyll a ratios were assessed every 6 weeks. The earliest signs of oxidative stress were detected in the activities of fungal copper/zinc-superoxide dismutase, algal iron-superoxide dismutase and water-soluble low-molecular-weight antioxidants, which increased significantly as early as 42 days after exposure to pollution. Catalase activity increased in lichens transplanted to the polluted sites after 90 days. All activities decreased towards the end of the experiment. The impact of air pollution on R. lacera, using the traditionally employed parameter of chlorophyll b/chlorophyll a ratio, was only detected after 6 months of exposure to air pollution. Our results indicate that antioxidant parameters may serve as improved early-warning indicators of air pollution stress in lichens.