Lichens as bioindicators of sulfur dioxide

Lichens are excellent indicators of sulfur dioxide levels in the lower trophosphere. With the advent of the industrial revolution, sulfur dioxide levels increased steadily throughout western Europe until legislation in the 1960's and 1970's resulted in reductions in recent decades. Parallel with these developments was a major impoverishment of the region's lichen flora, particularly in city centers and industrial regions. But as conditions ameliorated, reinvasion of many lichen species has occurred. Concomitantly, Lecanora conizaeoides, a species with an apparent major sulfur requirement, has declined in abundance in recent decades. Similar trends have been observed along SO2 gradients in other parts of the world. Although field situations can be complex with a variety of factors, including other air pollutants, influencing lichen community dynamics, a wide range of experimental investigations support the assertion that lichens do respond to sulfur dioxide. When moist, lichens can be a major sink for SO2 because of the high solubility Of SO2 in water. Approximately 70% of the SO2 absorbed by lichens can subsequently be leached as sulfate, but the retained 30%, present primarily as bisulfite and/or sulfite, can readily become toxic when accumulated to sufficient levels. Under chronic exposures, a variety of physiological parameters (photosynthesis, respiration, nitrogen fixation, retention of electrolytes, etc.) may decline and, under acute exposures, death may result. Retained sulfur can be reduced metabolically to H2S and released, but the magnitude of H2S release is relatively small. Most biochemical investigations emphasize the effects Of SO2 in terms of either acidification or oxidation potential due to the formation of free radicals. Observed differential sensitivity to SO2 among different lichen species can be partly understood in relation to the parallel, differential magnitude of antioxidants, such as glutathione or ascorbate, occurring in those lichens. Furthermore, differential activity of antioxidant enzymes, such as superoxide dismutase and peroxidases, in removing radicals apparently is also important in explaining the parallel differential sensitivity among lichen species.