Biomonitoring atmospheric heavy metals with lichens: Theory and application

Recent records of environmental contamination noted a moderate decrease of SO₂ pollution, whereas the burden of atmospheric heavy metals is still considerable. The present review refers to the entrapment, uptake, and accumulation of heavy metals by lichen thalli, made apparent by parameters of lichen vitality and stress. The particulate nature of airborne heavy metals is made evident by parameters referring to the entrapment of heavy-metal containing particles by lichen thalli. The mechanism of uptake of heavy metals, investigated by means of controlled experiments, refers to extracellular and intracellular uptake. The rate of absorption and the accumulation of heavy metals is dependent on morphological features of lichen thalli in addition to kind and intensity of emission sources and to nonanthropogenic factors such as climate and topography. The role of lichens as biomonitors is demonstrated by the case of lead. In contrast to data obtained by retrospective studies, using lichens as biomonitors of heavy metal pollution, performed in the 1970s, which indicated an increase of Pb as a result of the massive use of leaded gasoline, the subsequent disuse of this additive led to a decrease detected in later studies. The disparity of emission sources is illustrated by the case of Hg. Mercury is a product of anthropogenic activity in addition to its natural derivation. The dominance of the anthropogenic factors is made obvious by high levels of Hg recorded near chlor alkali plants and other industrial sites. The role of the substrate in the uptake and accumulation of heavy metals was investigated to detect its relative share. Airborne metals were, however, determinant factors in the metal content of lichen thalli. The interaction of contaminants and biomonitors has a definite physiological impact on the vitality of the biomonitors. Physiological processes of disintegration investigated in this context are degradation of cell membranes and chlorophyll, decrease of the quantum yield of photosynthesis, decrease of the photosynthetic rate, increase of stress-ethylene production, and severe ultrastructural change. Lichens exposed to heavy metal pollution exhibit changes of the spectral reflectance response, an increase of malondialdehyde (MDA), a decrease of ATP, and injury to enzymatic activities. A comparative analysis of data referring to the accumulation of airborne elements in lichens and of data referring to alterations in physiological parameters of lichen viability substantiates the validity of assessments of environmental quality.