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 cons iderable. The present review refers to the entrapment, uptake, and accumula tion of heavy metals by lichen thalli, made apparent by parameters of liche n vitality and stress. The particulate nature of airborne heavy metals is m ade evident by parameters referring to the entrapment of heavy-metal contai ning particles by lichen thalli. The mechanism of uptake of heavy metals, i nvestigated 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 demon strated by the case of lead. In contrast to data obtained by retrospective studies, using lichens as biomonitors of heavy metal pollution, performed i n the 1970s, which indicated an increase of Pb as a result of the massive u se of leaded gasoline, the subsequent disuse of this additive led to a decr ease detected in later studies. The disparity of emission sources is illust rated by the case of Hg. Mercury is a product of anthropogenic activity in addition to its natural derivation. The dominance of the anthropogenic fact ors 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 acc umulation of heavy metals was investigated to detect its relative share. Ai rborne metals were, however, determinant factors in the metal content of li chen thalli. The interaction of contaminants and biomonitors has a definite physiological impact on the vitality of the biomonitors. Physiological pro cesses of disintegration investigated in this context are degradation of ce ll membranes and chlorophyll, decrease of the quantum yield of photosynthes is, decrease of the photosynthetic rate, increase of stress-ethylene produc tion, and severe ultrastructural change. Lichens exposed to heavy metal pol lution exhibit changes of the spectral reflectance response, an increase of malondialdehyde (MDA), a decrease of ATP, and injury to enzymatic activiti es. A comparative analysis of data referring to the accumulation of airborn e elements in lichens and of data referring to alterations in physiological parameters of lichen viability substantiates the validity of assessments o f environmental quality.