Characteristic Features of Lichens Utilized in Biomonitoring Studies

Lichens has been recognized and successfully utilized as biological indicators of air quality. They are among the most valuable and reliable biomonitors of atmospheric pollution. Primarily lichens were utilized to monitor gaseous pollution, namely sulphur (SO.), nitrogen (NO., NH3, NO3. etc.) (Rao and LeBlanc 1967; Vestergaard et al. 1986) . Lichens show high sensitivity towards sulphur dioxide because their efficient absorption systems result in rapid accumulation of sulphur when exposed to high levels of sulphur dioxide pollution (Wadleigh and Blake 1999). The algal partner (5-10% of total thallus structure) is most affected by the sulphur dioxide as chlorophyll is irreversibly converted to phaeophytin and thus photosynthesis is inhibited (Upreti 1994) . Lichens also absorb sulphur dioxide dissolved in water (Hawksworth and Rose 1970) . Excessive levels of pollutants in the atmosphere, especially SO2, has detrimental effect on the physiology and morphology of sensitive species, causes extinction of the species, which ultimately results in changed lichen diversity pattern (Haffner et al. 2001; Purvis 2000).

Fig. 7.2 Growth forms oflichen, their sensitivity and biomonitoring potential

Change in the community structure of epiphytic lichens is a sensitive indicator of the biological effects of air pollutants (Richardson 1991; Pinho et al. 2004; Paoli and Loppi 2008). Rise in the number of nitrophillous lichens especially members of the Physciaceae family (including Phaeophyscia and Pyxine) has been attributed to climate change and also to dry and dusty conditions in urban centers (van Herk et al. 2002; Loppi and Pirintos 2000). In addition to lichen diversity changes, lichens are being widely used for monitoring accumulation of heavy metals (Loppi et al. 1998; Garty et al. 2003; Shukla and Upreti 2007a).

Lichens are employed for air quality assessment in three ways viz., by identifying and mapping all lichen species in an area (LDV and IAP studies), transplanting healthy lichens into a polluted area and measuring thallus deterioration and sampling an individual species and measuring contaminants accumulated within the thallus (Pfeiffer and Barclay-Estrup 1992; Garty et al. 2003; Loppi and Frati 2006; Pinho et al. 2004; Shukla and Upreti 2007a; Shukla et al. 2010b).

Mechanisms involved to accumulate substances in lichens from their ambient atmosphere include, particulate trapping, ion exchange, extra cellular absorption, hydrolysis, and intracellular uptake (Nieboer et al. 1978).

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