5 Lichens as bioindicators of air pollution

Lichens are considered the result of a symbiotic association of a fungus and an alga. More precisely the term “alga” indicates either a Cyanobacteriae or a Chlorophyceae; the fungus is usually an Ascomycetes, although on rare occasions it may be either a Basidiomycetes or a Phycomycetes. In this association, the alga is the part that is occupied with the formation of nutrients, since it contains chlorophyll (Chl), while the fungus supplies the alga with water and minerals. These organisms are perennial and maintain a uniform morphology over time. They grow slowly, have a large-scale dependence upon the environment for their nutrition, and – differently from vascular plants – they do not shed parts during growth. Furthermore, their lack of cuticle or stoma means that the different contaminants are absorbed over the entire surface of the organism [1–3]. As far back as 1866, a study was published on epiphytic lichens used as bioindicators [4]. Lichens are the most studied bioindicators of air quality [5]. They have been defined as “permanent control systems” for air pollution assessment [6]. During the last 30 years, many studies have stressed the possibility of using lichens as biomonitors of air quality in view of their sensitivity to various environmental factors, which can provoke changes in some of their components and/or specific parameters [3, 7–30]. For indeed, many physiological parameters are used to evaluate environmental damage to lichens, such as: photosynthesis [31–33]; chlorophyll content and degradation [34–37]; decrease of ATP; variations in respiration levels [34]; changes in the level of endogenous auxins; and ethylene production [38–40]. Furthermore, laboratory exposure to SO2 causes relevant membrane damage to lichen cells [41]. Many studies show a positive correlation between the sulphur content of lichens and SO2 present in the atmosphere [42–45]. Various authors report that the concentration of Chl a + b is altered by vehicle traffic pollution [46–48], and by urban emissions [49]. In general, lichens that are transplanted into areas with intense vehicle traffic show an increase in Chl a + b concentration that is proportional to increases in emissions. Such effects are WITPress_BMTA_ch005.qxd 12/14/2007 3:14 PM Page 111