Chapter 11 Biomonitors in use: lichens and metal air pollution

Lichens are reviewed for their potential to reflect metal air pollution. They are discussed both as being symbiotic plants and as biomonitor organisms. The attention is focused on their biological performance, the dynamics of metal uptake and release, on particle interception and on (physiological) processes underlying metal accumulation within the thallus. Laboratory-linked detailed physiological assessments of lichen behaviour are recognized as are the more pragmatic approaches often adopted in field work. Lichen morphology and physiology are regarded as of similar relevance in overall metal accumulation. A simplified approach is discussed in which the lichen is seen as essentially "homogeneous", in the sense that no symbiont-dependent differentiations are regarded, but where the lichen's morphological characteristics are seen as of importance in initial interception of airborne particles. Modelled results from this approach relative to field survey assessments indicate the potential of this view to a lichen. Discussed literature data on lichen physiology indicate that, in surveys, apart from lichen metal determinations, lichen behaviour may be critically viewed if not "normalized" by simultaneous analysis of selected lichen physiological parameters: apart from the assessment of the lichen's surface-to-mass ratio, a number of possible analyses are presented, among which the determination of leaching through damaged cell membranes, stress-ethylene, the rate of photosynthesis, the potential quantum yield of photosystem II, the lichen's spectral reflectance, or the lichen's chlorophyll content.