Since the last 25 years, lichens have been increasingly used as biomarkers of the acidic and metallic atmospheric pollution. Today, the value of lichens as biological monitors around metal-emitting industrial areas is widely recognized (Richardson, 1995). Lichens exhibit several characteristics which meet some requirements of ideal biological monitors: a widespread environmental occurence, a keen ability to uptake metals far beyond their physiological needs and without apparent toxicity, a slow growth, round year physiologic activity, longevity and roughness, a direct sensitivity to atmospheric pollutants due to the absence of roots and cuticle, and a limited retention time of metals in the thallus (Walther et al., 1990). For example, the concentration of lead in lichen growing near the A6 french motorway was found to have decreased from 1980 to 1992 due to the increasing use of unleaded gasoline (Deruelle, 1995).
The absorbing capacity of lichens for different heavy metals has been widely studied, but little is known on the localisation and retention mechanisms of metals in the thallus. Laboratory studies of metal sorption were generally restricted to metal accumulation from metal aqueous solutions, which differs from airborne originating metal conditions. The presence of metal-rich particulates in the medulla of naturally contaminated lichens was recognized by microscopy and X-ray diffraction. Eventually, three different uptake mechanisms are supposed to exist (Richardson, 1995): (1) entrapment of metal-rich particulates originating from atmospheric aerosols (2) extracellular ion exchange within cell walls, (3) intracellular uptake involving active transmembrane transport. Purvis (1984) pointed out the role of oxalic acid, an extracellular exudate which would keep lichen from toxicity by external immobilisation of metals. Later, she demonstrated the role of lichenic acids (substances exclusively produced by lichens and well known for their strong chelating properties) in metal complexation (Purvis, 1987).
There is a clear need to get better insights into these mechanisms in order to use lichens as reliable tools for pollution diagnosis and monitoring. The stakes are considerable for metal pollution assessment as lichens could act as an alternative to heavy and costly instrumentation. Determining metal speciation in lichens is a difficult task because
(i) analysis should avoid any perturbation of the biological system integrity, and (ii) metals are generally diluted. These difficulties are overcome by EXAFS spectroscopy as it allows to work on non-disturbed vegetals with a detection limit for trace elements presently comprised between 100-500ppm.
Three lichens contaminated by different sources of pollutions were studied by this method: Xanthoria Parietina contaminated by atmospheric rejections from a tetraethyl and tetramethyl lead factory, and two terricolous lichens, Cladonia Subulata and Diploschistes Muscorum collected on Zn tailings near a Zn extraction plant (soil and air pollution). EXAFS results showed that in Xanthoria Pb is retained as silicate (PbSiO3) and organic lead (Pb complexed with carboxyl groups). In terricolous lichens, Zn was found to be complexed by carboxylic organic ligands in Cladonia, and specifically by oxalate in Diploschistes. The amount of oxalic acid determined by enzymatic analysis can explain the complexation of one part of total Zn. Moreover, we demonstrated that Diploschistes's lichenic acids are able to complex Zn. We can assume that lead silicate particulates have an atmospheric origin. Carboxylic organic complexes could be explained by metal complexation with lichenic acids or with macromolecules of the cell wall, and Zn oxalate could result from complexation with oxalic acid exudate. Further RMN investigations are underway to find out the role of lichenic acids in metal complexation, and comparative studies are carried out to settle if oxalic acid naturally occurs in Diploschistes or if it is induced by Zn pollution stress.
Deruelle, S., Intern. Symposium on biological markers of pollution, ANPP Annales, 271-280 (1995).
Purvis, O.W., Elix, J.A., Broomhead, J.A. & Jones, G.C., Lichenologist 19, 193-203 (1987).
Purvis, O.W., Lichenologist 16, 197-204 (1984).
Richardson, D.H., Symbiosis 18, 119-127 (1995).
Walther, D.A., Ramelow, G.J., Beck, J.N., Young, J.C., Callahan, J.D., & Marcon, M. F., Water, air & soil pollution 53, 189-200 (1990).