Accumulation of Trace Metals in the Lichen Evernia prunastri Transplanted at Biomonitoring Sites in Central Italy

Abstract

Samples of the epiphytic lichen Evernia prunastri collected in an unpolluted area were transplanted to the urban environment of Teramo (central Italy). The influence of the transplant process on trace element content was found to be negligible and after only two months, concentrations of all elements (Cd, Cr, Cu, Pb, and Zn) were significantly higher than in control samples. It is suggested that motor traffic was the main source of atmospheric Cr, Cu, and Pb, while for Cd and Zn additional sources from phosphate fertilizers and pesticides used in the surrounding farmlands are hypothesized. The possible uptake and accumulation processes of these metals in relation to time of exposure are discussed. Lichens are known for their ability to accumulate airborne substances in concentrations far greater than those in the atmosphere (Nieboer & Richardson 1981). Accumulation at ion exchange sites and particulate trapping are two well documented mechanisms by which elevated levels of elements are achieved in lichens (Nieboer et al. 1978). Metal cations bind to extracellular anion exchange sites in the cell wall and on the plasma membrane surface (Brown & Beckett 1985). Because cell wall-bound elements are readily exchangeable, extracellular quantities and proportions reflect recent environmental inputs (Brown 1987). However, when elements occur in insoluble particulate matter, they cannot be exchanged through the cell wall and accumulate in the thallus in relation to the environmental availability of the particles and exposure time i.e., lichen age (Brown 1987). This last point was documented by Bargagli et al. (1987), who found that the central (older) part of the thallus of foliose lichens has higher concentrations of certain metals than the peripheral (younger) part. However, elements present in trapped particulates may be dissolved in water or solubilized by the lichen to some degree, and metal ions released by this process may have several fates e.g., become potentially capable of occupying cation bind sites in the cell wall or be taken up intracellularly (Kershaw 1985). According to Loppi et al. (1997), elements of limited metabolic significance (e.g., Al, Pb) have higher concentrations in the central parts, where they are presumably trapped in the medulla, whereas e ements essential for lichen metabolism (e.g., Co, Mo, Zn) have higher concentrations in the peripheral parts, but are easily displaced from one part of the thallus to another. Nevertheless, the different me abolic fates of the various metals suggest that lichens may selectively accumulate those elements that remain extracellular, whereas those elements that enter into the cell may be metabolized and eliminated or may lead to death of the lichen (Kershaw 1985). In the present paper, the results of a transplant experiment with Evernia prunastri are reported. The objective of the study is to investigate the uptake over time of Cd, Cr, Cu, Pb, and Zn after lichen samples were transplanted to an urban envi-