Accumulation of Trace Elements in the Peripheral and Central Parts of a Foliose Lichen Thallus

Abstract

Total concentrations ofAl, As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Mo, Ni, Pb, Sb, and Zn were compared in the peripheral (younger) and central (older) part of the thallus of the epiphytic lichen Parmelia caperata from a background area of central Italy. The results showed that the trace element content was highly variable. Concentrations were similar for several elements (As, Cr, Fe, Hg, Mn, Ni, Sb). Elements of limited metabolic significance (Al, Cd, Pb) had higher concentrations in the central parts, suggesting that they are trapped in the medulla, while elements essential for lichen metabolism (Co,. Cu, Mo, Zn) had higher concentrations in the peripheral parts, suggesting that they are easily displaced from one part of the thallus to another. Lichens are perennial, slow-growing organisms, that maintain a fairly uniform morphology in time, are highly dependent on the atmosphere for nutrients, and do not shed plant parts as readily as vascular plants. The lack of a waxy cuticle and stomata allows many contaminants to be absorbed over the whole lichen surface (Hale 1983). Contaminants are deposited on lichens by free-falling precipitation, occult precipitation such as fog and dew, dry sedimentation, and gaseous absorption (Knops et al. 1991). Occult precipitation occurs under stable atmospheric conditions and contains concentrations of nutrients and contaminants orders of magnitude higher than normal precipitation (Nash & Gries 1995). Lichens accumulate and retain many trace elements to concentrations that vastly exceed their physiological requirements, and tolerate these high concentrations by sequestering elements extracellularly as oxalate crystals or lichen acid complexes (Nieboer et al. 1978). Since the concentrations of trace elements in lichen thalli may be directly correlated with the environmental levels of these elements (Andersen et al. 1978; Herzig et al. 1989), lichens are useful for monitoring spatial and/or temporal deposition patterns of trace elements. When using lichens as monitors of metal deposition, samples of approximately the same age should be collected and analyzed in order to have comparable data. In polluted areas, the central part of the thallus of foliose lichens may have higher concentrations of certain metals than the peripheral parts, owing to the older age and longer exposure time (Bargagli et al. 1987a; Hale & Lawrey 1985; Schutte 1977; Schwartzman et al. 1987). However, this zonation pattern is not always evident, and several elements seem to be rather mobile within the thallus (Goyal & Seaward 1982; Pakarinen 1985; Seaward 1980). The aim of the present study is to compare the trace element content of the peripheral and central parts of lichen thalli in an unpolluted area. MATERIALS AND METHODS In September 1993, 5-10 whole thalli (about 5-10 cm in diameter) of the foliose lichen Parmelia caperata were collected in 10 sampling sites in a remote area of TuIscany (central Italy). This lichen species was chosen because of its wide distribution in the study area and most of Italy, and because it is commonly used in biomonitoring surveys. All samples were collected at a height of 1.5-2.0 m above ground level, from the trunks of isolated oak trees (Quercus cerris and Q. pubescens) having a circumference of at least 70 cm. Trace element concentrations in lichens were previously demonstrated to be similar in the two species of oak (Loppi et al. 1996). In the laboratory, lichen samples were air-dried and sorted to remove dead or senescent tissue and as much extraneous material (adhering bark, mosses, other lichen species, soil particles etc.) as possible. The outermost 34 mm of the thallus (distinguishable by color and lack of rhizinae) was detached and analyzed separately from the remaining central part bearing rhizinae. The samples were then powdered and homogenized and about 150 mg of material was mineralized in a pressurized digestion system with concentrated HNO3 for 8 hr at 1200C. Trace elements were determined by atomic absorption spectrophotometry (Perkin-Elmer 4100 ZL), using an oxyacetylene flame for Cr, Cu, Fe, Mn, and Zn; a graphite furnace for Al, As, Cd, Co, Mo, Ni, Pb, and Sb; and the cold vapor technique for Hg. Trace element concentrations were expressed on a dry weight basis. Precision of analysis was estimated by the coefficient of variation of four replicates and was found to be less than 10% for all elements. To check the accuracy of digestion and analytical procedures, the NBS Standard Reference Material 1572 'Citrus Leaves' was re-analyzed after every ten samples. Prior to statistical procedures, trace element concentrations were transformed to logarithms to correct for skewed distributions (Bailey 1981). Significance of differences between means was tested by one-way analysis of variance (ANOVA) using Scheffe's test, and correlations between element pairs by Pearson's product-moment coefficient. 0007-2745/97/251-253$0.45/0 This content downloaded from 207.46.13.124 on Wed, 22 Jun 2016 04:58:33 UTC All use subject to http://about.jstor.org/terms 252 THE BRYOLOGIST [VOL. 100 TABLE 1. Trace element concentrations (mean ? standard deviation, xg g-1 dry weight, and CV% in parentheses) in the peripheral and central parts of Parmelia caperata thalli from a background area of central Italy (n = 10). * = differences statistically significant at p < 0.05. Peripheral part Central part Al* 890 ? 430 (48.3) 1976 ? 1416 (71.7) As 1.009 ? 0.554 (54.9) 1.108 ? 0.494 (44.6) Cd* 0.298 _ 0.154 (51.7) 0.404 ? 0.222 (54.9) Co* 1.184 ? 0.279 (23.6) 0.834 ? 0.227 (27.2) Cr 4.76 ? 2.25 (47.3) 4.89 _ 4.34 (88.7) u* 9.2 ? 2.4 (26.1) 8.4 ? 3.6 (42.9) Fe 938 ? 404(43.1) 1148 ? 728 (63.4) Hg 0.170 ? 0.107 (62.9) 0.172 ? 0.051 (29.7) Mn 38.5 ? 21.4 (55.6) 42.3 _ 40.0 (94.6) o* 1.396 ? 0.765 (54.8) 0.947 ? 0.416 (43.9) Ni 4.78 _ 1.67 (34.9) 3.37 _ 1.64 (48.7) Pb* 5.43 ? 2.54 (46.8) 15.76 ? 11.54 (73.2) Sb 0.454 ? 0.210 (46.3) 0.550 ? 0.111 (20.2) Zn* 38.2 ? 10.0(26.2) 31.3 _ 8.8 (28.1)