Element accumulation performance of living and dead lichens in a large-scale transplant application

Abstract

In bioaccumulation studies, sample devitalization through acid washing or oven drying is commonly applied to enhance the element accumulation efficiency of moss sample. Such aspect, however, has never been considered in biomonitoring surveys using lichens. In this study, the trace element accumulation performance of living (L) and dead (D) samples of the lichen Pseudevernia furfuracea was compared by a side-by-side transplanting at 40 sites in a large, mixed land use area of NE Italy for 8 weeks. Devitalization was achieved without any physico-chemical treatments, by storing lichen samples in a dark cool room for 18 months. Health status of lichens was assessed before and after the sample exposure by chlorophyll fluorescence emission. Although elemental analysis of the two exposed sample sets revealed a similar trace element pollution scenario, the content of 13 out of the 24 selected elements was higher in D samples. By expressing results as exposed-to-unexposed (EU) ratio, D samples show a higher bioaccumulation signal in 80% of transplant sites for Al, Ca, Fe, Hg, Pb and Ti. Overall, the health status of lichen samples might lead to interpretational discrepancies when EU ratio is classified according to the recently proposed bioaccumulation scale.

Keywords: Bioaccumulation; Devitalization; Interpretative scale; Lichen transplants; Pseudevernia furfuracea.

Fig. 1

Principal component analysis (PCA) based on EU ratio data of elements (a) and Pseudevernia furfuracea samples (b). In the principal component space of elements, sample sets and land use categories are represented as supplementary variables (black dotted arrows: L—living samples, D—dead samples; grey dotted arrows: I—industrial, U—urban, R—rural, N—natural landcover; Supplementary Methods S1.2, Supplementary Table S1)

Fig. 2

Cluster analysis (CA) of elements (a) and sites (b) with bar charts showing mean EU ratio values for different element groups (a) and site clusters (b) (error bars indicate 95% confidence intervals). Black dotted lines show the EU thresholds of bioaccumulation classes for 8-week transplants (Supplementary Table S4). Letters above bars indicate among-group/cluster significant differences (Kruskal-Wallis ANOVA and Dunn’s post hoc test). Elements shared by matching groups are underlined and reported in bold

Fig. 3

Boxplots of EU ratio data of living (L, grey) and dead (D, white) Pseudevernia furfuracea samples for 24 target elements. Data refer to median, first and third quartiles, and non-outlier ranges (outliers and extreme values are highlighted by circles and stars, respectively). Asterisks next to the element name indicate significant differences between the sample sets (Wilcoxon test; Table 1). Background is coloured according to the EU range of bioaccumulation classes (Supplementary Table S4)