New insight on element bioaccumulation performance of two lichen biomonitors: When morpho-chemical details mark the difference

In biomonitoring of airborne elements with native lichens, two allopatric species could be used jointly only when bioaccumulation mechanisms are the same, and differences negligible. The present study aims at (i) investigating the bioaccumulation performance for 12 elements in one-year-old lobes of Flavoparmelia caperata and Xanthoria parietina (i.e. the two lichen native biomonitors most used in the peri-Mediterranean regions) collected side-by-side on the same trees at 17 sampling sites. Both species were characterized morphologically and by X-ray diffraction analysis to evaluate specific thallus area and oxalate content, respectively. Results show that Ca and chalcophile elements (Cd, Cu, Pb and Zn) were more concentrated in F. caperata, lithophile and siderophile elements (Cr, Fe, and Ti) in X. parietina, whereas Al, Li, Mn, and Ni were accumulated in similar amounts. The outstanding content of Ca-oxalate deposits in F. caperata underlies the higher content of Ca, Cd and Pb observed in this species. On the contrary, the micro-anatomy of the upper cortex of X. parietina, which is more porous than in F. caperata, allows a higher transfer of litophile and siderophile elements. The adoption of interspecific calibration factors for biomonitoring purposes is thoroughly discussed, and the choice of using a single native species made by the most recent guidelines for biomonitoring airborne elements with lichens is fully supported.