Competition between heavy metal ions for binding sites in lichens: Implications for biomonitoring studies

The competitive behavior of divalent heavy metals (Cd, Cr, Pb, Zn) during cation uptake was investigated in the foliose lichen Xanthoria parietina. Lichen thalli were incubated with solutions containing 10 and 100 μM of CdCl_2, CuCl_2, and ZnCl₂ as well as 5 and 50 μM of Pb(NO₃)₂, tested individually and in combination (Cd²⁺+Cu²⁺+Pb²⁺+Zn²⁺). The analysis of molar concentrations suggests that a competition between cations for binding sites in X. parietina does exist. The decrease in net uptake between single and mixed solutions ranged between 14 and 29% at the lowest concentration and between 38 and 68% at the highest concentration. Furthermore, the uptake was proportionally lower for richer solutions. Each metal may behave differently when uptook: some (toxic elements) are preferentially stored at extracellular level (Cd, Pb), while others (micro-nutrients) are also present at intracellular level (Cu and Zn). The proportion between extracellular and total content changed for those elements accumulated also at intracellular level (Cu and Zn), while for Cd and Pb almost all the uptake occurred by passive mechanisms mainly at extracellular binding sites. The competition between metals for binding sites in the lichen surface entails that bioaccumulation data might result in an underestimation of some element levels measured in biomonitoring studies.