Melanization Affects the Content of Selected Elements in Parmelioid Lichens.

Lichens belonging to Parmeliaceae are highly diversified, but most of them share an extremely conserved morpho-chemical trait: the lower cortex is heavily melanized. The adaptive value of this character is still uncertain. Melanins are ubiquitous compounds found in most organisms since they fulfil several biological functions including defense against UV radiation, oxidizing agents, microbial stress, and metal complexation. This work aims to establish whether melanization can affect the elemental content of lichen thalli. The relative abundance of macro- (Ca, K and S) and micro- (Fe, Mn and Zn) nutrients in melanized and non-melanized pseudotissues of nine species was first evaluated by a non-destructive micro-X-ray fluorescence elemental analysis on either the upper and lower cortex, and on the internal medulla, which was artificially exposed to the mechanical removal of the lower cortex. Afterwards, the total concentration of the same elements was measured in composite samples by inductively coupled plasma atomic emission spectroscopy after acidic digestion. In order to verify whether Fe and Zn are chemically bound to the melanized pseudotissues, a sequential elution experiment was performed on two species: the two-side heavily melanized Melanelixia glabratula and the one-side lightly melanized Punctelia subrudecta. The content of Fe and Zn was higher in the melanized species than in the non-melanized ones. Species deprived of their melanized lower cortex showed a sharp decrease in Fe but not in Zn, suggesting that the melanized lower cortex is involved in Fe complexation, whereas Zn is homogeneously distributed throughout the thallus.