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 melanised. The adaptive value of this character is still uncertain. Melanins are ubiquitous compounds found in most organisms since they fulfil several biological functions (defence against UV radiation, oxidising agents, and microbial stress, metal complexation, etc.). This work aims to verify whether melanisation 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 melanised and non-melanised 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 melanised pseudotissues, a sequential elution experiment was performed on two species: the two-side heavily melanised Melanelixia glabratula and the one-side lightly melanised Punctelia subrudecta. The content of Fe and Zn was higher in the melanised species than in the non-melanised ones. Species deprived of their melanised lower cortex showed a sharp decrease in Fe but not in Zn, suggesting that the melanised lower cortex is involved in Fe complexation, whereas Zn is homogeneously distributed throughout the thallus.