Metals and secondary metabolites in saxicolous lichen communities on ultramafic and non-ultramafic rocks of the Western Italian Alps

There is a long history of studies on lichens found in ultramafic habitats, but comparisons between lichen communities on different ultramafic lithologies are scant, and potential metabolic adaptations to the multiple edaphic stresses of ultramafic substrates have been widely neglected. The present work is the first to characterise differences in the abundance and structure of saxicolous lichen communities on different ultramafic lithologies (dunite, lherzolite, and serpentinite), analysed in two areas of the Western Alps (NW Italy). Differences between communities on various ultramafic lithologies were observed, including differences between a mafic control (Mg-Al metagabbro); however, factors other than the substrate were observed to drive more remarkable differences between lichen communities on ultramafics of alpine and pre-alpine areas. XRF analyses demonstrated that the mineral composition of different lithologies is reflected by metal contents in crustose lichens, with weathering processes accounting for relative shifts in elemental abundances between rocks and thalli. A thin layer cromatography screening of lichen secondary metabolites (LSMs), which are thought to regulate metal and pH homeostasis in thalli, revealed lithological vicariance among dominant lichen species with different LSM patterns and intraspecific variability in LSM production associated with differences in lithology and location. In particular, the presence or absence of norstictic acid in species or lineages/individuals on the different lithologies, in relationship to concentrations of Fe, Mg, and Ni in lichen thalli, was recognised as a metabolic adaptation to metal stress. Pull-up tests revealed that physical factors such as a differential surface disaggregation may contribute more towards differences observed in lichen abundance on the different lithologies investigated.