Role of lichens in weathering of granodiorite in the Sila uplands (Calabria, southern Italy)

This paper explores the role of five recurrent epilithic lichen species (Aspicilia intermutans (Nyl.) Arnold,Xanthoparmelia pulla (Ach.) O. Blanco, A. Crespo, Elix, D. Hawksw. & Lumbsch, Rhizocarpon lecanorinumAnders, Tephromela atra (Huds.) Hafellner and Lecanora bolcana (Pollini) Poelt), which encrust granodioritespheroidal boulders exposed in the Sila uplands (Calabria, southern Italy), in weathering of plutonic rocksin a typical mountainous Mediterranean environment. A detailed investigation was carried out on thelichen–rock interface of each species, by comparing them mutually and with lichen-free granodiorite samples.For this purpose, the lichen species were sampled together with the encrusted rock surface for detailedmineral-petrographic analyses performed in thin and ultra-thin sections. Optical and scanning electron microscopyof these sections and of bulk samples permitted us to highlight the peculiar modes of physicaland chemical attacks of lichen thalli and hyphae on and into the substratum for each species. Crack systemsoften parallel to the outer rock surface appear often intruded by hyphae, which cause rupture of primaryminerals, with detachment and progressive incorporation of their fragments into the thallus. In particular,the species L. bolcana and T. atra revealed an unexpected, partly endolithic behavior, presumably enhancedby the presence of rock fractures earlier generated by other physical breakage processes already affectingthe spheroidal boulders in the Sila mountains. Dissolution features often affect primary minerals (evenquartz), that may show very peculiar patterns which are suggestive of a biologically-induced control. Variousphyllosilicate clay minerals were identified using SEM–EDS microprobe analyses and FT-IR spectroscopy,which also enabled the identification of possible amorphous silica (or quartz micrograins), rhizocarpic acidand carotenoid at the encrusted granodiorite interface. In contrast, neither oxalic acids nor oxalates weredetected.