Micromorphological and ultrastructural investigations of the lichen–soil interface

The lichen–soil interface for three terricolous lichens, Baeomyces rufus (Huds.) Rebend., B. roseus Pers. and Cladonia rangiferina (L.) Web., from different alpine sites was studied using micromorphological methods (thick and thin sections) and ultramicroscopy (ultra-thin sections). Contacts between lichens and their substrates are mainly via their fungal symbionts. For Baeomyces, a close relationship between the lichen hyphae and soil minerals were revealed as well as associated physical processes (reorientation of mineral particles, biodegradation of the walls of plant debris, and granulo-fibrillar bonding between different elements). For Cladonia, the basal part of the thallus degenerates; fungal hyphae escape from it and are incorporated into the soil. The production of polysaccharide compounds by the hyphae seemed to play an important role in the formation of organo-mineral aggregates. Moreover, the lichen–soil relationships were complicated further by the activity of lichen-specific bacterial colonies. The results obtained at various observation scales showed that lichens were the principal agents of mineral weathering and the incorporation of humic compounds into the mineral substrate, at least under those extreme climatic conditions where they are abundant.