AN APPROACH TO STONE AND FRESCO LICHEN BIODETERIORATION THROUGH FOURIER-TRANSFORM RAMAN MICROSCOPIC INVESTIGATION OF THALLUS SUBSTRATUM ENCRUSTATIONS

The Fourier transform Raman spectra of thallus-substratum interfaces of Xanthoria parietina growing in situ on sandstone and gypsum were obtained for the first time. The presence of small amounts of calcium oxalate in thallus-substratum interfaces was confirmed spectroscopically and evidence for biodeterioration via lichen metabolic by-products was also noted. Although the oxalate determined spectroscopically was in good agreement with the results from thermogravimetric measurements there was also spectroscopic evidence for incorporation of stone particles into the lichen encrustation. Comparisons are made with Raman spectroscopic studies of the thalli of the high oxalate-producing lichen Dirina massiliensis forma sorediata. The Raman spectra Ochrolechia parella growing in association with Xanthoria parietina on maritime sandstone were also recorded and the spatial discrimination of the techniques for these high and low oxalate-producing lichens was demonstrated.