Raman spectroscopy of pigments and oxalates in situ within epilithic lichens:: Acarospora from the Antarctic and Mediterranean

Fourier Transform laser Raman spectroscopy was used to generate diagnostic spectra for pigments and biodegradative calcium oxalate in situ in two yellow-pigmented species of the lichen genus Acarospora from contrasting sites in the Antarctic and the Mediterranean. This non-intrusive technique was used to identify the photoprotective pigments rhizocarpic acid and beta-carotene by their unique Raman spectral fingerprints. The use of low energy near-IR excitation at 1064 nm eliminated interference from autofluorescence of photosynthetic pigments. The insensitivity of the technique to water permitted the use of field-fresh material. The dominant yellow pigment, rhizocarpic acid, gave a diagnostic pattern of corroborative bands at wavenumbers (nu) 1596, 1665, 1620 and 1000 cm(-1). It was possible to discriminate between hydration states of calcium oxalate; the monohydrate (whewellite) featured a nu(CO) stretching band at 1493 cm(-1) whereas the dihydrate (weddellite) had a contrasting nu(CO) stretching band at 1476 cm(-1). Fourier Transform deconvolution and intensity measurements were used to obtain relative quantitative data for rhizocarpic acid by using its nu(CO) and nu(CONH) amide modes, for carotenoid pigment by its nu(C = C) band at 1520 cm(-1) and for calcium oxalates by their nu(CO) bands, nu(CO), nu(CONH) and nu(C = C) are the vibrational stretching modes of the carbonyl C = O, protein amide 1 and alkenyl C = C moieties, respectively, in the pigments and metabolic products of the Acarospora lichens. The ability to determine the precise (20 mu m spot diameter) spatial distribution of these key functional molecules in field-fresh thallus profiles and variegations has great potential for understanding the survival strategies of lichens, which receive high insolation, including-elevated levels of UV-B, under extremes of desiccation and temperature in hot and cold desert habitats.