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

• Published in: New Phytologist
• Main Authors: HOLDER, J. M., WYNN‐WILLIAMS, D. D., RULL PEREZ, F., EDWARDS, H. G. M.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2000
• Subjects: Antarctic; The Antarctic; Antarc*
• Online Access: http://dx.doi.org/10.1046/j.1469-8137.2000.00573.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1046%2Fj.1469-8137.2000.00573.x; https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1469-8137.2000.00573.x; https://nph.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1469-8137.2000.00573.x

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 β‐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 (ν) 1596, 1665, 1620 and 1000 cm −1 . It was possible to discriminate between hydration states of calcium oxalate; the monohydrate (whewellite) featured a ν(CO) stretching band at 1493 cm −1 whereas the dihydrate (weddellite) had a contrasting ν(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 ν(CO) and ν(CONH) amide modes, for carotenoid pigment by its ν(C = C) band at 1520 cm −1 and for calcium oxalates by their ν(CO) bands. ν(CO), ν(CONH) and ν(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 μm 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.