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

• Published in: The New phytologist Vol. 145; no. 2; pp. 271 - 280
• Main Authors: HOLDER, J. M., WYNN-WILLIAMS, D. D., RULL PEREZ, F., EDWARDS, H. G. M.
• Format: Journal Article
• Language: English
• Published: Cambridge, UK Cambridge University Press 01.02.2000; Blackwell
• Subjects: Acarospora; Antarctic; Antarctic regions; Antarctica; beta-carotene; Biological and medical sciences; Calcium; calcium oxalate; chemical composition; Fundamental and applied biological sciences. Psychology; geographical variation; histochemistry; Lichenes; Lichens; Mediterranean; Mediterranean Sea; mortality; organic acids and salts; oxalate; Oxalates; oxalic acid; Pigments; Plant cytology, morphology, systematics, chorology and evolution; quantitative analysis; Raman scattering; Raman spectroscopy; rhizocarpic acid; spectral analysis; Spectral bands; Spectroscopy; Thallophyta; Thallus; UV‐radiation; Mediterranean region; Antarctica; Mediterranean; pigments; rhizocarpic acid; UV-radiation; Antarctic; oxalate; Raman spectroscopy; Acarospora; Lithophyte; Temperature; Desiccation; Hostile environment; Environmental factor; Survival; Spatial distribution; Light; Pigments; Lichenes; Habitat; Raman spectrometry; Betacarotene; Thallophyta; Adaptation
• ISSN: 0028-646X; 1469-8137
• DOI: 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.