Journal of Experimental Botany, Vol. 54, No. 384, pp. 1093-1100,
March 1, 2003
© 2003 Oxford University Press
Tissular localization of coumarins in the green alga Dasycladus vermicularis (Scopoli) Krasser: a photoprotective role?
Received 21 June 2002; Accepted 5 December 2002
Universidad de Málaga, Facultad de Ciencias, Departamento de Ecología, Campus Universitario de Teatinos s/n, 29071 Málaga, Spain
1 To whom correspondence should be addressed. Fax: +34 952 13 20 00. E-mail: jaguilera{at}uma.es
Cell distribution of coumarins, a group of UV-absorbing substances, was analysed by epifluorescence optical microscopy in the green macroalga Dasycladus vermicularis. Maximal concentration of 3,6,7-trihydroxycoumarin (THC), which corresponds to almost 100% of the total coumarins in D. vermicularis, was found in the apical part of the thallus, which is more exposed to solar radiation. At a cell level, two blue, highly fluorescent layers, corresponding to a large accumulation of THC, were found in the internal part of the cell wall and around the vacuolar membrane. The percentage of UV radiation absorbed by each THC layer could be measured from the in vitro total thallus concentration of THC and histological measurements of the layers. The THC layer close to the cell wall absorbed 88% of the incident irradiance at 346 nm (corresponding to the maximum of absorbance of THC in the UVA region), while that close to the vacuole membrane absorbed 87.5%. These results agree with the hypothesis of a natural sunscreen role, significantly reducing harmful UV radiation reaching the cell. Owing to the release of this substance into the medium under different stress conditions, its capacity as a UV filter for other macroalgae has been tested. The ecological relevance of the release process of this UV-absorbing substance in specific environments is discussed.
Key words: Coumarins, Dasycladus vermicularis, epifluorescence optical microscopy, fluorescence parameters, photoprotection.