Low-temperature-induced accumulation of xanthophylls and its structural consequences in the photosynthetic membranes of the cyanobacterium Cylindrospermopsis raciborskii:: An FTIR spectroscopic study

The effects of the growth temperature on the lipids and carotenoids of a filamentous cyanobacterium, Cylindrospermopsis raciborskii, were studied., The relative amounts of polyunsaturated glycerolipids and myxoxanthophylls in the thylakoid membranes increased markedly when this cyanobacterium was grown at 25degreesC instead of 35degreesC. Fourier transform infrared spectroscopy was used to analyze the low-temperature-induced structural alterations in the thylakoid membranes. Despite the higher amount of unsaturated lipids there, conventional analysis of the nu(sym)CH(2) band (characteristic of the lipid disorder) revealed more tightly arranged fatty-acyl chains for the thylakoids in the cells grown at 25degreesC as compared with those grown at 35degreesC. This apparent controversy was resolved by a two-component analysis of the nu(sym)CH(2) band, which demonstrated very rigid, myxoxanthophyll-related lipids in the thylakold membranes. When this rigid component was excluded from the analysis of the thermotropic responses of the v(sym)CH(2) bands, the expected higher fatty-acyl disorder was observed for the thylakoids prepared from cells grown at 25degreesC as compared with those grown at 35degreesC. Both the carotenoid composition and this rigid component in the thylakold membranes were only growth temperature-dependent; the intensity of the illuminating light during cultivation had no apparent effect on these parameters. We propose that, besides their well-known protective functions, the polar carotenoids in particular may have structural effects on the thylakoid membranes. These effects should be exerted locally-by forming protective patches, in-membrane barriers of low dynamics-to prevent the access of reactive radicals generated in either enzymatic or photosynthetic processes to sensitive spots of the membranes.