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 25°C instead of 35°C. 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 νsymCH2band (characteristic of the lipid disorder) revealed more tightly arranged fatty-acyl chains for the thylakoids in the cells grown at 25°C as compared with those grown at 35°C. This apparent controversy was resolved by a two-component analysis of the νsymCH2band, which demonstrated very rigid, myxoxanthophyll-related lipids in the thylakoid membranes. When this rigid component was excluded from the analysis of the thermotropic responses of the νsymCH2bands, the expected higher fatty-acyl disorder was observed for the thylakoids prepared from cells grown at 25°C as compared with those grown at 35°C. Both the carotenoid composition and this rigid component in the thylakoid 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.