Effect of solar irradiance on lipids of the green alga Ulva fenestrata Postels et Ruprecht

We examined variations in lipid composition of the marine green alga Ulva fenestrata grown under different irradiance conditions (80% of incident photosynthetically active radiation [PAR] and 24% PAR). The ratio of storage lipids (triacylglycerol) to major structural chloroplast lipids (glycolipids and phosphatidylglycerol) was influenced considerably by the irradiance. Triacylglycerol predominated in U. fenestrata exposed to 80% PAR (58.7+/-1.7% of total lipids). In contrast, when the alga was grown at 24% PAR, the amount of monogalactosyldiacylglycerol, sulfoquinovosyldiacylglycerol and phosphatidylglycerol increased two to three and a half times compared to algae cultivated at 80% PAR. The content of digalactosyldiacylglycerol and betaine lipid in U. fenestrata showed little dependence on light intensity. Each lipid class had a characteristic fatty acid composition. However, the relative proportions of fatty acids present in triacylglycerol, monogalactosyldiacylglycerol and sulfoquinovosyldiacylglycerol did not depend on irradiance conditions. The variation in fatty acid composition of digalactosyldiacylglycerol and phosphatidylglycerol and changes in the amounts of lipids were responsible for the differences in total fatty acid composition of U. fenestrata among light intensities. The most sensitive to the change in quantity of incident light were the 16:4n-3 and 16:0 acids. The highest level of 16:4n-3 acid was detected in algae grown under 24% PAR, whereas 16:0 acid was highest at 80% PAR. However, the total lipid content in the algae was relatively constant at different light intensities (5.0+/-1.0 mg/g fresh weight at 80% PAR and 5.2+/-1.1 mg/g at 24% PAR). These results suggest that changes in lipid composition of U. fenestrata can be considered as a mechanism of adaptation and survival of thalli subjected to variations in solar irradiance.