Increasing global temperatures are expected to increase the risk of extinction of various species due to acceleration in the pace of shifting climate zones. Nevertheless, there is no information on the physicochemical properties of membrane lipids that enable the adaptation of the algae to different climatic zones. The present work aimed to compare fatty acid composition and thermal transitions of membrane lipids from green macroalgae Ulva lactuca harvested in the Sea of Japan and the Adriatic Sea in summer. U. lactuca inhabiting the Adriatic Sea had bleached parts of thalli which were completely devoid of chloroplast glycolipids. The adaptation to a warmer climatic zone was also accompanied by a significant decrease in the ratio between unsaturated and saturated fatty acids (UFA/SFA) of membrane lipids, especially in bleached thalli. Hence, bleaching of algae is probably associated with the significant decrease of the UFA/SFA ratio in glycolipids. The decreasing ratio of n-3/n-6 polyunsaturated fatty acids (PUFAs) was observed in extra-plastidial lipids and only in the major glycolipid, non-lamellar monogalactosyldiacylglycerol. The opposite thermotropic behavior of non-lamellar and lamellar glycolipids can contribute to maintenance of the highly dynamic structure of thylakoid membranes of algae in response to the increasing temperatures of climatic zones.

An ability of marine macroalgae Ulva lactuca (L.) to adapt membrane lipid matrix to elevated seawater temperature depending on rate of warm acclimation was studied. Plants were collected in the Peter the Great Gulf of the Sea of Japan in winter at 4°С and acclimated to summer temperature (20°С) at different rates: 16 or 2°С/day. The fatty acid composition and the crystal – liquid crystal thermal transition of glycolipids (monogalactosyl diacylglycerol, digalactosyl diacylglycerol, sulfoquinovosyl diacylglycerol) and betaine lipid 1,2-diacylglycero-O-4'-(N,N,N-tri-methyl)-homoserine (DGTS) from U. lactuca were investigated by GLC, HPLC-MS and differential scanning calorimetry, respectively. The dependence of the results of temperature acclimation on its rate was shown. Thus, rapid warm acclimation induced the decrease of the ratios of n-3/n-6 polyunsaturated fatty acids and unsaturated/saturated fatty acids, as well as the unsaturation index of fatty acid composition of lipids, which corresponded to seasonal changes. However, this trend did not retain, or even reverse changes occurred at lower warming rate. Redistribution of the composition of molecular forms of glycolipids in acclimated algae led not only to maintain the peak maximum temperature of thermal transition (T_max), but also to phase separation of glycolipids. This contributed to the fit of the thermogram profiles to those of summer samples. The calorimetric transition of DGTS was most effectively changed: even with rapid acclimation, the T_max value of the summer sample was reached. In general, U. lactuca showed the ability to adapt lipid matrix of membranes at different rates of warming seawater.