The role of triacylglycerol as a reservoir of polyunsaturated fatty acids for the rapid production of chloroplastic lipids in certain microalgae

Many microalgae are known to accumulate triacylglycerols (TAGs), especially under nitrate starvation. Generally, these TAGs are predominantly constructed of saturated and monounsaturated fatty acids. In contrast, the TAGs of the red microalga Porphyridium cruentum are rich in arachidonic acid (AA, 20:4 omega6) and eicosapentaenoic acid (EPA, 20:5 omega3). A mutant of this alga, impaired of growth at suboptimal temperatures, was shown to have reduced levels of EPA and of the eukaryotic molecular species of monogalactosyldiacylglycerol (MGDG) and an elevated level of TAG. Labelling experiments have shown that labelling of wild-type TAGs decreased, whereas that of the mutant remained high. Contemporarily, eukaryotic MGDG of the mutant was less labelled. Similarly, TAGs of the green alga Tl2, which can grow at a low temperature, are extremely rich in AA. We have labelled exponentially growing cultures of T12 kept at 25 degrees C with radioactive AA and cultivated the cultures for a further 12 h at 25 degrees C, 12 degrees C or 4 degrees C. At low temperatures, labelled AA was transferred from TAGs to polar lipids. These findings may indicate that polyunsaturated fatty acids that can be incorporated into the membranes, enabling the organism to quickly respond to low-temperature-induced stress.