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 construc ted of saturated and monounsaturated fatty acids. In contrast, the TAGs of the red microalga Porphyridium currentum are rich in arachidonic acid (AA, 20 :4 omega6) and eicosapenta-enoic acid (EPA, 20:5 omega3). A mutant of th is alga, impaired of growth at suboptimal temperatures, was shown to have r educed levels of EPA and of the eukaryotic molecular species of monogalacto syldiacylglycerol (MGDG) and an elevated level of TAG. Labelling experiment s have shown that labelling of wild-type TAGs decreased, whereas that of th e mutant remained high. Contemporarily, eukaryotic MGDG of the mutant was l ess labelled. Similarly, TAGs of the green alga T12, which can grow at a lo w temperature, are extremely rich in AA. We have labelled exponentially gro wing cultures of T12 kept at 25 degreesC with radioactive AA and cultivated the cultures for a further 12 h at 25 degreesC, 12 degreesC or 4 degreesC. At lowtemperatures, labelled AA was transferred from TAGs to polar lipids. These findings may indicate that polyunsaturated fatty acids that can be i ncorporated into the membranes, enabling the organism to quickly respond to low-temperature-induced stress.