Glycerolipidome peculiarities of the smallest green picoeukaryote Ostreococcus tauri
Résumé
The pico-alga Ostreococcus tauri is a minimal photosynthetic eukaryote that has been implemented as model system. O. tauri is known to efficiently produce docosahexaenoic acid (DHA). We provide a comprehensive study of the glycerolipidome of O.tauri and validate this species as model for related picoeukaryotes. Ostreococcus lipids displayed unique features that combined traits from the green and the chromoalveolate lineages. The betaine-lipid diacylglyceryl-hydroxymethyl-trimethyl-β-alanine and phosphatidyldimethylpropanethiol, both hallmarks of chromalveolates, were identified as presumed extraplastidial lipids. DHA was confined to these lipids while plastidial lipids of prokaryotic type were characterized by the overwhelming presence of ωω-3 C18-polyunsaturated fatty acids (PUFA), 18:5 being restricted to galactolipids. C16:4, a FA typical of green microalgae galactolipids, was also a major component of O. tauri extraplastidial lipids while 16:4-CoA species was not detected. Triacylglycerols (TAGs) displayed the complete panel of FAs and many species exhibited combinations of FAs diagnostic for plastidial and extraplastidial lipids. Importantly, under nutrient deprivation 16:4 and ωω-3 C18 PUFAs accumulated into de novo synthesized TAGs while DHA-TAGs species remained rather stable indicating an increased contribution of FAs of plastidial origin to TAG synthesis. Nutrient deprivation further severely downregulated the conversion of 18:3 to 18:4 resulting in obvious inversion of 18:3/18:4 ratio in plastidial lipids, triacylglycerols as well as in acyl-CoAs. The fine-tuned and dynamic regulation of 18:3/18:4 ratio suggested an important physiological role of these FAs in photosynthetic membranes. Acyl position in structural and storage lipids together with acyl-CoA analysis further help to apprehend mechanisms possibly involved in glycerolipid synthesis.