Glycerolipid Characterization and Nutrient Deprivation-Associated Changes in the Green Picoalga Ostreococcus tauri

Abstract

The picoalga is a minimal photosynthetic eukaryote that has been used as a model system. is known to efficiently produce docosahexaenoic acid (DHA). We provide a comprehensive study of the glycerolipidome of and validate this species as model for related picoeukaryotes. lipids displayed unique features that combined traits from the green and the chromalveolate 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 ω-3 C18 polyunsaturated fatty acids (FAs), 18:5 being restricted to galactolipids. C16:4, an FA typical of green microalgae galactolipids, also was a major component of extraplastidial lipids, while the 16:4-coenzyme A (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 ω-3 C18 polyunsaturated FAs accumulated into de novo synthesized TAGs while DHA-TAG species remained rather stable, indicating an increased contribution of FAs of plastidial origin to TAG synthesis. Nutrient deprivation further severely down-regulated the conversion of 18:3 to 18:4, resulting in obvious inversion of the 18:3/18:4 ratio in plastidial lipids, TAGs, as well as acyl-CoAs. The fine-tuned and dynamic regulation of the 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 determine mechanisms possibly involved in glycerolipid synthesis.

Plastidic Δ6 fatty-acid desaturases with distinctive substrate specificity regulate the pool of c18-pufas in the ancestral picoalgaostreococcus tauri

“…Phosphate limitation is required for the maximal activity of the high-affinity-phosphate-transporter promoter (promHAPT) driving transgene overexpression. Furthermore, phosphate limitation has been previously shown to enhance the accumulation of 18:3n-3 at the expense of 18:4n-3 in WT (Degraeve-Guilbault et al ., 2017). Transgenics for each of the Δ6-DES were screened by luminescence recording of the luciferase reporter gene (promCCA1:Luc) and their FA-profiles were further assessed (Fig.…”

“…5G). Under phosphate limitation, DGTA is the prevailing extraplastidic structural lipid (Degraeve-Guilbault et al ., 2017) (Fig. S11A, S11B).…”

“…In O. tauri , as in cyanobacteria and other most green microalgae, plastidic lipids correspond to sn-1/sn-2 18:X/16:X species. In contrast, O. tauri extraplastidic lipids encompass sn-1/sn-2 SFA/16:4, VLC-PUFA/16:4 and di-homo-VLC-PUFA as major species (Degraeve-Guilbault et al ., 2017). These distinctive positional signatures strongly suggest that, in O. tauri , plastidic lipids synthesis is independent of ER synthesis.…”

“…Overexpression of each of the three Δ6-DES in O. tauri led to a similar increase of 20:4n-6 in DGTA and TAG and a specific increase of 18:4n-3 in DGTA for Ot05-OE. We previously reported that the acyl-CoA pool was enriched in 18:3n-3 and 18:4n-3, whose amounts were varying according to the plastidic C18-PUFA content (Degraeve-Guilbault et al ., 2017). Together with the discovery of pΔ6-DES, these previous observations support that Δ6-desaturation products are exported from the plastid to the acyl-CoA pool.…”

Optimizing bioreactor growth of the smallest eukaryote

“…Ostreococcus tauri was originally isolated from copper contaminated water and it can grow in a variety of media conditions including artificial salt water (ASW) brackish media 1,2 or with additives such as glycerol 3 . It has a highly condensed genome 4,5 , a simple cell ultrastructure 6 , is genetically tractable 7,8 , and is capable of significant triacylglycerol accumulation under various nutrient limitation conditions 9 . Additionally, it displays other favorable characteristics that can facilitate scale-up in bioreactors or downstream bioprocessing such as immotility, absence of a cell wall, and lack of annotated flocculating enzymes.…”