Characterisation of Unique Eukaryotic Sphingolipids with Temperature-Dependent Δ8-Unsaturation from the Picoalga Ostreococcus Tauri.

Sphingolipids are ubiquitous components of eukaryotic cell membranes and are found in some prokaryotic organisms and viruses. They are composed of a sphingoid backbone that may be acylated and glycosylated. Assembly of various sphingoid base, fatty-acyl and glycosyl moieties results in highly diverse structures. The functional significance of variations in sphingolipid chemical diversity and abundance is still in the early stages of investigation. Among sphingolipid modifications, the Δ8-desaturation of the sphingoid base occurs only in plants and fungi. In plants, sphingolipid Δ8-unsaturation is involved in cold hardiness. Our knowledge of the structure and functions of sphingolipids in microalgae lags far behind that of animals, plants and fungi. Original sphingolipid structures have been reported from microalgae. However, functional studies are still missing. Ostreococus tauri is a minimal microalga at the base of the green lineage, and is therefore a key organism for understanding lipid evolution. In the present work, we achieved the detailed characterisation of O. tauri sphingolipids and unveiled unique glycosylceramides as sole complex sphingolipids. The head groups are reminiscent of bacterial sphingolipids, as they contain hexuronic acid residues and can be polyglycosylated. Ceramide backbones show limited variety and sphingolipid modification is restricted to ∆8-unsaturation. The ∆8-sphingolipid desaturase from O. tauri only produced E-isomers. Expression of Δ8-sphingolipid desaturase and Δ8-unsaturation of spingolipids both varied with temperature, with lower levels at 24°C than at 14°C. Overexpression of the Δ8-sphingolipid desaturase dramatically increases the level of Δ8 unsaturation at 24°C and is paralleled by a failure to increase cell-size. Our work provides the first characterisation of O. tauri sphingolipids and functional evidence for the involvement of sphingolipid ∆8-unsaturation for temperature acclimation in microalgae, suggesting that this function is an ancestral feature in the green lineage.