Thesis presented December 18, 2023

Abstract:
Microalgae encompass unicellular photosynthetic organisms whose diversity is represented throughout the tree of life. They colonize a variety of environments and are regularly subjected to numerous biotic and abiotic stresses. In response to these stresses, lipid droplets (LDs) can be produced. LDs are organelles composed of a hydrophobic core, predominantly containing triacylglycerols (TAGs), surrounded by a monolayer of glycerolipids embedded with numerous proteins. LDs are present in nearly all living organisms and are involved in various functions such as carbon and energy storage, remodeling of membrane lipids, synthesis and storage of hydrophobic molecules, and more. Increasingly, new functions are attributed to them, and LDs are now considered distinct organelles, highly dynamic and flexible, both in their lipid composition and protein complement.
The biogenesis of LDs occurs within the endomembrane system and is under the control of complex molecular machineries. In this process, the protein Seipin plays a major role and has been characterized in numerous organisms (plants, animals, yeast). This transmembrane protein forms an oligomer inserted into the endoplasmic reticulum membrane and functionally influences various aspects of LD biogenesis. For example, it is involved in determining LD budding orientation, controlling LD size, and regulating the TAG flux within LDs.
In the oleaginous diatom Phaeodactylum tricornutum, an isoform of Seipin (PtSeipin) has previously been identified, and the research presented here aimed to characterize its function. By combining structural prediction, phylogenetic, microscopy, lipidomic, and proteomic approaches, we have highlighted several unique features of PtSeipin. We observed that Seipin in diatoms and more broadly in stramenopiles are phylogenetically closer to those in animals and fungi than to photosynthetic organisms like plants and green algae. We also established that the loss of PtSeipin function leads to a pronounced phenotype, with the formation of oversized LDs, as observed in other species. However, uniquely, we observed a dramatic accumulation of TAGs, particularly under stress conditions, suggesting that additional specific regulatory mechanisms are involved in LD formation in P. tricornutum. Finally, the analysis of purified LD proteomes in PtSeipin mutants revealed that the protein StLDP, described as the major LD protein, is no longer recruited. These results open up new avenues for fundamental research, particularly in the search for Seipin partner proteins and the detailed study of its functional domains. Additionally, these findings also provide new insights for applied research in the field of next-generation biofuels.

Keywords:
Seipin, Lipid droplet, Lipids, Triacylglycerols, Diatom, Microalgae