Thesis presented March 25, 2022

Abstract:
In the current context of ecological and energy transitions, microalgae are increasingly studied for their high lipid content, which can be used to produce biofuels. However, knowledge about the metabolism of microalgae is still limited. Higher plants and microalgae, although living in different places, are subject to environmental constraints, such as a lack of phosphate (Pi), a macronutrient essential for their development. To compensate for this deficiency, plants and microalgae modify the lipid composition of their membranes in order to remobilise the phosphate present in the phospholipids. This is why many data in the literature show an increase in non-phosphorus lipids to replace and compensate for a decrease in phospholipids. It is known that sulfoquinovosyldiacylglycerol (SQDG) replaces phosphatidylglycerol (PG), that digalactosyldiacylglycerol (DGDG) replaces phosphatidylcholine (PC), and in microalgae that lipid betaines appear to replace PC. Is the replacement of each phospholipid by a non-phosphorus lipid related to their similar structural properties in the membrane? It is within this framework of fundamental research into the understanding of lipid rearrangements that this thesis work is situated.
The use of neutron diffraction on lipid films is a powerful tool to determine many structural parameters of the membranes, such as their organisation, thickness and thickness of the water layer between the membranes, as well as their stiffness and compressibility. In this project, we were able to show that PG and SQDG have very similar biophysical properties explaining their interchangeability during phosphate deficiency. Secondly, we tried to find an answer to the deformation of the chloroplast envelope observed in deficient plants, which seems to be related to the increase of DGDG in the membranes. The results suggest that DGDG makes the membranes more flexible and promotes their juxtaposition, but there is still insufficient data to conclude with certainty. We then highlighted that the properties of DGTS (1,2-diacylglyceryl-3-O-4'-(N,N,N-trimethyl)-homoserine), present in microalgae, are different from those of CP, including a thicker bilayer and stronger interlayer repulsion, which could explain its absence in seed plant membranes. Plants with a different fatty acid composition than microalgae, rich in C20 fatty acids and carrying up to 5 unsaturations, would not be able to counteract the effects of the polar head of the lipid betaines. A close link between the fatty acid composition and the presence of betaine lipids in the organism thus seems to emerge. Finally, in some microalgae, the presence of acyl-SQDG (ASQD) in the membranes could favour the docking of the membranes and thus the stacking of thylakoids in the chloroplast. However, the data obtained do not yet allow us to validate this hypothesis. Thus, the study of lipids and membranes using a physical approach has enabled us to provide answers to biological questions, thus improving our understanding of lipid remodelling in plants and microalgae during phosphate deficiency.

Keywords:
Microalgae, Phosphate starvation, Lipids, Membranes architecture, Neutron diffraction

On-line thesis.