• Membre du conseil du laboratoire
• Direction d'équipe

• Planète et Univers/Océan, Atmosphère
  

The target of rapamycin (TOR) signaling pathway regulates fundamental intracellular functions critical for cell viability and proliferation. Manipulation of TOR in high lipid-producing microalgae may help overcome the trade-off between biomass production and lipid yield that still impairs the viable production of biofuel from microalgae. In this study, we inhibited the TOR kinase in the model diatom Phaeodactylum tricornutum using the selective TOR inhibitor AZD-8055, and analyzed cell proliferation, chlorophyll content, lipid synthesis and carbon metabolism. AZD-8055 inhibits cell proliferation in a dose-dependent manner compared to N deprivation which stops growth. Microscopy, flow cytometry, and quantitative analyses of lipids also demonstrated that AZD-8055 treatment strongly promotes triacylglycerol (TAG) accumulation while decreasing the quantity of sterols. The TAG productivity of AZD-8055 treated cultures was significantly higher than for N deprived cultures. Measurement of the activities of the key metabolic enzymes glyceraldehyde phosphate dehydrogenase (GAPDH), glucose-6-phosphate dehydrogenase (G6PDH) and malate dehydrogenase (MDH) revealed opposite effects for AZD-8055 treatment and N-starvation on the activity of the glycolytic enzyme GAPDH. This suggests that TOR inhibition and N starvation may have distinct impacts on general metabolism and lipid accumulation. Our main finding is that treating cultures with AZD-8055 results in higher TAG productivity than N starvation in P. tricornutum. The chemical or genetic manipulation of the TOR signaling pathway in P. tricornutum and other diatoms may lead to the development of strains or approaches suitable for the enhanced production of TAGs for biofuel.

After the exponential growth phase, variability in the scattering efficiency of phytoplankton cells over their complete life cycle is not well characterised. Bulk measurements are impacted by senescent cells and detritrus. Thus the analysis of the evolution of the optical properties thanks to their morphological and/or intra-cellular variations remains poorly studied. Using the Cytosense flow cytometer (CytoBuoy b.v., NL), the temporal course of the forward and sideward efficiencies of two phytoplankton species (Thalassiosira pseudonana and Chlamydomonas concordia) were analyzed during a complete life-cycle. These two species differ considerably from a morphological point of view. Over the whole experiment, the forward and sideward efficiencies of Thalassiosira pseudonana were, on average, respectively 2.2 and 1.6 times higher than the efficiencies of Chlamydomonas concordia. Large intra-species variability of the efficiencies were observed over the life cycle of the considered species. It highlights the importance of considering the optical properties of phytoplankton cells as a function of the population growth stage of the considered species. Furthermore, flow cytometry measurements were combined with radiative transfer simulations and biogeochemical and optical measurements. Results showed that the real refractive index of the chloroplast is a key parameter driving the sideward signal and that a simplistic two-layered model (cytoplasm-chloroplast) seems particularly appropriate to represent the phytoplankton cells.