Actuellement, plus de 70% des biomédicaments commercialisés sont des glycoprotéines recombinantes. Les coûts élevés de production de ces biomédicaments ont poussé les scientifiques à développer des organismes de production alternatifs. Récemment, les microalgues ont été proposées en tant que potentiel système de production compte-tenu de leur rapidité de croissance et de leurs faibles coûts de production. Cependant, avant de produire des biomédicaments industriels chez les microalgues, il est impératif de s’assurer que les modifications post-traductionnelles, comme la N-glycosylation, soit conservées et compatibles avec une utilisation thérapeutique. Dans ce contexte, l’étude de la Nglycosylation de deux microalgues modèles, Chlamydomonas reinhardtii (microalgue verte) et Phaeodactylum tricornutum (diatomée) a été réalisée. Dans un premier temps, l’ingénierie de la N-glycosylation de C. reinhardtii a été initiée en exprimant une Nacétylglucosaminyltransférase I (GnT I) hétérologue. Les résultats obtenus ont permis de réévaluer les voies de N-glycosylation de C. reinhardtii et de montrer que cette microalgue synthétise une structure glycannique linéaire qui n’est pas substrat de la GnT I. Dans un second temps, un protocole d’extraction et de caractérisation des précurseurs glycanniques de C. reinhardtii et P. tricornutum a été développé et appliqué pour déterminer la structure des précurseurs glycanniques dans ces espèces. Enfin, la caractérisation de deuxxylosyltransférases potentielles (XTA et XTB) de C. reinhardtii a été menée en utilisant des mutants d’insertion et des analyses des N-glycannes par spectrométrie de masse. Cette étude a confirmé les rôles spécifiques de XTA et XTB dans la voie de N-glycosylation de C. reinhardtii. / Currently, more than 70% of the commercialized biopharmaceuticals are glycoproteins. The high production costs lead scientists to develop alternative organisms suitable for such production. Recently, microalgae emerged as a potential interesting production system thanks to their quick growth rate and low production costs. However, prior to start industrial glycoproteins production in microalgae, protein post-translational modifications like Nglycosylation, must be carefully controlled. This PhD thesis focused on the analysis of the Nglycosylation pathway of two different microalgae, Chlamydomonas reinhardtii (greenmicroalgae) and Phaeodactylum tricornutum (diatom). In order to start N-glycan engineering, heterologous N-acetylglucosaminyltransferase I (GnT I) sequences were expressed in C.reinhardtii. This study demonstrated that C. reinhardtii synthetize a linear N-glycan unsuitable for GnT I activity and allows the reinvestigation of the C. reinhardtii N-glycosylation pathway. A second chapter of this work focus on the optimization of a protocol suitable for analyzing the structure of the Dolichol N-linked precursors of C. reinhardtii and P. tricornutum. Lastly, two potential xylosyltransferases (XTA and XTB) from C. reinhardtii were characterized using insertional mutants and N-glycomic analyses by mass spectrometry approaches. This work allows us to propose specific involvement of XTA and XTB in the xylosylation processing of C.reinhardtii N-glycans.

This thesis reports the rheological properties of algae slurries as a function of cell concentration for three microalgae species: Nannochloris sp.,Chlorella vulgaris, and Phaeodactylum tricornutum. Rheological properties ofalgae slurries have a direct impact on the agitation and pumping power requirements as well as process design for producing algal biofuels. This study measures the rheological properties of eight diff erent concentrations of each species ranging from 0.5 to 80 kg dry biomass/m³. Strain-controlled steady rate sweep tests were performed for each sample with an ARES-TA rheometer using a double wall couette cup and bob attachment. Shear rates ranged from 5 - 270 s⁻¹, corresponding to typical expected conditions. The results showed that Nannochloris sp. slurry behaved as a Newtonian fluid for concentrations up to 20 kg/m³. Samples with concentrations above 40 kg/m³ behaved as a shear thinning non-Newtonian fluid. The effective viscosity increased with increased biomass concentration for a maximum value of 3.3x10⁻³ Pa-s. Similarly, C. vulgaris slurry behaved as a Newtonian fluid with concentrations of up to 40 kg/m³, above which it displayed a shear thinning non-Newtonianf behavior and a maximum eff ective viscosity of 3.5x10⁻² Pa-s. On the other hand, P. tricornutum slurry demonstrated solely Newtonian fluid behavior, with the dynamic viscosity increasing with increasing biomass concentration for a maximum value of 3.2x10⁻³ Pa-s. The maximum observed e ffective viscosity occurred at a concentration of 80 kg/m³ for all three species. Moreover, an energy analysis was performed where a non-dimensional bioenergy transport e ffectiveness was de termined as the ratio of the energy content of the transported algae biomass to the sum of the required pumping power and the harvesting power. The results show that the increase in major losses due to increase in viscosity was overcompensated by the increase in the transported biomass energy. Also, cultivating a more concentrated slurry requires less dewatering power and is the preferred option. The largest bioenergy transport eff ectiveness was observed for the slurries with the largest initial dry biomass concentrations. Finally, the relative viscosity of algae slurries was modeled using a Kelvin-Voit based model for dilute and concentrated viscoelastic par- ticle suspensions. The model, which depends primarily on the packing factor of the algae species, agrees with the measured viscosity with an average error of 18%, while the concentrated particle suspension model was slightly more accurate than the dilute suspension model. / text

Algae play a key role in the aquatic environment as primary producers; however, they can form harmful algal blooms that are detrimental to the aquatic environment, people, and even the economy. It is generally believed that climate change will influence future algal blooms. The first study estimated algae community in three freshwater locations in Mississippi, Desoto Lake, White’s Creek Lake, and Columbus Lake, and one saline location, Western Mississippi Sound, using modern and new tools of the autonomous surface vessel, FlowCam, and EcoTaxa. From those results and leading into the second study was the application of four climate scenarios on the species Anabaena flos-aquae, a freshwater cyanobacteria, and Phaeodactylum tricornutum, a saltwater diatom. The four scenarios increased from current conditions, high climate mitigation, low mitigation, and no mitigation. Anabaena flos-aquae exhibited increased rates of growth with each climate condition. Phaeodactylum tricornutum only had minimal changes in the experimental conditions.

En condition de stress, les organismes photosynthétiques réorientent leur métabolisme vers la production de métabolites d’intérêt. Cette thèse vise à fournir de nouvelles informations sur ces processus chez les algues et les plantes supérieures. La 1ère partie est consacrée aux effets de 3 éclairements différents sur le métabolisme du carbone de la diatomée Phaeodactylum tricornutum. L'impact d’un éclairement supérieur à 300 µmol photons m-2 s-1 (ML) est tout à fait différent de celui obtenu pour un éclairement de 30 µmol photons m-2 s-1 tant au niveau physiologique que moléculaire. Dans nos conditions, la carence en carbone constitue la cause principale de l'apparition de la phase stationnaire. La synthèse des lipides est plus élevée sous un éclairement inférieur à ML. En revanche, la synthèse des protéines et de chrysolam-inarine augmente respectivement sous fort et faible éclairements. Les changements d’expression génique suggèrent que la conversion réversible du phosphoénol-pyruvate en pyruvate constitue une étape clé de l’orientation des intermédiaires dans les différentes voies de biosynthèse des molécules d’intérêt. L'état physiologique des cellules doit être considéré pour la comparaison d’échantillons. La 2ème partie du mémoire est consacrée aux effets de la quantité et de la qualité de la lumière délivrée par des LED (bleu, rouge, 70% de lumière rouge+30% de bleu et blanc) sur la régulation de la synthèse de métabolites secondaires chez 3 espèces de menthe prélevées dans la nature. La qualité de la lumière influence la stratégie d’utilisation du carbone par les plantes. Par exemple, la lumière rouge stimule le mieux la production de la synthèse de l'huile essentielle chez la menthe alors que le taux de fixation du carbone est similaire à celui trouvé dans les autres conditions d’illumination. Glomus mossae est le seul champignon mycorhizien à arbuscules à induire une augmentation supplémentaire de la production d’huile essentielle. / Under stress, photosynthetic organisms reoriente their metabolism toward the production of high added value molecules. Regarding the importance of this process, it is surprising that the processes on which the reorientation relies are still not better understood. This thesis aims to provide new information regarding these processes. The 1st part of this thesis is dedicated to the effects of 3 different light intensities on the carbon metabolism of the diatom Phaeodactylum tricornutum. The impact of light intensities higher than 300 µmol photons m-2 s-1 (ML) were different from those obtained with 30 µmol photons m-2 s-1 at the physiological and molecular levels. Carbon deficiency was responsible for the occurrence of plateau phase in cultures. Except lag phase, lipid synthesis was higher under ML In contrast, protein and chrysolaminarin syntheses increased under 1000 and 30 µmol photons m-2 s-1, respectively. Gene expression modifications suggest that the reversible conversion between phosphoenolpyruvate and pyruvate constitutes a key step for the orientation of intermediates to either high value molecules biosynthetic pathways. The physiological state of the cells should be taken into consideration when samples comparison is considered.The 2nd part of the report is dedicated to the effects of light quantity and quality (blue, red, 70% red+30% blue and white light delivered by LED) on the regulation of essential oil synthesis in 3 Mentha species collected in nature. The light quality impacts the strategy of carbon utilization by the plant. Typically, red light was the most effective for stimulating the production of essential oil in Mentha sp. while carbon fixation capacity was similar to that found in other ligthing conditions. Regardless the light intensity, Glomus mossae was the only arbuscular mycorrhizal fungus able to enhance additionally essential oil production.

Microalgae are gaining more attention for several reasons such as being potential producers of sustainable fuel, for use as health supplements and in skincare. Simris Alg is a Swedish company that produces Omega 3 supplements from a primary producer of these fatty acids - the algal diatom Phaeodactylum tricornutum, which is a sustainable alternative to Omega 3 derived from fish. Omega 3 fatty acids constitute a small fraction of the total algal biomass, and to increase profitability and utilize all of the biomass, the purpose of this thesis project is to present potential applications for the residual material that is left after oil extraction. A general composition study was made of Simris Alg algal residue material, and results are compared to those found in previous studies of P. tricornutum biomass. An optimization of the fractionation is needed to separate the storage carbohydrate chrysolaminarin and cell wall component glucuronomannan, followed by analysis for confirmation. Also, it would be interesting to separate chitin, if there is any, since despite the presence of chitin synthases, it is unclear whether the diatom actually produces chitin. When gathering information, no actual experimental characterization of carbohydrate active enzymes involved in synthesis of the main carbohydrates investigated were found. Such information would be useful to increase production of the carbohydrate of interest, if valuable applications are found. Potential applications of various cell components, such as carbohydrates, in skincare would be interesting to investigate, as well as optimizing fucoxanthin extraction for use as an additional high value product next to Omega 3.