The concentration of carbon dioxide in natural seawater is sufficient for microalgal growth, but insufficient for high algal-density culture due to limitation of photosynthesis in the artificial medium. This study was adjusted the initial pH to 5.5, 6.5 and 7.5 by adding CO2 or HCl in 1L flask cultured stagnantly with continuous illumination. The best growth of Isochrysis galbana was found in the culture (CO2-6.5i) with initial pH 6.5 using CO2 adjustment and maintaining the same during day 2 to day 5, while the worst was in CO2-5.5i. Furthermost, we adjusted the pH of cultures daily to the set values; the best growth was also found in CO2-6.5e having the cell number 221% of the blank. Initial addition of NaHCO3 with doses of 0.01 g/L, 0.05 g/L, 0.1 g/L, 0.5 g/ L and 1g/ L in the culture, showed the lowest cell number after 5 days culture is in the group of 1.0 g/L and no significant difference among the rest groups. Addition of 0.5 g/L NaHCO3 and adjusted the pH to 6.5 by HCl in the beginning promoted algal growth and resulted in the culture having 212 % cell number of the blank. Using the feedback control system, 100 L algal cultures with aeration and providing CO2 gas or HCl liquid to maintain the pH as 5.5, 6.5 and 7.5 or 7.5 individually were conducted to test the effect of pH control on the mass culture of I. galbana. Better growth was found in the culture with CO2 feedback control than HCl-control in duplicate experiments. It also showed significant difference among the groups adjusted pH between 6.5-7.5. The cell concentration could reach 1100-1400 x 104 cells/ml and was about double the amount of the blank without pH control cultured in 7 days. Meanwhile, the NO3-N concentration was nearly exhausted while the PO4-P still replete. This study reveals the high concentration and fast growth of I. galbana can be maintained under the suitable physical condition providing the carbon source in an optimal pH.

Les enzymes lipolytiques sont des ester hydrolases impliquées dans le métabolisme lipidique. Leurs caractéristiques se sont révélées être des atouts dans de nombreuses applications industrielles. Chez les microalgues, l’isolement et la caractérisation de ces enzymes d’un point de vue structural et fonctionnel restent des domaines de recherche peu explorés à ce jour.Certaines espèces bénéficient pourtant de contenus en lipides intéressants, source de matière première pour les industries de l’agroalimentaire ou de l’énergie. Par exemple, l’acide docosahexaénoique (DHA), un acide gras polyinsaturé de la série des omégas 3, est reconnu pour ses propriétés en santé humaine. Parmi de nombreuses espèces, Isochrysis galbana, une microalgue unicellulaire appartenant à la classe des Prymnesiophycées est considérée comme une source possible de DHA. La présence d’acides gras libres a été montrée par l’analyse des lipides, suggérant la présence d’enzymes lipolytiques potentiellement intéressantes pour leur sélectivité et leur spécificité de substrat.L’analyse d’une banque de marqueurs de séquences exprimées a permis l’identification de séquences susceptibles de coder des enzymes lipolytiques. Les ARN messagers ont été extraits et les ADN complémentaires ont été clonés.Ce travail de thèse présente l’analyse et le clonage de deux gènes codant une ester hydrolase putative et une thioestérase putative, issues de la microalgue Isochrysis galbana.Les deux séquences codent des protéines de poids moléculaires de 35,41 kDa et de 42,31 kDa. Elles montrent 30 à 40 % d’identité et de similarité avec des hydrolases notamment des carboxylestérasesde différents organismes. Les séquences protéiques ont permis l’identification du pentapeptide consensus Gly-X-Ser-X-Gly caractéristique des enzymes lipolytiques et les acides aminés Ser/Asp/His de la triade catalytique.Les deux séquences codantes ont été clonées et exprimées dans la levure Saccharomyces cerevisiae et la bactérie Escherichia coli. Le clonage dans E. coli a permis d’identifier à la taille attendue une protéine par Western blot. En présence de cette protéine, la composition en acides gras des lipides de la bactérie a été modifiée. L’analyse CPG a notamment montré une augmentation des proportions en acides gras C16 :1 et C18 :1 par rapport au témoin. Ce résultat permet de caractériser l’activité thioestérase pour IgTeCe. / Lipolytic enzymes present in all known species play a key role in lipid metabolism and are involved in several industrial processes. They catalyse lipid hydrolysis and synthesis. Actually and particularly in microalgae, isolation and characterization of this type of enzyme remains an unexplored research area.The potential of the lipidic content of microalgae in food industry or energy field requires specific lipolytic enzymes. Docosahexaenoic acid (DHA), an 3 poly insaturated fatty acid (3 PUFA) is well known for its beneficial effects on human health. Among many species, Isochrysis galbana, a unicellular marine microalga belonging to the Prymnesiophyceae class, is considered as a potential alternative source of DHA.Lipid analysis of I. galbana shows free fatty acids and suggests the presence of lipolytic enzymes with potential interesting selectivities and substrate specificities. Analysis of incomplete expressed sequence tag (EST) listed in the EST bank of Isochrysis galbana, identified incomplete genes that encode lipolytic enzymes. Messenger RNAs were extracted, characterized and cloned.This work describes the analysis and cloning of two genes encoding a putative ester hydrolase and a putative thioesterase in marine microalgae Isochrysis galbana. Sequences encode two proteins with predicted molecular weights of approximately 35,41 kDa and 42,31 kDa. Slight similarity and identity (from 30 to 40 %) were observed between the gene sequence and various  fold hydrolase found in diverse phyla (including carboxylesterase).Sequences also included the consensus Gly-X-Ser-X-Gly and the catalytic triad Ser/Asp/His. To characterize the predicted enzymatic functions, an experimental procedure was introduced: coding sequences were cloned into expression vectors and expressed in Saccharomyces cerevisiae and in Escherichia coli.Western blot identification of recombinant enzyme shows a convenient protein production in bacteria. Furthermore, the expression of the protein in E. coli shifted the fatty acid composition predominantly towards C16:1 and C18:1 fatty acids. The enzyme called IgTeCe showed a thioesterase activity.