Étude de l’immobilisation de la microalgue Raphidocelis subcapitata dans des billes d’alginate pour produire une quantité élevée de lipides

Abstract

Algal lipids are attracting much interest in the production of biodiesel and other bioproducts that can be used for nutritional and pharmaceutical applications. Lipids can also be used as emulsifiers in the food industry and as raw materials for the manufacture of cleaning products. However, the process of producing algal lipids presents high costs and certain significant challenges. Harvesting microalgae constitutes a limiting step, because these microorganisms are small and are diluted in the culture medium. Centrifugation is the most used technique on an industrial scale but it costs energy. The integration of immobilization technology in the cultivation of microalgae, in particular, trapping in alginate beads, would facilitate their harvest and therefore contribute to increasing the economic functionality of the microalgae. However, this technology is not yet perfectly mastered since certain problems remain to be resolved before scaling up, including the level of stability of the beads to promote the growth of microalgae and minimize cell leakage. The general objective of this project is the study of the immobilization of the freshwater microalga Raphidocelis subcapitata in alginate beads to produce a high quantity of lipids. This general objective has been subdivided into three specific objectives. The first objective aims to determine the alginate and CaCl2 concentrations which ensure the stability of the beads and promote the growth of the microalgae during 10 days of culture. To do this, the microalgae was cultivated in beads produced with different concentrations of alginate and CaCl2. The results showed that, among all the combinations studied, the 2 % alginate and 1 % CaCl2 combination was the best, because it ensures the stability of the beads and the growth of the microalgae, with a low concentration of free cells detected in the culture medium. The second objective is to examine the capacity of the immobilized microalgae to accumulate lipids under stress conditions. The production of biomass and lipids in the microalgae was quantified during its batch culture, with monitoring of the concentrations in nitrogen (N) and phosphorus (P) in the culture medium. The results showed that immobilized R. subcapitata was able to accumulate 24.7 ± 2.5 % of its dry weight in lipids when N and P were depleted. A lipid productivity PL = 29.8 ± 3.0 mg/L/day was recorded during the stationary phase of growth. The third objective aims to increase the lipid productivity of the microalgae by applying a twostep culture strategy. R. subcapitata was first cultivated under optimal N and P supply conditions to obtain maximum biomass production, then the beads were transferred to an N- deficient medium to induce lipid accumulation. By this strategy, immobilized R. subcapitata achieved 37.9 ± 3.8 % of its dry weight in lipids and lipid productivity PL = 40.3 ± 4.0 mg/L/day under N deficiency. Overall, the results obtained made it possible to achieve the general objective of the project. The lipid productivity of immobilized R. subcapitata can be further improved by varying the light intensity and light cycle. Finally, a transition to a semi-industrial scale is desirable before achieving industrial production with this technology.

Date	27 Mar 2024
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Robert Hausler (Supervisor)