Travail de fin d'études: "Control mechanism and recommendations for eliminating a predatory contaminant in Chlorella cultures."

Abstract

Extensive research spanning several years has focused on exploring the potential of Chlorella as a large-scale biomass source for various applications, including food, fertilizers, pharmaceuticals, chemical compounds, bioremediation, and energy production. However, a significant challenge arises from the susceptibility of Chlorella cells to being consumed by predators, resulting in decreased productivity. One particularly destructive predator in commercial Chlorella cultures is Poterioochromonas malhamensis, a mixotrophic flagellate known for rapidly causing the collapse of microalgal mass cultivation within a short period. Consequently, there is an urgent need for effective and safe methods to control and prevent contamination by the predatory organism. In this study, we explored the cultivation of Chlorella vulgaris in an ammonium nitrogen (AN) medium under alkaline conditions as a means to control P. malhamensis in laboratory conditions. Ammonium bicarbonate (NH4HCO3) was used as the source of AN. Our results demonstrated that NH4HCO3 concentrations corresponding to AN concentration of 120, 240, and 360 mg/L effectively controlled P. malhamensis, resulting in a remarkable 95% mortality within 24 hours. Furthermore, the NH4HCO3 concentration corresponding to a AN concentration of 240 mg/L exhibited the highest biomass concentration, comparable to that of the control group conducted in nitrate medium. To validate the effectiveness of the method, co-cultures of C. vulgaris and P. malhamensis was conducted using an AN concentration of 240 mg/L. The biomass yield of C. vulgaris in treated co-cultures with AN was similar to that of the monocultures conducted in nitrate medium, while the untreated co-cultures experienced a contamination event and complete crash. Considering the AN toxicity to P. malhamensis, the relative safety to C. vulgaris, and its low cost and chemical safety, cultivation of Chlorella in NH4HCO3 under alkaline pH conditions emerges as a technically and economically feasible, as well as safe approach for controlling P. malhamensis contamination in outdoor, large-scale cultivation of Chlorella.