Optimisation of lipid and carbohydrate synthesis in Tetradesmus obliquus UPSI-JRM02 under nitrogen stress condition for biofuel feedstock production

The optimisation of lipid and carbohydrate yields and productivity is important for enhancing microalgae biofuel production. This study aims to optimise the growth conditions of Tetradesmus obliquus UPSI-JRM02 for high lipid and carbohydrate production and to examine the synthesis of these biomolecules under nitrogen stress conditions. The optimisation study was carried out using response surface methodology with Design Expert software. Four variables (nitrogen concentration, temperature, pH, and light intensity) were optimised to enhance T. obliquus productivity. T. obliquus was then cultured in optimised conditions before being introduced in nitrogen stress condition to further increases the production of lipid and carbohydrate yields. The results indicated that the highest lipid (33 mg/L/day), carbohydrate (51 mg/L/day) and biomass (115 mg/L/day) productivity were produced at a nitrogen concentration of 400 mg/L NO3-at 36 oC with pH of 9.8, and light intensity of 23,500 lux. The lipid and carbohydrate yields obtained under optimised conditions were 24% and 41%, respectively. Meanwhile, the highest lipid and carbohydrate yields achieved under nitrogen stress conditions were 36.5% and 23%, respectively. These results suggested carbon was shifted into lipid synthesis under nitrogen stress condition. The fatty acid methyl ester synthesised under nitrogen stress condition was comprised of 54.07% polyunsaturated fatty acid, 43.33% saturated fatty acid, and 2.6% monounsaturated fatty acid. The fluorescence and transmission electron microscopy images further confirmed the presence of lipid bodies and starch granules in T. obliquus. As a conclusion, the yield and productivity of lipid and carbohydrate of T. obliquus UPSI-JRM02 had sucessfully increased at the optimum conditions. Meanwhile, the nitrogen stress condition improved only the lipid yield of T. obliquus. The implication of this study shows the potential of T. obliquus UPSI-JRM02 to produce promising biofuel feed stock for future application.