چكيده انگليسي :
Today, the world is facing a crisis of energy resources, and limited fossil fuel resources are unable to meet growing energy demand. In this study, during three experiments, the effects of different concentration of nitrogen, phosphorus and sulfur in the culture medium on biomass production and biochemical composition of Isochrysis galbana microalgae were investigated. After considering the different initial concentrations of nitrogen (0, 36, 72, 144 and 288 mg/l) in Valne culture medium, the minimum and maximum cells density (2×105 cells/ml) after 13 days Cultivation under different nitrogen concentrations was obtained at 0 and 144 mg of nitrogen (9×105 cells/ml) and (1.55×107 cells/ml), respectively. In terms of biochemical composition, with decreasing nitrogen concentration from 144 to 0 mg/l, chlorophyll a concentration ranged from 0.41 to 0.11 mg/l and chlorophyll b decreased from 0.56 to 0.18 mg/l, respectively. Microalgae grown at 0 mg/l showed the highest carbohydrate content with 47% cell weight. The maximum protein content (36.3%) was obtained at a concentration of 36 mg/l nitrogen, the total values of lipid content in the experimental unit with concentrations of 0, 36, 72, 144 and 288 mg/l nitrogen were 17.2, 0.25, 29.4, 30.6 and 21.6 percent, respectively. The results showed that the concentration of nitrogen in the culture medium caused a significant increase in the amount of lipid. Also reducing the nitrogen concentration from 72 to 0 mg/l, SFAs increased from 36.63 to 75.79% and at the same time MUFAs decreased from 25.82% to 18.65%. Also reducing the nitrogen concentration from 72 to zero mg/l, SFAs increased from 36.63 to 75.79% and at the same time MUFAs decreased from 25.82% to 18.65%. In addition, PUFAs (linoleic acid, linoleic acid, α-linolenic acid, and Eicosapentaenoic acid) under nitrogen deprivation were reduced by 6.78 times compared with adequate nitrogen 72 (mg/l) conditions. In the second part of this study, the growth of microalgae I. galbana under different phosphorus concentrations (0, 2, 4, 8 and 16 mg/l) was investigated. The initial concentration of algae 2×105 cells/ml changed significantly under different phosphorus conditions over a 13-day period. Cellular concentrations of minimum (3.75×106 cells/ml) and maximum (9.25×106 cells/ml) were obtained at 0 and 8 mg/l of phosphorus, respectively. Phosphorus restrictions reduce the amount of cellular pigments. By reducing the phosphorus concentration from 4 to 0 mg, the chlorophyll a concentration decreased from 0.66 and 0.18 mg/l, respectively, and the chlorophyll b decreased from 0.52 to 0.16 mg/l, respectively. The minimum and maximum total carotenoids at 0 and 8 mg/l of phosphorus were 0.84 and 0.03 mg/l, respectively. In study of the biochemical composition of this algae, cells grown at full phosphorus deprivation showed the highest carbohydrate content at 23.53%. The percentage of protein accumulated at different concentrations of phosphorus ranged from 20.14 to 34.56% of cell weight. The maximum protein content (34.56 %) was obtained at a concentration of 8 mg/l phosphorus, but significantly higher than the experimental units with a concentration of 4 and 16 mg/l of phosphorus. Total lipid content with concentrations of 0, 2, 4, 8 and 16 mg / l, respectively were 36.2%, 24.0%, 23.2%, 21% and 19.1% in the experimental unit, respectively. The results showed that a decrease in phosphorus concentration in the culture medium caused a significant increase in lipid content. The results also showed that the concentration of SFAs and MUFA was lower at concentrations lower than 8 mg/l of phosphorus. In general, according to the goals of algae breeding in order to use biomass for various industries such as aquaculture, food industry or biofuel production, various biochemical compounds can be obtained by manipulating the culture medium.
