The current study deals with the biological response variations on some biochemical properties of the freshwater green algae Chlorella vulgaris by exposure to different Mg²⁺ concentrations (5, 10, and 15) mg/L. Physiological and biochemical parameters, including growth curve, doubling time, photosynthesis pigments, total protein and carbohydrates were investigated. Moreover, enzymatic parameters such as catalase (CAT), superoxide dismutase (SOD), and reactive oxygen species (ROS) were also examined. The maximum growth rate was 0.482/day during the 9th day with 5 mg/L Mg²⁺, while the minimum was 0.019/day during the 13th and 14th days with 10 mg/L Mg²⁺. Furthermore, doubling time ranged between 15.501 during the 13th day with 10 mg/L Mg²⁺, and 0.624 during the 9th day with 5 mg/L. The maximum value of chlorophyll-a was 0.157 μg/mL during the 1st day with 10 mg/L Mg²⁺, while the minimum was 0.062 μg/mL during the 14th day with 15 mg/L Mg²⁺. The carotenoid ranged between 0.029 μg/mL during the 7th day with 15 mg/L Mg²⁺ and 0.002 μg/mL during the 14th day. The maximum protein value was 9.620 μg/L during the 1st day with 15 mg/L Mg²⁺, while the minimum was 1.772 μg/L during the 14th day with 15 mg/L Mg²⁺. The carbohydrate showed a maximum value of 0.824 μg/mL during the 1st day with 10 mg/L Mg²⁺, while the minimum was 0.293 μg/mL during the 14th day with 15 mg/L. Moreover, SOD ranged between 0.884 unit/L during the 1st day with 15 mg/L Mg²⁺, and 0.073 unit/L as the minimum value during the 14th day with 15 mg/L Mg²⁺. The maximum value of ROS was 3.627 mM/mL during the 1st day with 5 and 15 mg/L Mg²⁺, while the minimum was 1.674 mM/mL during the 14th day with 10 mg/L. The results show that values of CAT ranged between 0.200 unit/mL during the 7th day with 10 mg/L Mg²⁺, and 0.010 unit/mL during the 14th day with 15 mg/L. Overall, 5 mg/L for biomass production and 15 mg/L for protein and carbohydrate production were optimum doses.

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