چكيده لاتين
Dunaliella, a unique genus of unicellular green microalgae, has attracted considerable attention due to its significant ability to tolerate extreme environmental conditions, particularly high salinity. Its simple structure and physiological flexibility make it an ideal model organism for environmental and biotechnological studies. One of the most notable features of Dunaliella is its capacity to accumulate substantial amounts of β-carotene, a carotenoid pigment with strong antioxidant properties widely used in the medicinal, food, and cosmetic industries. Sodium bisulfite (NaHSO₃) is a sulfur-containing compound with reducing and antioxidant properties, commonly applied in the food and pharmaceutical industries as well as an additive in culture media. This study aimed to investigate the effects of sodium bisulfite on growth, photosynthetic pigments, respiration, photosynthesis, and oxidative damage under salt stress and its potential in enhancing the salt tolerance in Dunaliella cells. The cells were cultured in a growth medium containing 1.5M NaCl and different concentrations of sodium bisulfite ranging from zero, 0.75, 1.25, 2.5, 5, 7.5, 10, 12.5, 15, 17.5, and 20 mM. Based on growth rate, total chlorophyll, and β-carotene content, the concentrations of 1.25 and 2.5 mM of sodium bisulfite were identified as optimal for subsequent experiments. The effect of salt stress of 3.0 M NaCl was applied in the presence of two selected concentrations of sodium bisulfite. The results revealed that sodium bisulfite treatment significantly improved all measured parameters and effectively mitigated the adverse effects of salinity. For measuring the respiration and photosynthesis, algal cells were cultured in 1.5 M NaCl and treated with 1.25 or 2.5 mM sodium bisulfite. Algal culture was harvested after one week, centrifuged, and the pellet resuspended in phosphate buffer containing 1.5 M NaCl. The cells were subjected to salt stress from 1.5 to 3.0 M NaCl by direct addition of the 4.5 M NaCl phosphate buffer solution. The uptake or evolving O2 was measured using the polarography method before and after salt stress. Results show that sodium bisulfite improved photosynthesis and respiration, probably through enhancing cyclic electron flow, and could partially mitigate the adverse effects of 3 M NaCl stress. Total antioxidant capacity was assessed using the DPPH assay. Sodium bisulfite treatment, due to its antioxidant and oxygen-reducing properties, seemingly led to decreased oxidative stress and significantly reduced lipid membrane damage under salinity stress. In general, sodium bisulfite demonstrated a protective role by mitigating salt-induced oxidative damage and enhancing the physiological performance of Dunaliella cells under saline conditions.
