Deciphering salinity tolerance in the cyanobacterium Anabaena sphaerica: an evaluation of physiological and biochemical adjustments

Besides being a determinant of species distribution around the globe, salinity over the threshold limit imparts detrimental effects on life forms. The present study deciphered the tolerance of heterocytous cyanobacterium Anabaena sphaerica to 77.5, 100, and 200 mM NaCl by analyzing physiological and biochemical adjustments within the cells. Exposing the cyanobacterium to high NaCl reduces growth dynamics, photosynthesis, and membrane stability. Intracellular Na+ accumulation not only induces ionic imbalances by increasing Na+/Mg2+ and Na+/K+ ratios, but also causes oxidative burst in the cell.  Moreover increased glutathione, proline, and sucrose contents along with higher enzymatic antioxidants such as superoxide dismutase and catalase, delineated cyanobacterial resilience against both osmotic and oxidative stresses. Additionally, Fourier-transform infrared (FTIR) spectroscopy exhibited carbohydrate accumulation in the stressed cells as a function of reprogrammed carbon allocation, which might also occur as an adaptive measure. However, escalation in the activities of nitrate reductase and nitrite reductase at 77.5 and 100 mM NaCl depicted an increase in nitrate assimilation, indicating the ability of cyanobacterium to withstand at least 100 mM NaCl in the surrounding. Hence, A. sphaerica displays significant tolerance to salinity and, thus, can play a crucial role in ameliorating salinity from salt-affected paddy fields.