3.2. SA, Ca2+ and NO induces exopolysaccharides (EPS) secretion to protect cells from Ni2+-induced damage
The EPS secretion by cyanobacterial cells is a defense strategy to combat from toxicity; which was found to decreased by 23% under Ni2+-stress while upon SA, CaCl2 and SNP addition, a sharp increment in EPS content was noticed with respect to control (Fig.1B). The EPS synthesized by cyanobacteria either are secreted in surrounding medium or attached with cell wall, which mainly functions to protect the cell from environmental stresses like heavy metals and pesticides (Tchounwou et al., 2012; Tiwari et al., 2020). Due to its strong anionic nature (bearing sulphate groups and uronic acids), it is suitable candidate in blocking the entry of toxic heavy metals like cadmium (Jittawuttipoka et al., 2013). In addition, EPS also provides microenvironment to maintain the level of essential elements, which are in limited concentration in surroundings. Therefore, our findings suggests that SA, Ca2+ and NO might have induced the EPS level to restrict/ limit the Ni2+availability to the test organism as is also evident from biochemical detection for intracellular Ni2+ accumulation (Fig. 1D). But this increment in EPS content was further arrested upon c-PTIO and EGTA application thereby suggesting that in absence of Ca2+ or/ and NO, SA was unable to protect the cell from Ni2+-induced toxicity.