Exopolysaccharides (EPS) isolated from microalgae are promising immune cell proliferation agents, that could be potentially used as immunostimulants. In the current study, Scenedesmus acutus ( S. acutus ) was grown under varying nutrient (sulphur and phosphorus) concentrations to enhance the EPS production, and the isolated EPS were assessed for their effect on cell proliferation using peripheral blood mononuclear cells (PBMC). Five different concentrations of MgSO 4 (0, 0.25, 0.5, 1.0 and 1.25 g/L) and K 2 HPO 4 (0, 0.2, 0.6, 0.8 and 1.0 g/L) were taken as compared to the normal culture conditions (0.75 g/L MgSO 4 and 0.4 g/L K 2 HPO 4 ) with the intention to enrich EPS secretion. LC–MS, FTIR and NMR analysis revealed that isolated EPS have the characteristic spectrum of hetero-polysaccharides (octa-saccharides). Immunostimulatory property of EPS was demonstrated by their ability to augment PBMC proliferation as measured by MTT assay. Further, increase in the glucose content and proliferative index was observed for EPS obtained under higher concentrations of MgSO 4 (1 and 1.25 g/L) and K 2 HPO 4 (0.6 and 0.8 g/L) relative to normal culture conditions. Effects of the generated EPS under varying concentration of MgSO 4 ( r = 0.84–0.99) and K 2 HPO 4 ( r = 0.76–0.97) remained strongly correlated with cell count, chlorophyll content, total biomass, glucose, proliferative index and its scavenging activity. Collectively, our data not only showed that EPS generated by S. acutus under higher concentration of K 2 HPO 4 and MgSO 4 possess improved immunostimulatory properties, but also provides convincing evidence towards nutritional optimization of alga for enhanced EPS production with better bioactivities. Supplementary Information The online version contains supplementary material available at 10.1007/s13205-021-02720-z.

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