Enhanced production of Scenedesmus acutus extracellular polysaccharides exhibiting immunostimulation and antioxidant activity

Abstract

Microalgae biomass has been recognized and identified as a source of novel bioactive compounds for its great potential value with various industrial and health promoting applications in aquatic, human as well as animal lives. Scenedesmus acutus is a green freshwater microalgae which is known to sustain life even in low concentration of salts. Recently, the microalgae have been explored for the production of exopolysaccharides (EPS) with their promising applications in different industries. The scope of this study is assessment of antioxidant and immunostimulation activity on peripheral blood mononuclear cells of the EPS isolated from S. acutus and their enhanced production by cultivating the microalgae in nutritional stress conditions. Phenol-sulphuric acid assay confirm the presence of sugar content in alcoholic extraction rich EPS. The results of cell proliferation assay showed that EPS increased the cell proliferation of peripheral blood monocuclear cell (PBMC) in a dose dependent manner from 100 – 1000 μg/mL, after which the saturation for cell growth activity occurred. This result concluded the immunostimulatory activity of EPS extracted from S. acutus. Also DPPH assay have shown the anti-oxidant effect which increases with concentrations. Liquid chromatography-mass spectrometry results revealed the presence of different saccharide sugars composed of various hexoses (glucose, fructose, galactose, mannose) and pentoses (ribose, xylose, arabinose) with phenol groups (Glucogallin). Oligosaccharides (eight monosaccharide units) and tertasaccharides (four monosaccharide units) were also found to be present in EPS as per the mass spectrometry results. The presence of structure of alkyl halide and amine groups with anomeric carbon sugars were confirmed by 1H NMR spectrometry. FTIR analysis showed the presence of amine, sulphate and halo groups due to the detection of N-H, S=O and C-Br bonds. The enhanced production of EPS (approximately 8 fold increase) was obtained when the culture was grown in sulphate stress (1.25g/L) and phosphate stress (0.6 g/L). However, highest sugar content (11% increased) was observed in sulphate (stress (1.25g/L) as compared with normal conditions (MgSO4: 0.75 g/L and K2HPO4: 0.4 g/L) in BG-11 media. In few stress conditions, pigment (carotenoid) and lipids (waxes) were also observed to be increased. Hence, it is concluded that EPS isolated from S. acutus possess antioxidant and immunostimulatory activities that has the potential to hit the therapeutic industries.