Production and characterization of microalgal exopolysaccharide as a reducing and stabilizing agent for green synthesis of gold-nanoparticle: a case study with a Chlorella sp. from Himalayan high-altitude psychrophilic habitat

A Chlorella sp. was isolated from a brackish water high-altitude lake of the Northern-Western Himalayas and selected on the basis of its ability to secrete substantial amount of exopolysaccharide (EPS). The purified EPS was estimated to have molecular weight (Mw) of approximately 1.52 × 105 Da with polydispersity of 2.315 and showed amorphous dense morphology. The production of the EPS was maximized to ≅ 460 mg L−1 through nutrient optimisation. The monosaccharide composition of EPS showed the presence of hexose (glucose, galactose mannose), pentose (xylose, arabinose), deoxy (rhamnose), amino (galactosamine) and acid (galacturonic acid and glucuronic) sugars. The one-dimensional proton NMR and FT-IR analysis confirmed the presence of aliphatic groups. The pseudo-plastic rheology and the antioxidant activity of the EPS suggested its suitability for industrial applications. Importantly, the isolated polysaccharide exhibits its potential for easy and single pot biosynthesis of polysaccharide-capped gold-nanoparticles (AuNPs). The polysaccharide-capped AuNPs displayed exceptional stability at extended pH range and high salinity and suggest that this algal EPS can be utilized for its biotechnological application.