Dual Valorization of Indigenous Chlorella sp. RSA402: UVB–Induced Vitamin D Production and Extracellular Polysaccharide (EPS)-Mediated Green Synthesis of Zinc oxide (ZnO) Nanoparticles for Oil Emulsification

An indigenous microalgae Chlorella sp. RSA402 was screened for integrated biomass and Extracellular polysaccharides (EPS) valorization. Growth kinetics revealed the mid-log phase on day 7, determined from cell density, dry biomass accumulation, and chlorophyll a content. Biomass, at this phase corresponded with elevated biochemical constituents, including protein (400±30.02 mgL⁻¹) and carbohydrate (250±12.05 mgL⁻¹) concentrations. UVB (Ultraviolet B) irradiation (295 nm) of day-7 biomass resulted in a vitamin D yield of 1.5 mg mg⁻¹ dry weight, as confirmed by HPLC analysis. EPS production reached 1.5±0.7 gL⁻¹, comprising 17.5±1.9% carbohydrates, protein (6.17±1.2%) and uronic acids (5±0.9%) respectively, and was employed as a sole biogenic capping and stabilizing agent for green synthesis of ZnO nanoparticles. The synthesized EPS-ZnO exhibited reduced crystallite size (8.58 nm) with enhanced colloidal stability (zeta potential −27.1 mV; hydrodynamic diameter 232 nm by DLS). FTIR analysis confirmed biomolecular functional group interactions during nanoparticle formation, while TGA demonstrated improved thermal resistance with minimal weight loss (9.15%). EDAX spectra validated the elemental purity of ZnO. The EPS–ZnO nanocomposite achieved 67.34±0.12% efficiency on diesel emulsification. Overall, the study represented a sustainable nutraceutical–nanotechnology integrated biorefinery framework.