Salt-regulated triacylglycerol accumulation and biodiesel quality of Scenedesmus sp. BHU1 under two-stage cultivation

• Stage I control cells showed higher CO₂ fixation rate. • FTIR-SDS showed dominating lipid group and TGA confirmed stable thermal traits. • Stage II showed higher calorific value, theoretical methane potential, and fuel efficiency. • GC-MS revealed dominant C16–C18 saturated fatty acids. • Biodiesel met EN 14214 and ASTM D6751 standards. Microalgae efficiently sequester carbon and accumulate neutral lipids; however, challenges persist in achieving high triacylglycerol (TAG) and biofuel yields. This study presents a novel two-stage salt stress strategy that enhances neutral lipid accumulation, jet fuel production, and biodiesel quality in Scenedesmus sp. To address this, Scenedesmus sp. was cultivated under varying NaCl concentrations (0–0.4 M) and time intervals (0–12 days) using a two-stage process. In stage I, control cells (0 M NaCl) showed a higher CO₂ fixation rate (122.06 mg L⁻¹ d⁻¹), while 0.4 M NaCl-treated cells recorded the highest neutral lipid content (77.01%). In stage II under 0.4 M NaCl at 8 days, 1.48 kg biomass yielded 1 kg microalgal oil, and 2.59 kg biomass produced 1 kg jet fuel. Further, second derivative spectra from FTIR spectroscopy revealed shifts in functional groups favoring neutral lipid synthesis (peaks at 2923 cm⁻¹ and 1746 cm⁻¹), while thermogravimetric analysis (TGA) showed decomposition of lipids and carbohydrates during pyrolysis. Additionally, CHNS analysis revealed maximum higher heating value (25.64 MJ/kg), and lower heating values (24.71 MJ/kg) in 8 th -day biomass. GC-MS analysis revealed a predominance of saturated fatty acids (SFAs; 66.67%), particularly C16–C18, over monounsaturated fatty acids (MUFAs; 18%) and polyunsaturated fatty acids (PUFAs; 16%). Notably, biodiesel produced from 8 th day biomass met both the EN 14214 and ASTM D6751 standards, demonstrating the potential of Scenedesmus sp. for biodiesel feedstock. Notably, biodiesel produced from the 8 th -day biomass met both the EN 14214 and ASTM D6751 standards. Thus, current study highlights the potential of salt supplementation in Scenedesmus sp. BHU1 to enhance triacylglycerol, supporting its use as a biodiesel feedstock.