Enhancing the production of high-value bioproduct-enriched Spirulina biomass through salinity and phytohormone-stimulated two-stage cultivation in hydroponic effluent

• Two-stage salinity–phytohormone culture enhanced Spirulina productivity. • Food-grade phycocyanin obtained for colorant and nutraceutical use. • Lipid yield improved for both biodiesel and health product applications. • Over 85% nutrient removal achieved from hydroponic wastewater. • CO₂ fixation >762 mg/L/day supports carbon capture and bioeconomy goals. This study aimed to enhance biomass yield and the accumulation of phycocyanin and lipids in Spirulina BSF using a two-stage cultivation system under blue light, with hydroponic effluent (HE) as a low-cost growth medium, compared with a one-stage cultivation over 10 days. In the two-stage cultivation, cultures were exposed on day 8 to varying salinity levels (0–3.5%), salicylic acid (SA, 0–200 µM), and indole-3-acetic acid (IAA, 0–3 mg/L), whereas the one-stage cultivation lacked these stressors. The combined treatment of 2% salinity, 100 µM SA, and 0.2 mg/L IAA significantly increased biomass (4.53 g/L) by 132%, phycocyanin (4.41 g/100 g) by 261%, and lipid content (17.22 g/100 g) by 50.8% compared with one-stage cultivation, while maintaining high phycocyanin purity (1.45), suitable for food-grade applications. Spirulina BSF lipids showed high polyunsaturated fatty acid (PUFA) content (25.33%) and met biodiesel standards (EN 14214, ASTM D6751 , TH2020 ). Cultivation removed >85% of nutrients from HE and fixed CO₂ at 762.5 mg/L/day, demonstrating effective wastewater remediation and carbon capture. This integrated approach highlights the potential of combining abiotic stress and phytohormones to convert agricultural wastewater into high-value products, supporting sustainable biorefinery and circular bioeconomy development.