Optimizing Indigenous Spirulina platensis Culture: Unveiling the Role of Potassium Nitrate in Growth, Pigment, and Protein Synthesis

Spirulina platensis is a filamentous cyanobacterium widely utilized as a human dietary supplement and as a protein-rich feed ingredient in aquaculture. This study evaluated the effect of potassium nitrate (KNO₃) on the growth performance, biomass yield, pigment composition, and protein content of a native strain of S. platensis. The experiment was conducted in nitrogen-free Zarrouk medium supplemented with four KNO₃ concentrations (1.0, 1.5, 2.0, and 2.5g L^-1). Cultures were maintained under photoautotrophic batch conditions for four days, with continuous illumination (4,500 lux) and salinity of 15 ppt. Results showed that KNO₃ concentration significantly (P< 0.05) influenced growth rate, dry weight, protein, chlorophyll-a, and carotenoid levels. The highest specific growth rate (0.839 day^-1) and biomass concentration (1.392g L^-1) were achieved at 2.5g L^-1 KNO₃, representing a 1.3-fold increase compared to the lowest concentration (1.0 g L^-1). Similarly, chlorophyll-a, carotenoid, and protein contents peaked at 2.5 g L^-1 KNO₃, though values were not significantly different (P> 0.05) from those observed at 2.0 g L^-1. These findings suggest that KNO₃ concentrations between 2.0 and 2.5g L^-1 provide optimal conditions for maximizing biomass production, protein accumulation, and pigment synthesis in S. platensis. This range may serve as a practical benchmark for scaling up cost-effective Spirulina cultivation for nutritional and industrial applications.