Optimization of Phycocyanin Production from Arthrospira platensis: Response Surface Modeling and FTIR Characterization

Arthrospira platensis provides a cyanobacterial source of C-phycocyanin (C-PC). We evaluated 15 Box–Behnken experimental runs across 13 distinct factor combinations, varying NaHCO3 addition (5–20 g L−1), illuminance (3500–9500 lux), and macronutrient stock dose (10–30 mL L−1), followed by treatment-level response surface analysis and exploratory ATR-FTIR of three terminal samples. Crude extract C-PC concentration ranged from 0.0272 to 0.4188 mg mL−1; recovery calculated for 0.1 g of dry biomass extracted in 10 mL was 2.72–41.88 mg g−1. The quadratic model had R2 = 0.972 and 15-fold run-level cross-validation Q2 = 0.591. A reduced model achieved Q2 = 0.698 only when its structure was fixed before cross-validation; repeating AICc selection within each training fold reduced Q2 to 0.463, indicating model selection uncertainty. The highest predicted concentration within the investigated bounds was 0.4326 mg mL−1 near 12.34 g L−1 NaHCO3, 7285 lux, and the upper tested macronutrient dose (30 mL L−1). Three archived validation readings averaged 0.4378 ± 0.0082 mg mL−1 (mean ± SD), with 1.2% relative deviation; independent culture identity could not be verified from the available records. ATR-FTIR showed similar protein-associated fingerprints among T1, T8, and T15 but did not establish biochemical reproducibility or biocatalytic function. Consequently, this is an exploratory within-domain optimization of crude extract pigment concentration; extrapolation, spectrophotometric linearity, and proposed enzymatic valorization require further validation.