A Sustainable Seawater-Based Cultivation Strategy for Arthrospira platensis: RSM-Based Process Evaluation and Biodiesel Potential Assessment

Sustainable microalgal biodiesel requires cultivation approaches that lower resource demand without compromising biomass performance. Here, Arthrospira platensis was cultivated in natural Marmara seawater collected at different times of the year and in synthetic media, and its response to salt and potassium stress was examined. Biomass production was assessed through a Central Composite Design within a Response Surface Methodology framework. Lipid measurements were retained as descriptive experimental data because the corresponding response model did not reach statistical significance. Optical-density measurements were converted to biomass concentration using a gravimetrically established OD600-to-dry-biomass factor. Total lipids were recovered by a modified Bligh and Dyer extraction, fatty acids were quantified by GC-FID, and cetane numbers were estimated from the FAME profiles. Salt had the strongest statistical association with biomass (p < 0.0001), and potassium was also significant (p = 0.0043). Although the quadratic biomass equation was significant (p = 0.0002; R2 = 0.9537), its highly significant lack of fit (p < 0.0001) restricts its predictive use. Within the tested factor space, the largest predicted biomass value was 1.317 g L−1 at 16.8 g L−1 salt and 0.5 g L−1 potassium; because both settings coincide with the design limits, this point is treated as a boundary maximum rather than a stationary or global optimum. Measured lipid contents spanned 14.67–18.43%. Among the cultivation conditions, the October 2025 seawater culture gave the highest mean estimated cetane number (57.78 ± 0.04; n = 3). The experiments confirm that A. platensis can grow in Marmara seawater, suggesting a possible means of decreasing freshwater use. Since the seasonal seawater batches were not fully characterized before cultivation, however, biological differences among them cannot be assigned to individual seawater constituents. Further work should therefore combine broader experimental ranges and improved model adequacy with contemporaneous chemical characterization of each seawater batch.