Enhancing astaxanthin production in Haematococcus pluvialis QLD by a pH steady NaHCO₃-CO₂-C/NH₄Cl-N culture system

Haematococcus pluvialis is considered as the best source of natural astaxanthin, while its application is limited due to the high production cost. Exploring cost-effective culture systems can be an efficient way to improve economic feasibility. In this study, it was found that biomass concentration and astaxanthin content of H. pluvialis QLD with ammonium as nitrogen source and pH control were similar to the treatment with nitrate as nitrogen source, indicating that the low-cost ammonium can be appropriately utilized when pH is well controlled. Subsequently, the concentration of CO₂ was optimized, and 1% CO₂ was found to be desirable for cell growth and astaxanthin accumulation. Further, a pH steady NaHCO₃-CO₂-C/NH₄Cl-N culture system was explored to improve cell growth and astaxanthin production in H. pluvialis QLD. The results showed that pH was stabilized at 7 to 8 with enhanced biomass concentration and astaxanthin content under a bicarbonate-C/ammonium-N ratio of 2.5:1 and 1% CO₂. Besides, optimal ammonium concentration was assessed, and maximal biomass concentration was obtained at 4.50 mM NH₄Cl. Moreover, light intensity was optimized to enhance astaxanthin production in the culture system. Maximal biomass concentration (3.77 g/L), astaxanthin content (56.87 mg/g), astaxanthin production (216.56 mg/L), and astaxanthin productivity (12.63 mg/L/d) were obtained under a light intensity of 480 μmol/m²/s. The performance of this study was greater than most reports under phototrophic batch cultivation, indicating that the NaHCO₃-CO₂-C/NH₄Cl-N culture system is promising for cost-effective astaxanthin production.