From lab to pilot scale: a low-cost fertilizer approach for astaxanthin production in Haematococcus pluvialis

Haematococcus pluvialis is one of the main commercially cultivated microalgae used for the production of natural astaxanthin. This potent antioxidant is naturally accumulated under stressful conditions, including high light intensity, nutrient deprivation, and osmotic stress. Nevertheless, high production costs, primarily associated with culture medium ingredients, remain a major bottleneck in industrial production. This study first evaluated carbon supplementation (NaHCO₃) and increased nitrogen (NaNO₃) concentrations to identify conditions that enhance biomass production, followed by the substitution of conventional N, P, K, and Mg sources with agricultural fertilizers to develop a cost-effective formulation for pilot-scale applications. The highest astaxanthin accumulation was achieved when 1.5 g/L carbon was added to the growth medium. Nitrogen supply had a lesser impact. Agricultural fertilizers, including ammonium nitrate (NH₄NO₃, 33.5% N), monoammonium phosphate (MAP, 12% N and 61% P₂O₅), phosphoric acid (PA, 86% H₃PO₄), potassium nitrate (KNO₃, 13.5% N and 46% K₂O), potassium sulfate (K₂SO₄, 51% K₂O), and magnesium sulfate (MgSO₄, 16% MgO), were subsequently used to replace conventional nutrient sources and develop low-cost formulations for outdoor cultivation. Among the tested formulations, F4, containing ammonium nitrate, potassium sulfate, and monoammonium phosphate, showed the best growth performance, reaching 9.85 × 10⁵ cells/mL and a biomass concentration of 3.15 g/L after 45 days, surpassing the carbon-supplemented and standard BBM conditions by 7% and 43%, respectively. When this formulation was used for pilot-scale cultivation, it maintained high performance during the green phase in closed photobioreactors and the red stage in open raceways, reaching a biomass concentration of 2.5 g/L and an astaxanthin yield of 350 mg/L after 45 days of cultivation. This formulation reduced medium-related biomass production costs by approximately 97.2-99.6% and offers a promising strategy for cost-effective, large-scale natural astaxanthin production.