Application of carbon dioxide for Thermosynechococcus FA-CM-4201 cultivation for thermostable phycocyanin

Phycocyanin, a natural blue dye, is widely used in the food, beverage, and cosmetic industries, but its use is limited due to heat sensitivity. Therefore, thermostable phycocyanin has been sought to expand its application. Thermosynechococcus sp. FA-CM-4201, an isolated microalgae from a hot spring in northern Thailand, has the potential to be a source of thermostable phycocyanin. However, the production cost should be optimized for cost-effective cultivation. Since microalgae can sequester CO₂ as a carbon source for photosynthesis and subsequently reduce CO₂ emissions, this study aims to investigate the effects of CO₂ concentration in different growth medium on Thermosynechococcus FA-CM-4201 growth and phycocyanin production. The cells were grown in modified BG-11 and modified Zarrouk medium with 1 and 5% CO₂ feeding, 50 ^oC, and 80 µmol photons.m^-2.s^-1 00for 7 days. The results indicated that CO₂ stimulated growth and phycocyanin accumulation. Applying CO₂ to modified BG-11-grown cultures significantly increased growth, phycocyanin content, and yield compared to standard medium with ambient air. At 1% CO₂, cells achieved the highest growth rate (μ = 0.062 h−1), phycocyanin content (206.01 mg g^-1), and biomass yield (1.37 g L^-1). Whereas the modified Zarrouk-grown cells required 5% CO₂ to achieve the highest phycocyanin content and yield. These findings demonstrate that Thermosynechococcus could grow by replacing standard medium carbon with CO₂ without decreasing algal growth. Therefore, the reduction in carbon from commercial fertilizer will pave the way to reduce the thermostable phycocyanin production cost.