Enhanced Carbon Dioxide Biofixation and Lipid Production of Chlorella sp. Using Alkali Absorber and Strategic Carbon Dioxide Supply

As carbon dioxide (CO₂) emissions rapidly increase, alternative strategies are needed to capture and mitigate carbon dioxide using microorganisms. To enhance CO₂ fixation and biomass production in microalgae, achieving the optimum concentration of dissolved carbon in the culture medium is essential. This study focuses on enhancing biomass production and CO₂ biofixation efficiency in Chlorella sp. BRE5 by increasing dissolved inorganic carbon (DIC) through the strategic use of sodium hydroxide (NaOH) and CO₂. Under shake flask study, the highest specific growth rate of 0.195 day⁻¹, biomass productivity of 123.2 mg/L/day, and CO₂ biofixation rate of 231.6 mg/L/day were found at NaOH dose of 0.25 g/L with CO₂ (1%, v/v) supplementation. Further, optimized NaOH with different supply strategies of 1% CO₂ was conducted in a photobioreactor (PBR) study. The best result was observed in PBR, where 1% CO₂ strategically sparged (3-day intervals) with optimum NaOH dose. Under this condition, biomass yield, CO₂ consumption rate, lipid productivity, and lipid content were found to be 2.25, 2.25, 4.19, and 1.87 times higher than the control. The outdoor cultivation of microalgae using a DIY bottle bioreactor (DIY BBR) was performed, resulting in less biomass and lipid productivity than that of the PBR study due to uncontrolled environmental conditions. The fatty acid methyl ester (FAME) profile comprised C16-C18 (84.86–90.69%), indicating the suitability for biodiesel production. This strategic supply of combined NaOH and CO₂ enhances DIC in the medium, facilitating both the CO₂ biofixation rate and biomass production.