Enhanced CO₂ Fixation Through Continuous Cultivation of Microalgae in a Two-Stage Photobioreactor System

The integration of microalgae cultivation into biorefinery systems represents a promising strategy to enhance carbon dioxide (CO₂) sequestration and support sustainable biomass production for diverse biotechnological applications. This study reports the CO₂ fixation performance and biomass productivity of a two-stage photobioreactor (PBR) system employed for the continuous cultivation of Haematococcus pluvialis, utilizing biogenic CO₂ generated from a heterotrophic culture. The first stage (ST1) operated as an autotrophic chemostat (ST1 PBR) with variable dilution rates (0.18-1.00 d^-1), while the second stage (ST2) consisted of sequential high-irradiance columns that promoted biomass and carotenoid accumulation. The two-stage PBR (ST1 + ST2 PBR) achieved a maximum CO₂ assimilation capacity of 0.747g/L/d and up to 94% CO₂ fixation efficiency. The daily CO₂ assimilation rate reached 6.53 g/d, representing a 10% improvement compared with the single-stage system (ST1 PBR). The maximum overall biomass productivity reached 0.332 g/L/d when both PBR stages were operated together, whereas the second stage alone achieved up to 1.18 g/L/d under continuous operation and a maximum biomass concentration of 3.33 g/L during batch-induced carotenogenesis. Nutrient uptake analysis revealed an increasing fraction of unconsumed major nutrients with higher dilution rates. Continuous operation under high irradiance did not induce pigment accumulation, whereas batch mode successfully triggered carotenogenesis over 13 days, resulting in visible pigment production and high biomass yield. Overall, the two-stage PBR system demonstrated high CO₂ capture efficiency, enhanced biomass productivity, and operational flexibility, offering a scalable and sustainable approach for integrating microalgal cultivation into biorefineries while contributing to carbon mitigation and circular bioeconomy goals.