Role of culture solution pH in balancing CO₂ input and light intensity for maximising microalgae growth rate

The interplay between CO₂ input and light intensity is investigated to provide new insight to optimise microalgae growth rate in photobioreactors for environmental remediation, carbon capture, and biomass production. Little is known about the combined effect of carbon metabolism and light intensity on microalgae growth. In this study, carbonated water was transferred to the microalgae culture at different rates and under different light intensities for observing the carbon composition and growth rate. Results from this study reveal opposing effects from CO₂ input and light intensity on the culture solution pH and ultimately microalgae growth rate. Excessive CO₂ concentration can inhibit microalgae growth due to acidification caused by CO₂ dissolution. While increasing light intensity can increase pH because the carboxylation process consumes photons and transfers hydrogen ions into the cell. This reaction is catalysed by the enzyme RuBisCO, which functions optimally within a specific pH range. By balancing CO₂ input and light intensity, high microalgae growth rate and carbon capture could be achieved. Under the intermittent CO₂ transfer mode, at the optimal condition of 850 mg/L CO₂ input and 1089 μmol/m²/s light intensity, leading to the highest microalgae growth rate and carbon fixation of 4.2 g/L as observed in this study.