Developing a CO₂ bicarbonation absorber for promoting microalgal growth rates with an improved photosynthesis pathway

In order to solve the problems of the short residence time and low utilization efficiency of carbon dioxide (CO₂) gas added directly to a raceway pond, a CO₂ bicarbonation absorber (CBA) was proposed to efficiently convert CO₂ gas and sodium carbonate (Na₂CO₃) solution to sodium bicarbonate (NaHCO₃), which was dissolved easily in the culture medium and left to promote the microalgal growth rate. The CO₂ gas reacted with the Na₂CO₃ solution (initial concentration = 200 mM L^-1 and volume ratio in CBA = 60%) for 90 min at 0.3 MPa to give the optimized molar proportion (92%) of NaHCO₃ product in total inorganic carbon and increase the microalgal growth rate by 5.0 times. Quantitative label-free protein analysis showed that the expression levels of the photosystem II (PSII) reaction centre protein (PsbH) and PSII cytochrome (PsbV2) in the photosynthesis pathway increased by 4.8 and 3.4 times, respectively, while that of the RuBisCO enzyme (rbcL) in the carbon fixation pathway increased by 3.5 times in Arthrospira platensis cells cultivated with the NaHCO₃ product in the CBA at 0.3 MPa.