Effect of excess CO2 on semi-continuous microalgae systems: Carbon biofixation

Microalgae-based CO2 fixation is a promising solution for achieving carbon neutrality. However, the low rate of carbon fixation is a significant issue due to gas transfer. A closed cultivation system was employed to overcome this challenge, incorporating headspace micro-positive pressure and elevated total CO2 (TCO2) levels to minimize CO2 loss and improve CO2 mass transfer. Microalgae could thrive and maintain a higher level of system pH within the TCO2 range of 0.12 to 0.49 mol CO2/L absorbent, and high TCO2 levels over 0.36 mol CO2/L absorbent were the optimal range. The semi-continuous microalgae system removed up to 8.72 ± 0.85 g CO2/L/day. Simultaneously, it exhibited phosphate and nitrate removal rates of 50.95 ± 10.71 and 182.14 ± 30.71 mg/L/day, respectively. The study found that 80% of the removed carbon was fixed as organic biomass. An increased level of TCO2 resulted in the accumulation of more lipids within the cells. Moreover, microalgae released carbon fixation products into the extracellular environment, and the dissolved organic carbon fixation rate reached as high as 14.58 ± 1.89 mmol/L/day. These findings suggest that microalgae use multiple pathways for carbon fixation, including biomass organic carbon accumulation, extracellular organic carbon secretion, and inorganic carbon precipitation. Improving carbon fixation rates may benefit the industrial application of microalgae technology, which aims to mitigate carbon emissions.