This study reports A. platensis capacity to capture and convert CO2 into biomass. The cyanobacterium was cultivated under controlled conditions with a 5 % v/v CO₂-enriched atmosphere at different photoperiods (light/dark): 12 h/12 h, 20 h/4 h, and 24 h (continuous light). The photoperiod influence on biomass growth and its effect on CO₂ uptake was evaluated by analyzing the supernatant dissolved inorganic carbon and nitrate concentration. The experimental dissolved inorganic carbon values were compared with numerical computations obtained from an established model. While the control without biomass stabilized at 36 ± 1.07 mgC/L of dissolved inorganic carbon, the A. platensis cultures stabilized at 675 ± 44.5 mgC/L on average. These results suggest that the alkalinity induced by photosynthesis and nitrate bio-assimilation enhances CO₂ dissolution in the culture medium. The photoperiod increase from 12 h/12 h to 24 h proved to boost the biomass productivity and CO₂ fixation rate from 0.05 to 0.13 g/L/d and 71.92 to 216.84, respectively.