Microalgae-based photobioreactors (PBRs) have gained attention as a sustainable solution for indoor air quality (IAQ) control. This study investigates indoor CO₂ absorption performance of Spirulina maxima (S. maxima) in NaHCO₃-limited cultivation (standard: NaHCO₃-free medium = 1:1 v/v%) of a lab-scale PBR system. Cultivation performance of three medium amendments (standard, 50% NaHCO₃, and NaHCO₃-free) was compared by observing cell growth for 30 days in a controlled environment. Empirical examinations were conducted to evaluate the algal CO₂ uptake, and overall system performance in the culture volumes of 2, 4, and 7 L and natural indoor CO₂ concentration of ~1100 ppm. We found CO₂ was reduced by ~55%, in an air chamber of 0.064 m³, showing the greatest mitigation rate (~20%) on Day 4. Under a high concentration of CO₂ (10,000 ppm), the CO₂ levels were decreased up to ~90% before saturation. This research provides valuable insights into the development of S. maxima-activated IAQ control systems for airtight buildings.