CO₂ capture by spirulina platensis RIA2 in bubble column photobioreactors

Abstract

    Practical carbon‑capture options are needed alongside source abatement to meet mid‑century climate goals. Microalgae offer a biological route that operates at ambient conditions while generating valuable biomass; within this portfolio, Spirulina is robust under high pH/alkalinity and compatible with simple photobioreactors. This study evaluates CO₂ capture by Spirulina platensis RIA2 cultivated for 21 days in 8‑L bubble‑column photobioreactors (working volume 5 L) under two aeration modes: ambient Air and CO₂‑supplemented (≈10,000–30,000 ppm). We monitored biomass growth, nitrate and phosphate uptake, and CO₂ fixation (Level A, biomass‑equivalent). CO₂ enrichment accelerated early growth (doubling time ≈ 2.92 days) and produced an earlier biomass peak (1.576 g L⁻¹ on day 11), whereas Air reached a higher peak (3.064 g L⁻¹ on day 16) and maintained a substantially higher cycle‑average biomass. Over 21 days, cumulative fixed CO₂ was 1.047 g CO₂ L⁻¹ with CO₂ supplementation versus 0.864 g CO₂ L⁻¹ with Air, but peak CO₂‑fixation productivity (PCO₂) was higher under Air (0.551 vs 0.395 g CO₂ L⁻¹ day⁻¹). Nitrate decreased more strongly in Air (≈88.9%) than in CO₂ (≈54.5%), while phosphate removal was comparable. Overall, CO₂ supplementation trades early acceleration for reduced late‑phase stability unless pH/nutrient control is applied, whereas Air favors steadier long‑term performance. These findings clarify operational trade‑offs for low‑complexity bubble‑column systems and support pilot integration of algae‑based CO₂ capture near point sources.