Experimental carbon dioxide biofixation and biomass production by microalgae

Biological carbon capture from CO₂-enriched air (0.04-20 vol.% CO₂) representative of flue-gas CO₂ fractions was evaluated using Chlorella vulgaris cultivated in BG-11 under controlled reactor conditions while varying key operating factors. Inlet CO₂ concentration, temperature, initial pH, aeration rate, and light intensity were examined within effective ranges. Biomass productivity and CO₂ biofixation were quantified from mass accumulation using a representative biomass carbon fraction. The effects of the selected parameters on microalgal concentration and fixed carbon were analyzed in detail. Results showed that CO₂ supply dominated performance. After a short acclimation, 20% CO₂ consistently delivered the highest biomass (final ≈5-6 g L⁻¹) and peak daily fixation of ≈1.0 g CO₂ L⁻¹ d⁻¹. Temperature displayed a right-skewed thermal response with an optimum at 28°C; 31-34°C accelerated early growth but flattened later. Initial pH exerted a strong, persistent effect with pH 9-10 > pH 5-7, consistent with bicarbonate-supported carbon-concentrating mechanism (CCM) operation. Aeration exhibited a process optimum at 0.5 vvm. Higher flows improved mid-run mass transfer but reduced late-phase gains due to CO₂ stripping and hydrodynamic stress. Light responses were near-linear up to saturation, with ∼280 µmol photons m⁻² s⁻¹ outperforming 350 µmol photons m⁻² s⁻¹ (mild photostress). The findings are interpreted via Rubisco carboxylation/oxygenation balance, CCM regulation, and inorganic-carbon speciation, providing actionable set points for scaling microalgal biofixation toward real-world applications.