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.