Bioprospecting microalgal strains cultivated using bicarbonate under a pH-regulated regime for enhanced carbon capture and utilization

Microalgae offers significant potential as a sustainable resource for biofuel production. However, an inefficient supply of gaseous CO₂ can lead to carbon limitation due to poor mass transfer efficiency, which poses a significant challenge in large-scale microalgal cultures. This study investigates pH-stat bicarbonate-based cultivation of various microalgal strains aimed at carbohydrate and lipid production. Four green algal strains were assessed for their capability to utilize bicarbonate ions as their inorganic carbon source for photosynthesis. Among the strains, Chlamydomonas sp. JSC4 attained the highest biomass concentration (1.05 ± 0.05 g/L), biomass productivity (242 mg/L/day), lipid content (42.7 ± 0.85 %), and carbohydrate content (39.16 ± 0.09 %) under conditions of 50 mM KHCO₃, pH 8, and 7 days of cultivation. The biomass productivity attained in this study is higher than those in the reported literature. The predicted biodiesel quality for all strains met the ASTM D6751 and EN 14214 specifications. Increasing the bicarbonate concentration to 100 mM and employing a fed-batch mode elevated the biomass to 1.45 ± 0.32 g/L, carbohydrate accumulation to 58.25 ± 1.28 % DCW, and carbon fixation efficiency to 63.8 ± 3.23 %, which is significantly higher than that of the control cultivated with CO₂ supply (0.30 ± 0.04 %). This study highlights the potential of bicarbonate ion supplementation as an effective strategy for maximal biomass production with high productivity and biomolecule accumulation, aiding in the realization of cost-effective microalgal biofuel production.