Towards optimal inorganic carbon delivery to microalgae culture

Microalgae offer the potential to sequestrate CO₂ and reduce the world's dependency on fossil fuels, thereby mitigating the greenhouse gas effect, which has resulted in climate change over time. Carbon is one of the critical nutrients for microalgae growth and product formation; the choice of carbon and pH control techniques has been identified as the key to improving the sustainability of microalgae cultivation. However, supplying CO₂ in the gas phase is not cost-effective and could be counterproductive as a high percentage could be lost back to the atmosphere due to low solubility in the medium. Additionally, using conventional pH control techniques such as acid, base, and buffers is expensive at scale. This study assessed freshwater Chlorella sorokiniana cultivation using NaHCO₃ solely as the inorganic carbon and for pH modulation. The investigation focuses on the effect of bicarbonate feed rates and culture starting pH in the acidic, neutral, and basic regimes on the pH modulation, biomass accumulation, and product yield in a fed-batch system. The final pH increased beyond the upper limit threshold for each pH regime, with the highest level of control achieved in the basic regime (7.50–8.48 ± 0.11). The maximum biomass accumulation reached 2.49 g/L and a doubling time of 49 h. Evaluation of the pigment showed a higher chlorophyll a content, 25.85 mg/L, similar to a CO₂-fed culture and higher than the bicarbonate culture reported. The lipid contents showed a fatty acid composition suitable for biodiesel production with a higher percentage of saturated fatty acids (SFAs) (62.93 %–90.86 %) across all conditions. The sole use of bicarbonate for simultaneous inorganic carbon supply and pH control can be a cost-effective measure without compromising biomass and lipid productivities.