Bicarbonate-based microalgal cultivation technologies: Mechanisms, critical strategies, and future perspectives

Conventional carbon fixation by microalgae involves the direct introduction of CO₂ as the carbon source for microalgal growth. However, CO₂ displays low solubility in water and a low mass transfer rate into microalgal cells, limiting the efficiency of CO₂ capture. Microalgae production using bicarbonate-based cultivation has gained significant attention recently due to its potential applications in biofuel production, wastewater treatment, and carbon capture. Microalgae can uptake and assimilate bicarbonate as an inorganic carbon source based on their capability for intracellular and extracellular interconversion of carbon dioxide and bicarbonate. This interconversion, catalyzed by carbonic anhydrases, is an integral part of carbon acquisition using carbon concentrating mechanisms. In contrast to photoautotrophic growth using CO₂, the pH of the culture medium increases significantly during bicarbonate assimilation. Thus, pH-stat cultivation needs to be adapted to attain maximal biomass production. Other strategies, such as optimization of bicarbonate concentration, combining bicarbonate as an additional carbon source in mixotrophic cultivation, and bicarbonate supplementation in wastewater treatment, can be applied to harness the potential of a highly soluble inorganic carbon source in an algal culture system. This comprehensive review presents bicarbonate-based microalgal cultivation technologies, particularly their underlying mechanisms, benefits and challenges, essential strategies, and prospective outlook. Prospective avenues for future research are outlined to support the advancement of cost-effective and green, microalgae-based integrated carbon capture, biofuel generation, high-value biochemical production, and wastewater treatment systems, thereby promoting a circular and sustainable net-zero carbon society. © 2024 Elsevier B.V.