High-CO₂ tolerance in microalgae: Possible mechanisms and implications for biotechnology and bioremediation

Recent developments in the field of microalgal biotechnology, including CO₂ biomitigation and the discovery of new species of microalgae that are tolerant to extremely high CO₂ levels (40-100 vol%), have renewed interest in the physiological effects and mechanisms of high-CO₂ tolerance in photoautotrophs. Photosynthetic apparatus state transitions that increase ATP generation, upregulation of H⁺-ATPases pumping protons out of the cell, rapid shutdown of CO₂-concentrating mechanisms, and adjustment of membranes' fatty acid composition are currently believed to be the key mechanisms governing cellular pH homeostasis and hence microalgae's tolerance to high CO₂ levels, which is especially characteristic of extremophile and symbiotic species. The mechanisms governing acclimation to high CO₂ comprise the subject of this review and are discussed in view of the use of CO₂ enrichment to increase the productivity of microalgal cultures, as well as the practice of carbon capture from flue gases.