Augmented CO₂ tolerance by expressing a single H⁺-pump enables microalgal valorization of industrial flue gas

Microalgae can accumulate various carbon-neutral products, but their real-world applications are hindered by their CO₂ susceptibility. Herein, the transcriptomic changes in a model microalga, Chlamydomonas reinhardtii, in a high-CO₂ milieu (20%) are evaluated. The primary toxicity mechanism consists of aberrantly low expression of plasma membrane H⁺-ATPases (PMAs) accompanied by intracellular acidification. Our results demonstrate that the expression of a universally expressible PMA in wild-type strains makes them capable of not only thriving in acidity levels that they usually cannot survive but also exhibiting 3.2-fold increased photoautotrophic production against high CO₂ via maintenance of a higher cytoplasmic pH. A proof-of-concept experiment involving cultivation with toxic flue gas (13 vol% CO₂, 20 ppm NO_X, and 32 ppm SO_X) shows that the production of CO₂-based bioproducts by the strain is doubled compared with that by the wild-type, implying that this strategy potentially enables the microalgal valorization of CO₂ in industrial exhaust.