Enhanced carbon capture and polyunsaturated fatty acid production by Phaeodactylum tricornutum after stepwise CO2 acclimation

Industrial flue gas serves as a major source of CO₂ emissions. Currently, the exploration on how to use microalgae to absorb it and produce more lipids and polyunsaturated fatty acids (PUFAs) has emerged as a promising research direction. However, maintaining microalgal growth rates under high CO₂ concentrations remains a key bottleneck in research. In response to this, we pioneered a stepwise CO₂ acclimation approach, whereby Phaeodactylum tricornutum—a model diatom isolated from the South China Sea with rapid growth and high PUFA content—were sequentially acclimated to 2 %, 5 % and 7.5 % CO₂. The results demonstrated that this approach enabled more stable growth. Furthermore, after acclimation, larger cell diameter, more carbon accumulation, higher carbon capture rate and lipid production were attained. The coupling of 7.5 % CO₂ and nitrogen deficiency (HCLN) resulted in 69.1 % higher carbon capture rate at 6 h and induced higher lipid productivity by 32 %, 46 % and 44 % at 6 h, 12 h and 24 h, respectively compared to 0.04 % CO₂ & high nitrogen (ACHN). In addition, the production of PUFAs was also promoted, especially for eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). The maximum promoting effect of HCLN on EPA and DHA appeared at 6 h with increases of 64 % and 280 % respectively compared to ACHN. This study suggests that the stepwise CO₂ acclimation strategy is an effective approach for microalgae to capture CO₂ and produce lipid and PUFAs, which is beneficial for the combination of flue gas treatment and product markets (e.g., biofuel and health care).