Isolation and characterization of Chlorella sp. mutants with enhanced thermo- And CO₂ tolerances for CO₂ sequestration and utilization of flue gases

Background: The increasing emission of flue gas from industrial plants contributes to environmental pollution, global warming, and climate change. Microalgae have been considered excellent biological materials for flue gas removal, particularly CO₂ mitigation. However, tolerance to high temperatures is also critical for outdoor microalgal mass cultivation. Therefore, flue gas- and thermo-tolerant mutants of Chlorella vulgaris ESP-31 were generated and characterized for their ability to grow under various conditions. Results: In this study, we obtained two CO₂- and thermo-tolerant mutants of Chlorella vulgaris ESP-31, namely, 283 and 359, with enhanced CO₂ tolerance and thermo-tolerance by using N-methyl-N-nitro-N-nitrosoguanidine (NTG) mutagenesis followed by screening at high temperature and under high CO₂ conditions with the w-zipper pouch selection method. The two mutants exhibited higher photosynthetic activity and biomass productivity than that of the ESP-31 wild type. More importantly, the mutants were able to grow at high temperature (40 °C) and a high concentration of simulated flue gas (25% CO₂, 80-90 ppm SO₂, 90-100 ppm NO) and showed higher carbohydrate and lipid contents than did the ESP-31 wild type. Conclusions: The two thermo- and flue gas-tolerant mutants of Chlorella vulgaris ESP-31 were useful for CO₂ mitigation from flue gas under heated conditions and for the production of carbohydrates and biodiesel directly using CO₂ from flue gas.