Spermidine protects Chlorella sp. from oxidative damage caused by SO2 in flue gas from coal-fired power plants

To resist the negative effects on Chlorella sp. growth caused by SO2 impurities and 15% CO2 in flue gas from coal-fired power plants, exogenous spermidine, which possesses two amino-terminal domains, was used to enhance antioxidant reactions, improve photosynthesis, protect the cell structure, and repair the extracellular environment. The protective effect on Chlorella sp. growth exhibited by spermidine became more obvious when the cells experienced more serious damage due to higher SO2 concentrations. The superoxide dismutase activity increased from 2.58 to 18.41 U/mg, which effectively alleviated cell damage from reactive oxygen species and toxicity. Spermidine inhibited HSO3-/SO32- penetration through cell walls, as well as protected the photosynthetic PSII structure and thylakoid membrane. Accordingly, chlorophyll synthesis recovered from 0.5 to 8.5 mg/L and the maximum photochemical efficiency improved from 0.41 to 0.7. As a result, the Chlorella sp. biomass yield recovered from 0.12 to 1.52 g/L in the presence of exogenous spermidine, while the lipid content of the biomass recovered from 5.28 to 19.12%, which showed the significance of spermidine in protecting microalgal cells in the presence of 15% CO2 flue gas with SO2 impurities.