The optimization of intracellular polysaccharides (IPSs )extraction from Chlorella sp.GTD4C-2 was performed using response surface methodology based on a central composite design.The optimal conditions for extraction were determined as follows:an extraction time of 1 h,an extraction temperature of 80 ℃,and a solid-to-liquid ratio of 1∶25 (g/mL).Subsequently,the extracted IPSs were further purified through ion exchange chromatography.Structural analysis was carried out using high-performance gel permeation chromatography,gas chromatography-mass spectrometry,and Fourier-transform infrared spectroscopy.Additionally,the DPPH radical scavenging activity and hydroxyl radical scavenging capacity of the IPSs were evaluated.The results revealed two stable molecular weight acidic polysaccharides:IPS-2 (53.1 kDa) and IPS-3 (25.6 kDa),both containing glucose and galactose as monosaccharide components.The sulfate content was found to be (5.69±0.11)% for IPS-2 and (8.46±0.03)% for IPS-3.In vitro antioxidant activity showed that vitamin C exhibited the highest antioxidant capacity followed by the crude extract of IPSs,then IPS-3,and finally IPS-2.In conclusion,polysaccharides extracted from Chlorella sp.GTD4C-2 demonstrated significant antioxidant activity with stronger antioxidative properties observed in the crude extract compared to isolated compounds (IPS-2 and IPS-3).These findings suggest promising applications for Chlorella sp.GTD4C-2 polysaccharides in various fields such as food products,pharmaceuticals,and health supplements.