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Aims Polysaccharides synthesized by microalgae can be used as the functional ingredients of food or drugs. Here, we investigated the physicochemical properties and bioactivities of the polysaccharide from microalgae Pavlova viridis , and indicated the structure–activity relationship. Main methods The polysaccharides (PPS0) were degraded with H 2 O 2 -vitamin C assisted by ultrasonic waves. The functional group content, monosaccharide composition, and average molecular weight (avg-MW) were detected by chemical or chromatographic method. The immunomodulatory activities were evaluated in vitro by detecting nitric oxide (NO) emission, neutral red uptake and macrophage proliferation. Antitumor activities of degraded fragments were detected using S180-tumor-bearing mouse model by intragastric administration. Key findings Degraded polysaccharides PPS1 and PPS2 were obtained at avg-MW of 386.96 and 54.99 kDa. The sulfate group content of polysaccharide was 16%, and the uronic acid content was 5.88 and 8.48%. PPS mainly consisted of fructose, glucose and mannose. All the degraded PPSs could increase phagocytosis and proliferation of macrophages, and stimulated NO emission in a dose-dependently way. PPS2 in Low-MW fragments had the strongest immunoenhancing activities. Different doses of PPS all could inhibit the growth of implanted S180 tumor. At dose of 200 mg/kg/day, the tumor inhibition rate of PPS2 was 57.06%, about 23.6% less than that of CTX-treated group. Different-MW PPS significantly increased lymphocyte proliferation. At 200 mg/L, the proliferation index of PPS2 was 1.37, 2.03 times higher than that of CTX-treated group. Significance The polysaccharides of Pavlova viridis had potential antitumor activities by improving immune response. Moreover, the bioactivities depend on their molecular weight. [ABSTRACT FROM AUTHOR]

Highlights: [•] Physicochemical and configurations characteristics of polysaccharide were clarified. [•] Relations between bioactivity and MW or properties of polysaccharide were studied. [•] Low-MW degraded fragments showed more obvious radical scavenging effects. [•] Higher anti-lipid-peroxidation activities were observed in low-MW polysaccharides. [•] Low-MW fragments had higher inhibited effect on H2O2-induced hemolysis of RBCs. [ABSTRACT FROM AUTHOR]