Structural characterization and immunomodulatory activity of sulfated polysaccharides from the green microalgae Tetraselmis suecica.

Marine microalgae, including Tetraselmis suecica, are promising sources of bioactive sulfated polysaccharides (SPs) with potential applications in functional foods and biomedicine. However, the structural characteristics and immunomodulatory activities of SPs from T. suecica remain largely unexplored. In this study, water-soluble polysaccharides were extracted from T. suecica, fractionated by anion-exchange chromatography into two major fractions (F1 and F2), and evaluated for their structural characteristics and immunomodulatory effects on RAW264.7 macrophages. The crude polysaccharide contained 62.7% carbohydrates, 14.1% proteins, 8.0% sulfates, and 0.9% uronic acids. The purified fractions (F1 and F2) contained 58.3-72.0% carbohydrates, 3.2-6.2% proteins, 3.1-13.7% sulfates, and 1.3-2.6% uronic acids. All three polysaccharide samples contained sulfate groups but differed markedly in sulfate content and structural characteristics. Compared with F1, F2 contained substantially higher sulfate (13.7% vs. 3.1%) and uronic acid (2.6% vs. 1.3%) contents and exhibited a distinct molecular weight distribution. Structural characterization revealed that F2 consisted predominantly of α-(1 → 4)-linked d-glucopyranosyl residues with occasional α-(1 → 4,6)-linked glucopyranosyl branch points and that the sulfate group was proposed to be attached at the O-6 position. Functionally, only F2 significantly enhanced nitric oxide (NO) and IL-6 and IL-10 production in RAW264.7 macrophages. Antibody neutralization assays suggested that this activity is mediated primarily through complement receptor 3 (CR3). To the best of our knowledge, this is the first study to structurally characterize sulfated polysaccharides from T. suecica and demonstrate their CR-3-mediated immunomodulatory activity, highlighting their potential as natural immunomodulatory ingredients for functional foods and biomedical applications.