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The 1o3;1o6-E-D-glucan chrysolaminaran with molecular weight 8.5 kDa and 3.6:1 ratio of 1o3:1o6 bonds was isolated from cultivated diatom alga Synedra acus and characterized. It was shown that the isolated chrysolaminaran inhibited growth and colony formation of human colon tumor cells.Keywords: diatom alga, chrysolaminaran, 1o3;1o6-E-D-glucan, antitumor activity. Diatom algae are unicellular or colonial eukaryotic organisms that inhabit seas, oceans, and freshwaters and have adapted to habitats of plankton and benthos. Diatom algae and terrestrial plants are responsible for about 40% of the primary production on Earth [1]. Polysaccharides make up from 10 to 80% of the organic mass of diatoms [2, 3] and play an important role in their metabolism. Polysaccharides of diatom algae are divided into three groups depending on their location in the cell: storage, structural, and extracellular [4,5]. Each of these fulfills its own specific function.Storage polysaccharides are localized in vacuoles and represent the final photosynthesis products [6]. They are E-D-glucans, are called chrysolaminarans, and consist of short chains of 1o3-bonded E-D-glucopyranoses (n = 20-60) with branching at C6 and/or C2 [7-9].The study of polysaccharides from diatom algae is important for both an understanding of the functional processes of these organisms and their practical use. Analogs of chrysolaminarans, 1o3;1o6-E-D-glucans, isolated from yeast, fungi (lentinan), and culture filtrates of Schizophyllum commune (schizophyllan) have been used in cancer immunotherapy [10,11], act as immunomodulators, and increase the resistance to infections [12]. This makes them an attractive alternative to cytotoxic drugs. In contrast with other natural biopolymers, purified E-1,3-glucans retain their biological activity over time so that the mechanism of their action on the cellular and molecular level can be studied. It is currently known that the biological action of E-1,3-glucans is mediated through two membrane receptors, CR3 [13] and Dectin-1 [14].The biological activity of algae polysaccharides (laminarans and chrysolaminarans) has been studied less than that of glucans from yeast and fungi.Our goal was to isolate chrysolaminaran from cultivated diatom alga Synedra acus and to study its structure and antitumor activity.A chrysolaminaran preparation consisting of a polysaccharide-protein complex with a 1:8 protein:carbohydrate ratio was isolated from S. acus cultivated under laboratory conditions by extraction with hot water. According to the literature, the protein:carbohydrate ratio in similar complexes can vary from 0.33 to 6.5 depending on the physiological condition of the cell [15]. Removal of protein from the S. acus complex by the Sevage method [16] did not completely free the polysaccharide from the protein. A sample of the chrysolaminaran without protein could be obtained using hydrophobic chromatography over Polykhrom-1. The chrysolaminaran yield was 0.35% of the moist alga weight.