**7. Polyphenols of Algae**

The composition of the polyphenol fraction of brown algae is characterized by the predominant content of phorotannins, which are unique complex biopolymers of marine origin and the main cytoplasmic components of these hydrobionts. These compounds are contained inside the cell in both the free and bound state [100]. Phlorotannins have antioxidant, hepatoprotective, anti-allergic, anti-tumour, anti-inflammatory, anti-bacterial, and anti-diabetic properties [101]. Ryu et al. [102] purified from the brown alga *Ecklonia cava* phlorotannin, which proved to be an effective selective inhibitor of the NA of influenza virus (72% at a dose of 30 μg/mL). By fractionating the ethyl acetate layer, five phlorotannins were obtained, identified as phloroglucinol, ecol, 7-phloracol, fluorofucofuroecol, and diecol. The inhibitory activities of these components were assessed against influenza viruses from NA group 1 (A/Bervig\_Mission/1/18[H1N1]), A/PR/8/34[H1N1]) and group 2 (A/Hong Kong/8/68[H3N2), A/Chicken/Korea/MS96[H9N2]). All five phlorotannin derivatives were found to be selective inhibitors of NA. Fluorofucofuroecol showed the strongest inhibitory activity against NA viruses of group 1 (IC50 of 4.5 and 14.7 mmol, respectively); diecol inhibited the NA of influenza virus strains of group 2 more effectively. All derivatives of phlorotannin enhanced the NA-inhibitory effect of ozaltamivir.
