Neuroprotective and antibacterial effects of phlorotannins isolated from the cell walls of brown algae Fucus vesiculosus and Pelvetia canaliculata

The Phaeophyceae (brown algae) essentially contribute to biotopes of cold and temperate seas. Their thalli are rich in biologically active natural products which are strongly and universally dominated with phlorotannins – polyphenols of complex and diverse structure based on multiple differently arranged phloroglucinol units. These electron-rich compounds are strong antioxidants with antimicrobial, anti-inflammatory and neuroprotective activities. In the algal cells phlorotannins can either accumulate in cytoplasm or can be secreted into the cell wall (CW) with subsequent covalent binding to the alginate network. The biological activities of easily extractable intracellular phlorotannins were comprehensively characterized, whereas the properties of the CW-bound polyphenol fraction are still mostly unknown. Recently, we identified dibenzodioxin bonding as the principal structural feature of the CW-bound phlorotannins of fucoid algae, whereas soluble intracellular phlorotannins relied on aryl- and ether bonds. However, profiles of biological activity associated with these structural differences are still unknown. Therefore, here, to the best of our knowledge, for the first time, we address the antioxidant, cytotoxic, neuroprotective, and antibacterial properties of the CW-bound phlorotannin fractions isolated from two representatives of the order Fucales - Fucus vesiculosus and Pelvetia canaliculata. The CW-bound phlorotannins appeared to be softer antioxidants, stronger antibacterial agents and were featured with essentially less cytotoxicity in comparison to the intracellular fraction. However, the neuroprotective effects of both sub-cellular phlorotannin fractions of F. vesiculosus and P. canaliculata were essentially similar. Thus, due to their lower cytotoxicity, CW-bound phlorotannins can be considered as promising antioxidants and neuroprotectors.