Phlorotannins

Polyphenolic secondary metabolites are a large and diverse group of chemical compounds which exist both in terrestrial and aquatic plants. Polyphenols from terrestrial plants are derived from gallic and ellagic acids, whereas the algal polyphenols are derived from polymerized phloroglucinol units (Fig. 8.1). These algal polyphenols are termed as phlorotannins and they are biosynthesized via acetate malonate pathway (Arnold and Targett, 2002). The monomeric units are linked through aryl-aryl bonds, and diaryl ether bonds are forming different subgroups of phlorotannins (Glombitza and Pauli, 2003). Their molecular size ranges between 162 Da (phloroglucinol) and 650 kDa (Breton et al., 2011).

In brown algae (Phaeophyceae), the only group of tannins present is phlorotannins and may constitute up to 15% of the dry weight (Arnold and Targett, 2002). The phlorotannins are localized in physodes of the algae which are membrane-bound cytoplasmic vesicles, and the fusion of physodes with cell membranes results in a secretion of phlorotannins (Li et al., 2009). The highest levels of phenolic compounds in brown algae are found either in meristematic or in reproductive regions of the thallus (Breton et al., 2011). The phlorotannin's concentration exhibits seasonal variations and has reported that highest polyphenolic content can be obtained in summer (Connan et al., 2004). Many brown algal species are popular food mainly in East Asia, and the presence of phlorotannins may affect the palatability due to their astringent taste. And also medicinal values of the brown algae are also related to the presence of phlorotan-nins. Medicinal values of phlorotannins are related to their structure and especially to the degree of polymerization, where oligo-phenols generally considered more active than highly polymerized compounds (Toth and Pavia, 2001).

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