Physode

Phlorotannins are a type of tannins found in brown algae such as kelps and rockweeds or sargassacean species, and in a lower amount also in some red algae. Contrary to hydrolysable or condensed tannins, these compounds are oligomers of phloroglucinol (polyphloroglucinols). As they are called tannins, they have the ability to precipitate proteins. It has been noticed that some phlorotannins have the ability to oxidize and form covalent bonds with some proteins. In contrast, under similar experimental conditions three types of terrestrial tannins (procyanidins, profisetinidins, and gallotannins) apparently did not form covalent complexes with proteins.

These phenolic compounds are integral structural components of cell walls in brown algae, but they also seem to play many other secondary ecological roles such as protection from UV radiation and defense against grazing.

Most of the phlorotannins' biosynthesis is still unknown, but it appears they are formed from phloroglucinols via the acetate-malonate pathway.

They are found within the cell in small vesicles called physodes, where the soluble, polar fraction is sequestrated, and as part of the cell wall, where they are insoluble and act as a structural component. Their concentration is known to be highly variable among different taxa as well as among geographical area, since they respond plastically to a variety of environmental factors. Brown algaes also exsude phlorotannins in surrounding seawater.

It has been proposed that phlorotannins are first sequestered in physodes under their polar, reactive form before being oxidized and complexed to the alginic acid of brown algla cell wall by a peroxidase. To this date (2012), not much is known about phlorotannins synthesis. The formation of physodes, vesicles containing phenolic compounds, have been investigated for many years. These cytoplasmic constituents were thought to be synthesized in the chloroplast or its membrane, but more recent studies suggest that the formation may be related to the endoplasmic reticulum and Golgi bodies.

...
Wikipedia