Phlorotannin
In contrast, under similar experimental conditions three types of terrestrial tannins (procyanidins, profisetinidins, and gallotannins) apparently did not form covalent complexes with proteins.
[6] 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.
[9][10] 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.
[5][12] 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 algal cell wall by a peroxidase.
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.
[16] The ultrastructural localization of physodes can be examined through transmission electron microscopy in samples primarily fixed in 2.5% glutaraldehyde and with postfixation with 1% osmium tetroxide.
[28] Cytoplasmic as well as exuded phlorotannins seem to play a role in algal reproduction, by contributing to the formation of the zygote's cell wall[10] and perhaps avoiding multiple fertilization by inhibiting spermatozoid movement.
[11] However, results form other studies suggest that the deterrent role of phlorotannins on herbivory is highly dependent on both algae and herbivore species.
[31] In Fucus vesiculosus, it is galactolipids, rather than phlorotannins, that act as herbivore deterrents against the sea urchin Arbacia punctulata.
Studies also demonstrated that sunlight intensity is related to phlorotannins production in Ascophyllum nodosum and Fucus vesiculosus natural populations.
[35] Further studies with Lessonia nigrescens[35] and Macrocystis integrifolia[36] demonstrated that both UV-A and UV-B radiation can induce soluble phlorotannins and that there is a correlation between induction of phlorotannins and reduction in the inhibition of photosynthesis and DNA damage, two major effects of UV radiation on vegetal tissues.
