Brown Algal Phlorotannins: An Overview of Their Functional Roles

Abstract

Phlorotannins are polyphenolic compounds, relatively hydrophilic, formed by polymers of phloroglucinol and found exclusively in brown algae. These molecules are located in vesicles denominated physodes (the soluble fraction) and also complexed to polysaccharides in the cell wall (the insoluble fraction). Well known as potential grazer deterrents, one of the most striking characteristics of these compounds, due to a number of hydroxyl groups, is their antioxidant potential, which opens promising perspectives for pharmaceutical and biotechnological uses. In Antarctic brown algae, especially endemic species of Desmarestiales, constitutively high levels of phlorotannins (up to 12% of dry weight) have been measured. Although translocation has not been conclusively confirmed, the differential allocation of phlorotannins in meristematic and reproductive tissues in some species suggests their involvement in chemical defenses protecting essential metabolic functions. Due to their UV-absorbing properties and peripheral localization in cells and tissues, phlorotannins have been related with the increased tolerance to UV radiation in various Antarctic brown algae. However, no induction of phlorotannins by UV has been demonstrated, which strongly supports the idea that these molecules are constitutive biochemical components of a suite of mechanisms against multiple stressors. Due to their structural role as primary compounds, phlorotannins are essential for various morpho-functional processes that in the case of Antarctic algae allow them to thrive under extreme conditions. Overall, the significance of phlorotannins in this group of algae has largely been recognized; however, fundamental aspects of their molecular expression, synthesis, and regulation still need to be addressed, especially considering the climate change-driven environmental scenarios.