The functional roles of biosynthesis pathways and the diversity of polyphenolic compounds, the products of the secondary metabolism of macro- and microalgae, are discussed. Phenolic compounds are included in the integrated system of regulation of biochemical and bioenergetic processes in the plant cell. A wide range of biological effects of polyphenols is associated with their antioxidant properties. They are involved in protecting the plant cell from stress factors and detoxification of reactive oxygen species such as superoxide (O2-•), hydrogen peroxide (H2O2), hydroxyl radical (OH-), singlet oxygen (1O2), and hydroperoxyl radical (HO2-). Often, the accumulation of polyphenolic compounds in the cell is considered as an indicator of physiological stress. According to the chemical structure of polyphenols, there are several classes, such as phenolcarboxylic acids (hydroxybenzoic acids, hydroxycinnamic acids), flavonoids (flavones, flavonols, flavanones, flavanonols, flavanols, anthocyanins), isoflavonoids (isoflavones, coumestans), stilbenes, lignans, and phenolic polymers (proanthocyanidins — condensed and hydrolyzable tannins). The diversity of phenolic compounds in higher plants, which has arisen in the process of evolution, is associated with their landfall and the need to form protective systems from ultraviolet irradiation. Macroscopic brown (Phaeophyceae) and red (Rhadophyta) seaweeds, containing large amounts of polyphenols. The content of phlorotannins, which are polymers of phloroglucinol (1,3,5-trihydroxybenzene) of different size and composition, may amount to 25% of the dry biomass of Phaeophyceae. The phlorotannin molecules absorb solar radiation in the middle and far regions of the UV spectrum, which explains the photoprotective role of these compounds. Part of the synthesized phlorotannins is excreted into the extracellular space, and soluble forms accumulate in the cellular compartments, mainly in particular vacuoles – physodes; under the light microscope, they look like small refractive inclusions. Red algae accumulate polyphenolic compounds containing bromine (bromophenols); they are poisonous to mollusks and protect these seaweeds from being eaten. Unlike Phaeophyceae and Rhadophyta, microscopic algae synthesize polyphenolic compounds in small quantities. Although microalgae are evolutionarily more primitive than higher plants, or can even belong to completely different evolutionary branches, they are able to synthesize relatively complex polyphenols. Available data suggest that the processes of biosynthesis of flavonoids in microalgae are less complex than those of higher plants, although they are not inferior in diversity to representatives of Bryophyta. Due to the large number of phenolic groups, the molecules of flavonoids, phlorotannins, and bromophenols effectively bind heavy metal (HM) ions, which contribute to the accumulation of divalent metals inside cells, and their extracellular forms are involved in chelation of HM, reducing their toxicity. Phenolic compounds are involved in the antioxidant protection of algae and in the formation of an adaptive response to oxidative stress.
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