*3.5. Phenolic Compounds*

Phenolic acids, tannins, flavonoids, and catechins are some of the phenolic compounds found in marine algae. The method of phenolic chemical extraction and the yield are strongly dependent on seaweed species. Brown seaweeds (Pheophyceae: P) are known for their high content of phlorotannins, complicated polymers made up of oligomers of phloroglucinol (1,3,5-trihydroxybenzene), while red or green seaweeds (Rhodophyceae: R) are known for their phenolic acids, flavonoids or bromophenols [10]. Polyphenols extracted from seaweeds were linked to variety of biological functions, containing antimicrobial, anticancer, antiviral, anti-obesity, antitumor, antiproliferative, antidiabetic, anti-inflammatory, or antioxidant effects [10]. Previous studies [101,188] demonstrated the anti-inflammatory activity of polyphenol-rich fraction derived from Rhodophyceae. Furthermore, phlorotannins and bromophenols derived from green or red algae possess strong inhibitory activity upon in vitro cancer cell proliferation or in vivo tumor growth, as well as antidiabetic and antithrombotic activities in vitro.

The phenolic active ingredients in seaweeds differ depending on whether they are red, green, or brown. Different phyla create different chemicals; for example, brown seaweeds produce phlorotannins, but red seaweeds produce a greater range of mycosporine-like amino acids (MAAs) than green species [189,190]. As a result of cellular mechanisms and genetic codification, the synthesis and diversity of phenolic chemicals are intimately tied to the seaweed taxonomic group and individual species [191]. Furthermore, phenolic acids such as benzoic acid, p-hydroxybenzoic acid, salicylic acid, gentisic acid, protocatechuic

acid, vanillic acid, gallic acid, and syringic acid have been found in the genus Gracilaria (Rhodophyta, red alga) [192,193]. Phlorotannins are well-known phenolic chemicals that brown seaweeds produce [194]. Flavonoids such as rutin, quercitin, and hesperidin were detected in many Chlorophyta, Rhodophyta, and Phaeophyceae species [195]. Chondrus crispus and *Porphyra*/*Pyropia* spp. (Rhodophyta), as well as *Sargassum muticum* and *Sargassum vulgare* (Phaeophyceae), may synthesis isoflavones, as can daidzein and genistein [196]. Furthermore, several flavonoid glycosides were found in the brown seaweeds *Durvillaea antarctica*, *Lessonia spicata*, and *Macrocystis pyrifera* (also known as *Macrocystis integrifolia*) [195].

Terpenoids are belonging to secondary metabolites discovered in seaweeds [190]. Meroditerpenoids (such as plastoquinones, chromanols, and chromenes) were discovered in brown seaweeds, primarily from the Sargassaceae family (Phaeophyceae). These compounds are produced in part from terpenoids and are distinguished by the presence of a polyprenyl chain connected to a hydroquinone ring moiety [197]. In Rhodomelaceae, red seaweeds manufacture phenolic terpenoids such as diterpenes and sesquiterpenes. *Callophycus serratus*, for example, synthesizes a particular diterpene called bromophycolide [198]. Some studies revealed the existence of phenolic and flavonoids acids in marine algae as seen in Figure 5 and the chemical structure of phenolics also presented in Figure 6.

**Figure 5.** Several seaweeds synthesize phenolic substances. Adapted from ref [194] obtained from mdpi journals. (**A**)—*Ascophyllum nodosum* (P); (**B**)—*Bifurcaria bifurcata* (P); (**C**)—*Fucus vesiculosus* (P); (**D**)—*Leathesia marina* (P); (**E**)—*Lobophora variegata* (P); (**F**)—*Macrocystis pyrifera* (P); (**G**)—*Asparagopsis armata* (R); (**H**)—*Chondrus crispus* (R); (**I**)—*Gracilaria* sp. (R); (**J**)—*Kappaphycus alvarezii* (R); (**K**)— *Neopyropia* sp. (R); (**L**)—*Palmaria palmata* (R); (**M**)—*Dasycladus vermicularis* (Chl); (**N**)—*Derbesia tenuissima* (Chl); (**O**)—*Ulva intestinalis* (Chl); P—Phaeophyceae, R—Rhodophyta; Chl—Chlorophyta.

**Figure 6.** Chemical structures of different types of phenols in seaweeds.
