## 2 Characterization and Types of Phenolic Compounds  
The extraction and characterization of phenolic compounds from marine algae constitute interesting results, with those reported in the literature [75; 76; 77; 78; 79]. These compounds and their biological action are commonly correlated. However, some phenolic extracts have an interesting property but have not been fully characterized. Antioxidant activities have been reported in green seaweed-derived bromophenols and flavonoids. [PERSON] et al. [80], and [PERSON] et al. [81] studied and proved the high radical scavenging activities of various green (Chlorophyta) species such as _Ulwa calthrata_, _U. compressa_ (formerly known as _Enteromorpha compressa_), _U. intestinalis_, _U. linza_, _U. flexuosa_, _U. australis_ (formerly known as _Ulwa pertusa_), _Capsosiphon fulvescens_, and _Chuetomorpha monililigera_. In their findings, antibacterial and cytotoxic effects on breast ductal carcinoma cell lines were verified in the phenolic fraction of _U. calthrata_ and _U. flexuosa_[82; 83]. In a more recent study, [PERSON] et al. [84] reported _C. socialis_-derived phenolic compounds, such as 2,3,8,9-tetrahydroobenz[c]chromen-6-one (Figure 2a), 3,4,3\({}^{\prime}\),4\({}^{\prime}\)-tetrahydrooxy-1,1\({}^{\prime}\)-biphenyl (Figure 2b), and cladophorol C (\(\alpha\)-hydro-\(\omega\)-[3,4-dihydroxyroxyphenyl]octa[oxy(2-hydroxyphen-4-yl)]) (Figure 2c), have been identified with antibacterial activity against methicillin-resistant _Staphylococcus aureus_ (MRSA). The functions of phenol compounds in red marine algae have barely been studied, but they probably have multipurpose actions in cell life, such as antioxidant, chelation, and anti-infection actions, as well as cofactors or hormones [85]. However, some research is not with the isolated phenolic compounds but with an extract enriched in polyphenols [85]. More than 8000 different structures of phenolic compounds are found in marine macroalgae, because of their importance in organisms' growth, survival, and defense. The classification of phenolic compounds according to their chemical structures is depicted in Figure 3.  
Moreover, the chemical structures of some phenolic compounds are illustrated in Figure 4. These compounds can be synthesized by various metabolic pathways such as the pentose phosphate pathway (PPP), and the phenylpropanol and shikimate pathways. Figure 3: Classification of phenolic compounds listed according to their chemical structures. Source: [PERSON] et al. [36]. Figure 2: **(a-c). ** Chemical structures of _Cladophora socialis_ (green)-derived phenolic compounds. **(a)**: 2,3,8,9-tetrahydroxybenzoic[c]chromen-6-one; **(b)**: 3,4,3\({}^{\prime}\),\({}^{\prime}\),\({}^{\prime}\)-Tetrahydroxy-1,1\({}^{\prime}\)-biphenyl; **(c)**: New phenolic natural compound. Source (permission obtained from the authors): [PERSON] et al. [84]. Figure 4: Chemical structures of some phenolic compounds. (**A**): Catechol, (**B**): Gallic acid, (**C**): Gentisic acid, (**D**): Phloroglucinol, (**E**): Hydroquinone (HQ), (**F**): Chlorogenic acid, (**G**): Coumaric acid, (**H**): 4-Hydroxbenzoic acid, (**I**): (+)-Catechin, (**J**): Caffeic acid, (**K**): Ferulic acid, (**L**): Sinapic acid, (**M**): 7-Hydroxy coumarin sulfate, (**N**): Eckol, (**O**): Dieckol, (**P**): Eckstolonol, (**Q**): Triphloroethol-A, (**R**): Epigallocatechin, (**S**): Catechin 3-O-gallate. [PERSON] [86], and [PERSON] and [PERSON] [87] reported varieties of phenolic compound classification to comprise a large number of heterogeneous structures from simple to highly polymerized structures. A simple phenolic group is formed that reveals hydroxyl groups at different positions: ortho, meta, and para (1,2-, 2,3-, and 1,4-), respectively. Catechol, HQ, and phloroglucinol are examples of simple phenolic compounds found exclusively in macroalgae [88]. One of the previous studies showed catechol in twenty-seven Japanese green and red seaweeds [89]. It is quite common to find this simple phenol with a bromine substituent that is bromophenol.