## 4 Natural Antioxidants and Their Role in Cosmetics
### Algae-Derived Antioxidants  
Algae, an ecological group of organisms with a highly diverse structure, belong to different kingdoms from a systematic classification perspective. They are commonly classified into four major seaweed classes: Cyanophyceaeae = Cyanobacteria (blue-green algae), Phaeophyceaeae (brown algae), Chlorophyceae (green algae), and Rhodophyceaeae (red algae). The broad biochemical composition of seaweeds gives them great potential as antioxidant additives suitable for commercial applications. The most promising biologically active components in their composition include sulfated polysaccharides, phlorotannins, and tyrosinase inhibitors. The main molecules with antioxidant and photoprotective properties in algae are mycosporine-like amino acids (MAAs) (Figure 6), the pigment scytonemin, and numerous phenolic compounds. Given these properties, algae have significant potential as cosmeceutical ingredients [180; 181; 182]. Algae contain high concentrations of polyphenols, making them excellent antioxidants. Polyphenols, chlorophyll, and carotenoids exhibit antioxidant activity, protect against the harmful effects of UV radiation, and possess anti-inflammatory properties [181; 182; 183; 184; 185; 186]. Recent research has demonstrated the UV-protective effects of various algae groups, which have long been incorporated into commercial sunscreens [180; 181; 184; 187; 188; 189; 190]. Brown algae contain unique compounds not found in terrestrial sources and exhibit a higher antioxidant potential compared to Rhodophyceaeae (red algae) and Chlorophyceaeae (green algae) [185]. Cyanobacteria extracts have shown greater antioxidant activity than those of red macroalgae [181]. Given these properties, bioactive compounds derived from algae represent promising cosmeceutical agents for skincare applications. A number of algae-based skin products have been marketed, such as Algenist (an anti-aging moisturizer containing microalgae oil and algoronic acid from algae); Helionori(r) by Gelyma (Marseille, France) and Helioguard365(r) (Mibelle Biochemistry, Buchs, Switzerland) (a sunscreen product containing MAAs from red seaweed _Porphyna umbilicalis_); Prottines(r) by Exsymol S.A.M., Monaco (an anti-aging agent from protein-rich extract of _Arthospira_); and _Dermochlorella_ by Codif, St. Malo, France (an anti-wrinkle agent from _Chlorella vulgaris_ extract) [191]. Cosmetic preparations for protection from tobacco smoke included in an international index of cosmetic ingredients (INCI) are derived from marine algae--brown algae, _Laminaria digitata_, _Sargassum muticum_, and _Undaria pinnatifida_, and red algae, _Rissella verruculosa_[192]. Interestingly, phycocyanin (usually found in red algae and cyanobacteria) is accepted as a natural color additive in food and cosmetics by the Food and Drug Administration (FDA) due to its non-toxic and biodegradable activities [191]. 4.10.1. Cyanobacteria (Blue-Green Algae)  
Cyanobacteria are a diverse group of photosynthetic prokaryotes that inhabit both aquatic and terrestrial environments. They produce a variety of secondary metabolites that are effective against reactive oxygen species (ROS), particularly pigments such as carotenoids and polyphenols, including phenolic acids and flavonoid compounds, which have great potential as pharmacologically active agents. The bioactive compounds isolated from various _Cyanobacteria_ species exhibit antioxidant, photoprotective, antimicrobial, anti-inflammatory, and immunostimulant properties [181; 193]. In cosmetic applications, cyanobacteria are particularly valuable as a source of substances that absorb UV radiation, making them ideal for sunscreen formulations. One of the primary mechanisms by which cyanobacteria provide photoprotection is through the production of natural sunscreens, including scytonemin (a pigment unique to cyanobacteria) and myxosporine-like amino acids (MAAs), both of which effectively absorb harmful radiation. High concentrations of scytonemin, MAAs, and polyphenols have been identified in _Sydonema_ sp.