**3. Bioactive compounds present in marine seaweeds**

Marine seaweeds are categorized into three algal classes; red (*Rhodophyceae*), green (*Chlorophyceae*), and brown algae (*Phaeophyceae*) based on the presence of natural pigments. *Phaeophyceae* contains brown color fucoxanthin pigment, whereas *Rhodophyceae* possess red color pigments phycoerythrin and phycocyanin, and *Chlorophyceae* is rich in green color pigment chlorophyll. Among the three varieties, most marine algae are referred to as "edible" that can be used for human consumption. Asians and South Africans mainly consume these edible seaweeds as a promising complementary and alternative medicine [19].

Recently, marine seaweeds have been identified as a rich source of bioactive secondary metabolites with human health benefits. In particular, polyphenols, sterols, alkaloids, flavonoids, tannins, proteins, peptides, essential fatty acids, enzymes, vitamins, and pigments are extensively synthesized by marine seaweeds. These compounds exhibit significant chemical and biological properties such as anti-diabetic, antioxidant, cytotoxic, anti-fungal, anti-bacterial, anti-coagulant, anti-inflammatory, and antiproliferative activities, etc., [19, 20]. The marine seaweeds are a rich source of sulfated polysaccharides (**Figure 1**), which have been reported to possess beneficial human effects. Fucoidan, alginates, and laminarans

#### **Figure 1.**

*Chemical structures of sulfated polysaccharides present in marine seaweeds.*

are sulfated polysaccharide found in brown seaweeds and reported to exhibit anti-diabetic, antioxidant, and anti-inflammatory activities [21]. Carrageenans and agarans are sulfated polysaccharides found in red seaweeds. Similarly, ulvan is the sulfated polysaccharide found in green seaweeds [22]. The sulfated polysaccharides are known to possess anti-viral, anti-tumor, and anti-coagulant activities [23].

Marine seaweeds are rich in polyphenolic compounds, including flavonoids, bromophenols, phlorotannins, mycosprine-like amino acids, and phenolic terpenoids (**Figure 2**). The mycosprine-like amino acid is a small molecule with hydroxylated aromatic rings. Phlorotannins are polyphenolic metabolites found in brown seaweeds. They can be classified into six subgroups; fuhalols, phlorethols, fucophlorethols, and fucols, eckols, and carmalols based on their linkage between phloroglucinol units and hydroxyl groups. Flavonoids, bromophenol, phenolic terpenoids, phenolic acids, and mycosporine-like compounds are reported to possess antioxidant, anti-diabetic, anti-inflammatory, anti-allergic, and anticancer properties [24–26].

**Figure 2.** *Organic structures of phlorotannins and bromophenols.*

*Advanced Pharmacological Uses of Marine Algae as an Anti-Diabetic Therapy DOI: http://dx.doi.org/10.5772/intechopen.96807*

Among the bioactive compounds present in marine seaweeds, marine algaederived accessory pigments are important as they possess beneficial biological activities [27]. Fucoxanthin is the most abundant accessory pigment found in brown seaweeds and reported to have potent biological activities such as anticancer, antioxidant, anti-diabetic activities due to the presence of unusual allenic bond and a 5, 6-monoepoxide in its structure [21]. Phycobiliproteins, a water-soluble accessory pigment found in red seaweeds, can be divided into three main categories; phycocyanins, allophycocyanins, and phycoerythrin. Phycoerythrins are abundantly found in red seaweeds and reported to possess immuno-modulating and anticancer activities. Similarly, chlorophylls are found in green seaweeds and are said to have antioxidant activity [27].

Similarly, marine algae-derived peptides have been identified to possess a wide range of biological activities such as antioxidant, anti-diabetic, anti-microbial, antihypertensive properties, etc. Hence, most algal-derived proteins have been widely used in food and pharmaceutical industries [28]. The protein content of the marine seaweeds differs depending on the seasonal period and type of species. The brown seaweeds usually contain low protein content compared to the red and green seaweeds. Despite this, some brown algal species such as *Choonospora minima*, *Padina gymnospora*, *Dictyota menstrualis*, and *Sargassum vulgare* possess high protein content up to 10–15%. According to the reported studies, green seaweed contains an average protein content level, ranging between 10–26%. In contrast, the highest protein content was reported in red seaweeds such as *Phorphyra tenure* and *Palmaria palmata*, which was around 47% [29].
