**2. Alginate biosynthesis in algae**

For many years, alginate has attracted great interest in food, cosmetic, biomedical and pharmaceutical industries, and therefore the potential sources of alginate have been extensively studied to meet the commercial demand. Brown seaweeds also known as the marine macroalgae are recognized as the main producer of commercial alginate. These seaweeds (class *Phaeophyceae*) are algal species comprising complex multicellular brown algae with a wide range of sizes and morphologies [15, 16]. Their cell wall has a unique structure that contains phenolic compounds, proteins and high amount of carbohydrates. Among these components, alginate is the fundamental polysaccharide, which is found in the form of insoluble mixed salts of calcium, magnesium, sodium, barium, and potassium. There is also a large amount of alginate located in the intercellular matrix of algae and thus, total alginate content of biomass reaches up to 40% of dry weight [17]. The composition and the characteristics of alginate depend on the type of species, growth conditions, harvesting season, and extraction methods. In the work of Li et al. [18] it was shown that the chemical composition of the *Sargassum fusiforme* strain extensively varied during harvest and the highest alginate content was observed in June, whereas the alginate with maximum molecular weight and viscosity was obtained in May. In another study, a brown macroalgae *Treptacantha barbata* was cultured under four colors of light-emitting diode (LED) light including blue, red, green, and yellow and the blue LED light produced the highest sodium alginate content [19].

Today, all commercial alginates are obtained from algal sources and their composition varies among the species. The main genera containing a high amount of alginate are *Laminaria*, *Sargassum*, *Macrocystis*, *Ascophyllum*, *Lessonia*, *Ecklonia* and *Alaria* [20, 21]. Different macroalgae species and their alginate compositions are summarized in **Table 1**.

Previous metabolic studies have focused on the investigation of biological pathway of alginate synthesis in brown algae and bacteria that is another source of alginate. Despite the advances in molecular biology and genomic studies, the biosynthesis pathway and regulatory mechanism of alginate in algae have been poorly characterized. However, several studies of bacterial and algal alginate production have shown striking similarities in the basic pathway and thus these findings may provide strong clues regarding the mechanism in seaweeds [35, 36]. The molecular bases of alginate production begin with the fructose-6-phosphate and it is converted to guanosine di-phosphate-mannuronic acid (GDP-ManA) with a series of enzymatic transformations. Various enzymes including, mannose-6-phosphate isomerase (MPI), phosphomannomutase (PMM), mannose-1-phosphate guanylyltransferase (MPG), GDP-mannose/UDP glucose-6-dehydrogenase (GMD/UGD) are responsible for the synthesis of alginate precursor [35, 37]. GDP-ManA is then transferred across the cytoplasmic membrane and polymerized to the polymannuronate by the membrane-anchored proteins. After this stage, it may contain some residues unrelated to the alginate structure and it undergoes a modification step consisting of epimerization and degradation. The epimerization process is carried out by the mannuranoate C5-epimerases (MC5E) conducting the isomerization from mannuronic acid to guluronic acid. It should be underlined that the alginate synthesis route in bacteria differs from algae with the O-acetylation process that protects the produced alginate from degradation [38]. Finally, alginate polymer, composed of α-l-guluronic acid and the β-d-mannuronic acid, is formed, exported through the outer membrane, and released from the cell. Evidence for the biosynthesis of this polysaccharide within brown macroalgae come from a few studies with a limited number of species


#### **Table 1.**

*The alginate content of various algae species.*

such as *Ectocarpus siliculosus*, *Saccharina japonica* and *Laminaria digitate* [35, 37, 38]. Therefore, further research is needed to understand the metabolic route of alginate synthesis and to control the mechanism in different algae strains.
