α(1→3) α(1→4) (residues)
*EPS: Exopolysaccharides.
Seaweeds can typically contain 2–13% (w/w) of crude cellulose, with brown algae leading the group [10]. In comparison, cotton is a well-known example of natural pure cellulose fiber with more than 90% (w/w) cellulose content [11] and widely used in the textile industry, while plant fibers can contain ~26–85% (w/w) cellulose [12]. For commercial purposes, cellulose obtained from bacterial fermentation is more appropriate due to the higher growth rate of bacteria and the possibility of genetic and/or media manipulation for enhancing the content.
Chemically, cellulose consists of glucose residues linked to β (1→4); it is a water-insoluble, linear molecule deprived of branching or coiling. Cellulose can be broke into its constituent glucose moieties by mineral acid treatment or enzymatic hydrolysis [2].
3.3.2 Chitosan
Chitosan, a partly deacetylated form of chitin obtained by heating it with a heavy alkaline, as sodium or potassium hydroxide is a poly-β (1–4)-2-amino 2-deoxy-d-glucopyranose. Chitin is the main structural component in crustacean (shrimps, lobsters, crabs) scales and exoskeleton; it is produced as waste from the seafood industry from where it is collected for producing analytical grade chitin and chitosan. After cellulose, it’s the second richest natural polysaccharide on the earth [13] and is a major component of fungal cell walls. Chitin is a N-acetyl glucosamine polymer (GlcNAc), chemically aligned with β (1→4) binding. The alkali treatment partially removes some of the acetyl groups in chitin, producing chitosan. Chitosan is a polymer with residues connected by β (1→4), of N-acetylglucosamine (GlcNAc) and glucosamine (GlcN). The reaction of deacetylation leads to shift in molecular weight and its extent is calculated by the deacetylation degree (DD) definition, described as the molar fraction of glucosamine in chitosan (composition of N-acetyl glucosamine and glucosamine). Both chitin and chitosan belong to the aminoglucopyran family of polysaccharides [14, 15].
It is noteworthy that, due to the strong hydrogen bonding, the chitin-based components are stable in nature [16], while, in the presence of other cross-linking molecules, the chitosan-based substances are relatively easier to treat or handle [17]. Chitosan is processed in various ways, such as powder dust, glue paste, fabric and therefore is extensively used in the industrial applications [18]. The presence of various polar functional groups (–OH, –NH2 and C–O–C) implies that these polysaccharides have a high water retentivity capacity. Primary amine groups and primary alcoholic groups are present at C-2 and C-6 position along with secondary hydroxyl group [14]. The hydrogen bond insolubility of chitosan in bulk is insoluble at neutral and alkaline pH, the extent of which is determined by the DD (~60–100%). When in contact with anions, chitosan starts gelling, forming bead like structures under mild conditions [14, 19].
Chitosan are derived and processed from chitin either by chemical or enzymatic treatment as the glycosidic bonds are sensitive to these treatments. Enzyme treatment of chitin leads to the production of oligomers and comparatively, a more stable, well-defined chitosan is formed [20]. The mean chitosan molecular weight varies ~3.5–20 kDa (66–95% deacetylated) [21].
Use of chitosan as an effective flocculant due to factors like high ionic strength, abundantly available compound, biodegradable in nature and low rate of toxicity compared to any chemical flocculant is well established [22]. Chitosan consists of positive charges due to its high charge density ratio and microalgal cells are negative charge, therefore it is easily absorbed by the cells resulting in destabilizing the microalgae. Firstly, it neutralizes charges on microalgae, electrostatic charges are weakened and thus, reducing the interparticle repulsion leading to charge neutralization. The prices of biopolymer (chitosan) are substantial and expensive, at almost $21 per kg (Qingdao Yunzhou Biochemistry Co., Ltd) [23]. Although the cost of chitosan compared to the chemical inorganic flocculants are higher, it is a preferred methodology for harvesting the microalgal biomass. Chitosan, due to its divergent ability and properties like biodegradability, biocompatibility, bioactivity, etc., is employed in various applications like wastewater treatments, food processing, biomedical engineering, drug delivery, etc. [24, 25].
3.3.3 Alginate
Alginates, a common name for alginic acid salts, is a category of anionic polysaccharides occurring primarily in brown marine algae (species belonging to Laminaria, Macrocystis, Ascophyllum, Sargassum) and other bacterial organisms (such as Pseudomonas, Azotobacter) [26, 27]. Bacterial alginates can be secreted by the strains as extra cellular material. However, at present, commercial alginate is derived from farmed brown seaweeds, since they can contain up to 40% (w/w) alginates in the form of an intracellular gel complexed with ions such as calcium, magnesium, and sodium [28, 29]. These are 1,4-linked linear polymers of β-D-mannuronic acid and α-L-guluronic acid [30]. In the midst of crosslink divalent (namely Ca, Ba and Sr) ions, alginates can form transparent gels that play an important role in nutrition and medicines.
3.3.4 Carrageenan
Carrageenans are hydrocolloids (water-soluble gums) present in the intracellular matrices of Rhodophyta (red seaweeds); its function in land plants is similar to that of cellulose. These comprise of linear high-molecular polysaccharides (100–1,000 kDa) consisting of D-galactose and a sulfatic substitution of 3,6-anhydro-D-galactose units with methyl ethers [31]. Carrageenans are divided in to six classes: [i] kappa (κ); [ii] lambda (λ); [iii] iota (ι); [iv] mu (μ); [v] nu (ѵ); [vi] theta (θ). κ-carrageenan, sourced from Kappaphycus alvarezii, consists of D-galactose sulfated at C4. It reacts in existence of K+ with milk proteins and form gels. ι-carrageenan, sourced from Eucheuma denticulatum, is composed of C4 sulfated D-galactose linked to a C2 sulfated anhydro-galactose. In the midst of Ca2+, it usually gels. Λ-carrageenan is composed of C2 sulfated D-galactose linked to C2, C6 sulfated D-galactose [32]. It does not have gelling capabilities; hence, itis used in dairy products as thickening agents. Generally, carrageenans are highly anionic polymers, with food-based applications due to their gelling, thickening, and stabilizing characteristics. They are also a potential feedstock for third generation biofuels [33, 34]. Carrageenans are hygroscopic however, in organic compounds are insoluble, an important feature for their gelling ability; the solubility varies according to pH and temperature. The gels are reversible, i.e. they melt at specific temperatures and reform after cooling [35, 36].
3.3.5 Agar
In the cell wall of agarophytes (red algae), Agar is a hydrophilic complex polysaccharide, which includes members of Gelidium and Gracilaria sp. It is a linear co-polymer L-galactopyranose with alternate associations of α (1→3) and β (1→4) units. Based on the purification process, residues of its parent porphyrane, D-galactose and L-galactopyranose6-sulfate may also be detected [37]. It is composed of two different fractions: agarose (neutral, gelling fraction, composed of galactose) and agaropectin (surface charged, non-gelling, composed of agarose, 3–10% (w/w) sulfate, varying amounts of ester sulfate, D-glucuronic acid, and pyruvic acid). The proportion of these fractions varies according to the seaweed [38]. Compared to other hydrocolloids, agar has excellent gelling and solubility characteristics. Due to repeating alternate sugars with α(1→3) and β(1→4) bonds, agar can form helical dimers similar to that of carrageenans. However, due to a comparatively lower content of anionic sulfates, agar forms nearly 2–10 times stronger gels with a melting point close to the boiling point of water [39].