3.3.7 Fucoidan
Fucoidan, or fucose-containing sulfated water soluble heteropolysaccharides, is in brown algae cell walls. Fucoidan would provide the cells with structural integrity and protect against dehydration [44, 45]. There are two groups of fucoidan based on the structures, one is composed of -L-fucopyranose (1→3) and the other is comprised of -L-fucopyranose residues (1→4). Sulfation or sulfate esters at 2,3 and/or 4 position are based on the glycosidic linkage and less than 10% are monosaccharides [46]. The exact composition varies according to the types of linkage, substitution, branching, sulfation, species, strain and geographical pattern; the biopolymer contains fucose, galactose, mannose, arabinose, xylose and sulfate. Algal fucoidans are low molecular weight (<30 kDa), formed by depolymerization [44, 47].
Fucus vesiculosus, a brown alga, contains 20% of its dry weight as fucoidans while algal cell walls are made up of 40% (w/w) of fucoidans. Strains commonly employed and used for the extraction of fucoidans are Ulva pertusa, Porphyra haitanensis, Laminaria sp., Sargassum glaucescens, S. horneri etc., as sulfated polysaccharides. The marine macroalgae are classified under mucopolysaccharides of brown macroalgae other than frame and storage polysaccharides [21].
Extraction of fucoidan carried out by several classical or novel methods, like, various chemical methods, water as solvent method, pretreatment, chromoatographic methods, microwave/ultrasound assisted extraction, enzyme extraction, etc. Pretreatments step is a requirement for removing chlorophyll, mannitol, salts and some compounds [48]. There is a long list role played and highlighted by fucoidan in human health applications, due its properties like anticancer, antiviral and anticoagulant. It is also found to stimulate immune systems in many ways like by altering the surface properties of cells or oral intake directly inhibiting, immunomodulating by promoting recovery of cells, acting as anti-inflammatory agents, etc. One assumption is that in fucoidans these bioactivity is because of presences of branched units within the sugar backbone [44].
3.3.8 Ulvan
Ulvan is a polysaccharide compound, derived from Ulva and Enteromorpha sp. with key constituents of rhamnose, xylose and glucuronic acid. The main repeating unit is a disaccharide known as ulvanobiouronic acid. Ulvans are typically known for the broad distribution of molecular weight. Its solutions are with low viscosities, which is indicative of its polydisperse nature with a higher proportion of short chain polysaccharides or a branched structure. Ulvans are able to form gels; although the mechanism for it is still unknown. They are also known for their ion binding abilities, which contributes to UIva sp. being an effective heavy metal binder [49].
3.3.9 Exopolysaccharides From Microalgae
Exopolysaccharides are polydisperse, polymers consisting of heterogeneous polysaccharides such as glucose, galactose, rhamnose, xylose, iduronic acids, methylated sugars like fucose and methyl galactose; they either adhere to the cell surface or are secreted by microbes into their environment [50]. The polydispersity index is sometimes used as an indication of their molecular weight distribution; a higher index means a broader distribution. It means that the polymers contain a high number of polysaccharides of different chain lengths [51]. Therefore, these natural biopolymer materials are capable of holding cells in proximity, forming an heterogenous matrix in the environment. They protect the cell against environmental conditions (salinity, drought) and are also carbon and energy pools for their hosts [52]. When present as extracellular excretions, they are called exopolysaccharides (EPS); those from algal cells are called algal EPS. The physicochemical properties of microalgal cells are significantly affected by the presence of these EPS [53, 54]. Basically, polysaccharides are categorized into three groups, based on their structure, function and localization, namely, intra-polysaccharides (also called storage polysaccharides), and extra polysaccharides (also called as structural). The latter is the most studied and consist of complex structure and are called as EPS. It could be categorized as cell bound (BPS) polysaccharides, sub-grouped into sheath, capsule and slime; they can also be known as released (RPS) polysaccharides, which are generally colloidal and consists of exudates with low molecular mass. Generally, EPS are sulfated polysaccharides and consist of extracellular polymeric substances [55, 56]. The EPS of green alga S. acuminatus consists of fucose- and mannose-(1→2/1→4) linked residues whereas those of C. vulgaris consist of 1→6 linked galactose where 67% of the total polysaccharide backbone is composed by β-1,6-D-galactopyranose [57].
Among 18 species in Chlamydomonas the distribution pattern of monosugars were identical [58]. The composition of EPS changes with different species; e.g., in C. vulgaris EPS comprises of either fucose (or) arabinose whereas it lacks xylose in C. ellipsoidea [59]. The EPS secreted from Ankistrodesmus densus [60] and Botryococcus braunii does not contain sulfate [61]. Galactose, followed by fucose, rhamnose and glucose along with methyl sugars are the major sugars found in B. braunii EPS. These changes in the sugar composition among different species and strains could be because of the different experimental set-up and analysis methodologies adopted. Red marine microalgae, under nitrogen stress condition with specified ratio of nitrogen/phosphorus content, produces EPS. Ascophyllum, Palmaria and Porphyra sp. consist of highest polysaccharide contents whereas Ulva sp. constitutes approximately 65% polysaccharides per dry weight content [21]. The EPS from Porphyridium and Rhodella sp. are anionic and sulfated [62]. The maximum EPS productivity reported in literature review is by cyanobacteria 2.9 g l−1 d−1 (Anabaena sp.). Microalgae, grown either in heterotrophic or autotrophic growth conditions, produce approximately 0.36 g l−1 d−1 (Chlorella sp.) and 0.19 g l−1 d−1 (Porphyridium sp.) EPS [55, 56, 63]. On an average, the maximum EPS concentration varies between 0.5 and 1 g/L among strains of cyanobacteria, (for example, Cyanothece sp.) and microalgal strains like B. braunii and Chlorella sp. Commercially cultivated marine and brackish species like, D. salina, Isochrysis glabana, Nannochloropsis salina, Phaeodactylum tricronutum, etc. are also considered as potential EPS sources [64, 65].
3.4 Applications of Polysaccharides
Diversity in the entire polysaccharide kingdom is due to the evolutionary pressures faced by these source organisms since millions of years; this diversity is the basis of utilization of these biopolymers for human use. Polysaccharides have found specific and critical applications in food, biomedical, food, pharmaceuticals, and agriculture sectors. Although an extensive discussion of each and every application would be outside the scope of the present text, the main applications of each of the polysaccharide have been summarized below.
3.4.1 Biomedical Applications
3.4.1.1 Cellulose
The biodegradable and biocompatible nature of cellulose along with its natural origin and availability resulted in its application as tissue scaffold, bio composite for wound healing and dressing, drug delivery system and blood purification [66]. Purification could be carried out both by membranes and microbead shaped composites [67, 68]. It also spawned the development of commercial products for skin grafting, dental implants, temporary skin, cornea scaffolds, artificial blood vessels, and wound dressing material [69]. Cellulose has also been used for drug delivery [70–74]. Owing to its rigid matrix, cellulose is adept at supporting cellular structures on its framework. This property along with its biocompatibility leads cellulose a promising prospect for tissue engineering. For instance, Ninan et al. have reported cellulose bio composite as an effective support for fibroblast cell line [75] while Muller et al. used cellulosic support for cartilage cell growth [76]. Besides, cellulose nanocrystals are attracting attention for their applications in tissue engineering [77] while nanoscale cellulose materials are being studied as well [78].
