Читать книгу Polysaccharides - Группа авторов - Страница 66

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].