**1.3 Cyanobacteria**

In recent years, there has been a continuous search for new water-soluble polysaccharides, particularly those produced by microorganisms including cyanobacteria [4]. Cyanobacteria or blue-green algae are Gram-negative prokaryotes that perform oxygenic photosynthesis and are unicellular or filamentous. They are capable of movement by gliding when in contact with the substrate [5] and also possess the ability to survive desiccation, extremes of temperatures, high pH, and salinity [6]. They are widely distributed in diverse habitats. During their life cycle, cyanobacteria exocellularly secrete outer investments mostly constituted by heteropolysaccharides, which are frequently associated with small amounts of non-carbohydrate substituents, such as peptide, DNA, and fatty acids [7]. These exopolysaccharidic secretions

#### *Green Synthesis of Silver Nano-Particle from Cyanobacteria and Effect on Microalgal Growth… DOI: http://dx.doi.org/10.5772/intechopen.106039*

are metabolites that accumulate on the surface of microbial cells. Their presence is considered as a boundary between the microbial cell and its immediate environment serving as a barrier to successfully cope with environmental constraints against high or low temperature and salinity or against possible predators and desiccation. The production of exopolysaccharides from cyanobacteria is considered to be a good alternative for polysaccharides produced by other organisms including higher plants, bacteria, fungi etc. This is owing to the versatile nature of cyanobacteria which are able to grow in any adverse environmental conditions. Their photosynthetic mode of nutrition and simple cultural requirements further add to the convenient growth of these organisms for large-scale production. In addition, the yield of the products obtained from these organisms can be enhanced by manipulating the culture conditions [8].

#### **1.4 Cyanobacterial EPS**

There are two categories in which cyanobacterial EPS can be grouped, the first one being those which are associated with the cell surface known as cell-bound or capsular polysaccharides (CPS) and the other being the released polysaccharides (RPS) referring to those that are discharged into the surrounding environment. Depending on the thickness, consistency, and appearance, the EPS associated with the cell surface can be termed sheaths and slimes [1]. The sheath is a thin, dense layer loosely surrounding the cells or cell groups usually visible in light microscopy without staining. The slime, on the other hand, refers to the mucilaginous material dispersed around the organism but does not reflect the shape of the cells. On the contrary, the RPS is soluble aliquots of polysaccharidic material released into the medium, either from the external layer(s) or derived biosynthetically which can be easily recovered from liquid cultures.

The cyanobacterial EPS are high molecular weight complex hetero-biopolymer of 10 kDa–2 MDa. This complexity is due to the presence of branching among the monomers and frequently with other macromolecules [9]. These high molecular weight heteropolysaccharides are made up of linear or branched repeating units comprised of 2–10 monosaccharides such as hexoses, pentoses, uronic acids, and deoxy-sugars. While other important substituents include phosphate, sulfhate, acetate, pyruvate, proteins and lipids form the side chains. EPS are attached to the cell surface via hydrogen bonds, hydrophobic and electrostatic interactions.

Certain characteristic features are exhibited by the cyanobacterial EPS which are rarely found in the EPS produced by other microbial groups. For instance, the presence of uronic acid and sulfhate groups contribute to the anionic nature of the cyanobacterial EPS, conferring a negative charge and a "sticky" behavior to the overall macromolecule [1, 10]. The anionic charge plays an important role in building the affinity of these EPS towards cations, notably metal ions. Furthermore, many cyanobacterial EPS are also characterized by a significant level of hydrophobicity due to the presence of ester-linked acetyl groups, peptidic moieties and deoxysugars such as fucose and rhamnose. In the past decades, several factors controlling the production of cyanobacterial EPS have been identified. These include energy availability and the C: N ratio [11]. However, other important factors such as the effect of other nutrients as well as growth conditions such as light intensity, salinity, and temperature have not been much focused. Hence, EPS production by variation of different growth parameters becomes an important area of study.

### **1.5 Role of cyanobacterial EPS**

Cyanobacterial EPS plays a major role in protecting cells from various stress conditions in extreme habitat by serving as boundary between the cell and the surrounding environment. EPS are considered to maintain the structure and function of the biological membrane, hence, protecting them from irreversible and lethal changes during desiccation. They possess hydrophobic/hydrophilic characteristics, owing to which they are able to trap and accumulate water; thus creating a gelatinous layer around the cell that regulates water uptake and loss and stabilizes the cell membrane during the periods of desiccation. Cyanobacterial sheath formed by EPS protects the cells from the detrimental process of biomineralization [12].

Polysaccharidic layer around the cell, in addition, prevent the cell from direct contact with toxic heavy metal present in the surrounding. Being negatively charged, these cyanobacterial EPS plays an important role in sequestration of metal cations and also create a microenvironment enriched in those metals that are essential for the growth of the cell which is otherwise present in low concentration in certain environments. The slime layer surrounding the cyanobacterial cell prevents the inactivation of nitrogenase enzyme, an enzyme responsible for nitrogen fixation which otherwise gets inhibited in presence of atmospheric oxygen. Cyanobacterial sheath also contains some UV absorbing substances such as scytonemim and mycosporine-like amino acid which protects the cell from the harmful effect of UV rays. Another important role of exopolysaccharides is that it helps in the gliding movement of cyanobacteria and also acts as an adhesive for cyanobacterial cell that lives in association or symbiosis with higher plant.

#### **1.6 Applications of cyanobacterial EPS**

Cyanobacterial exopolysaccharides possess potential applications in various fields such as food, cosmetics, environmental improvement, pharmaceutical, and water treatment industries [13, 14]. Due to the presence of both hydrophilic and hydrophobic groups in the macromolecules these exopolysaccharides act as emulsifying agent or biofloculant. Another interesting industrial application is that they have the ability to bind with the water molecules due to the presence of charged groups, finding their application in the cosmetic industry for product formulations [10]. These charged RPSs also have the capability to trap metal ions which may be used in the removal of toxic metal from polluted waters.

The most common industrial use of microbial polysaccharides is that they act as thickening agents because of their ability to modify rheological behavior of water, [15], and also to stabilize the flow properties of their aqueous solutions under drastic changes in temperature, ionic strength, and pH [1, 10]. These exopolysaccharides are water-soluble and can be used as swelling agents in the food industry due to the presence of cations such as Ca+2, Fe+3, Al+3, Cu+2, and Co+2. The cyanobacterial exopolysaccharides also find their use as soil conditioners due to the N2-fixing ability of some cyanobacterium colonies. Microbial exopolysaccharides can also be considered bioactive substances due to their possession of biological activities, such as antibacterial, anticoagulant, anti-oxidative, anticancer, and anti-inflammatory activities. This is because of the presence of sulfhate group in the molecules which interfere with the absorption and penetration of another microorganism thereby preventing or inhibiting the activity of that microorganism.

*Green Synthesis of Silver Nano-Particle from Cyanobacteria and Effect on Microalgal Growth… DOI: http://dx.doi.org/10.5772/intechopen.106039*

#### **1.7 Extraction**

As discussed above, whilst some EPS are tightly bound to the cell structure, others are free and directly released (RPS). Therefore, there exist some differences in their extraction methodologies. RPS can be separated using physical methods such as high-speed centrifugation and ultra-sonication whereas, firmly cells-associated EPS requires chemical methods for extraction. EPS cross-linked by divalent cations can be released from the biofilm matrix by complexing agents such as ethylenediamine tetraacetic acid (EDTA), cation-exchange resins such as Dowex or by formaldehyde treatment with or without sodium hydroxide [11].

#### **1.8 Characterization**

The monosaccharides forming the cyanobacterial biopolymers consist of many isomers and show limited absorption in UV-Vis regions making the analysis of polysaccharides very difficult in terms of detecting or identifying the macromolecule using absorbance or mass spectrometry. Total carbohydrates content can be determined by using the phenol-sulfuric method [16]. For analysis of carbohydrate composition, high-performance liquid chromatography (HPLC), however, remains the most widely used technique because of its high selectivity, sensitivity, and reliability compared to other analytical methods [7].

Though present in lower concentrations, other non-carbohydrate constituents (like protein, lipid, nucleic acid, etc.), also impart very important characteristics to the EPS due to their unique linkage to sugar moieties. Hence, the determination of these components is also of vital importance. In this regard, Fourier Transformed Infrared (FTIR) spectroscopy can be used to characterize the vibrationally active functional groups within polysaccharides.
