**3. Cyanobacteria as a source of bioplastics**

The PHB accumulation in cyanobacteria was first reported by Carr G.N. in 1966 with up to 10% (dcw) in *Chlorogloea fritschii* [41]. In photoautotrophic culture *Artrospira platensis (Spirulina)* accumulated a maximum PHB of 6%, and it is very less for exploiting cyanobacteria for PHA thermoplastics production. However, PHA biosynthesis in, *Synechocystis sp*. PCC 6803*, Nostoc muscorum*, and *Synechococcus sp.* MA19 produced up to 38, 46, and 55% (dcw), respectively, under different limiting culture conditions reported in studies [9, 42, 43]. The production of PHA reported in several other strains in photoautotrophic and also with supplementation of acetate or other organic carbon sources are profoundly lower compared to heterotrophic bacteria. Chen et al. reported a maximum accumulation of poly(3-hydroxybutyrate-co-3 hydroxyhexanoate) [P(3HBco-3HHx)] co-polymer up to 50% (dcw) in *Aeromonas hydrophila* 4AK4 grown in 5% glucose medium with 5% lauric acid under phosphorous limitation with a productivity of 540 mg.L<sup>1</sup> .h<sup>1</sup> [44]. Despite the high PHA accumulation, bacterial PHA thermoplastic has commercial limitations since the organic carbon substrate itself accounts for 30–50% of the total cost of production on a large-scale [15]. For example, PHB production of up to 77% (dcw) was reported in recombinant *Escherichia Coli* using glucose as substrate with a productivity of 3200 mg.L<sup>1</sup> .h<sup>1</sup> ; however, the carbon source used accounts for 38% of the overall cost of production [45]. Compared to heterotrophic bacteria (4–5% carbon substrate), cyanobacteria required significantly lower carbon substrate at about 0.4% [44, 46]. Thus, cyanobacteria are a more promising candidate for the large-scale production of bioplastics.

#### **3.1 General Cultivation of cyanobacteria**

Cyanobacteria are photosynthetic prokaryotes found in both fresh and marine water, soil, etc., and they have a unique physiology that makes them survive even in *Cyanobacteria as a Source of Biodegradable Plastics DOI: http://dx.doi.org/10.5772/intechopen.110376*

harsh ecological habitats such as deserts, hot springs, volcanic substrates, and even in alkaline basins. The cyanobacteria can be cultivated in three different culture systems an open-raceway pond (mostly preferred), a closed system (photobioreactor), and a hybrid system (combination of both open and closed systems) [47]. The widely used media for the cultivation of cyanobacteria is BG11 having the following composition (1500 mg.L<sup>1</sup> NaNO3, 31.4 mg.L<sup>1</sup> K2HPO4, 36 mg.L<sup>1</sup> MgSO4, 36.7 mg.L<sup>1</sup> CaCl2.2H2O, 20 mg.L<sup>1</sup> Na2CO3, 1 mg.L<sup>1</sup> NaMgEDTA, 5.6 mg.L<sup>1</sup> citric acid, 6 mg.L<sup>1</sup> ferric ammonium citrate, and 120 mgL-1 NaHCO3) (himedialabs). The components of the modified BG-11 used in the reactors are (K2HPO4, NaNO3, NaHCO3, CaCl2.2H2O, NaOH, Na2EDTA, and NaHCO3). The optimum pH and temperature for the growth of cyanobacteria are 7.5–9 and 30 2°C, respectively. The culture takes up to 7 days to reach the log phase, and the complete growth cycle ends in 20 days (after reaching the death phase).

#### *3.1.1 Open systems*

Open ponds are the natural ecosystem in which the algae tend to grow and develop. Open systems are classified into two types—natural (lakes and ponds) and artificial (containers and artificial ponds). There are several advantages of growing cyanobacterium in open systems which include low investment, construction of the pond being easier, and easy maintenance. Some of the drawbacks include a requirement for large land, poor light penetration, and low biomass productivity [48].

#### *3.1.2 Closed systems*

Photobioreactors are considered to be the closed system for the cultivation of cyanobacterium. By using this culture system, the drawbacks of the open system can be neglected. There are several advantages of using a closed system for algal cultivation which include control over culture parameters (pH, temperature, etc.,), low level of contamination, and good mixing that induces high gas exchange within the culture. There are various types of closed system available for the culture of algae which includes vertical column, tubular bioreactor, flat-plate bioreactor, etc [48].

#### *3.1.3 Hybrid system*

A combination of both open and closed systems is known as a hybrid system. There are two stages of cultivation in which the first stage involves a closed system and the second stage occurs in the open-raceway system. By utilizing this system, the advantages of both open and closed systems are possible. Many ongoing studies are designing a commercial-scale hybrid reactor that can be economical and can be easy to handle [49].

### **4. PHA from cyanobacteria**

#### **4.1 Biosynthesis of PHA in cyanobacteria**

For decades it was believed that cyanobacteria possess an incomplete Kerbs cycle like some prokaryotes due to the absence of the 2-oxoglutarate dehydrogenase

complex which performs the conversion of 2-oxoglutarate to succinyl-CoA in the TCA cycle [50]. Since the TCA cycle is incomplete, it is assumed that the breakdown of PHB polymers generating acetyl-CoA could be utilized neither for the production of cell components nor for energy generation [51]. It was hypothesized that this cycle was closed by the glyoxylate stunt of aspartate transaminase reactions [52]. However, recent studies reported that the Kerbs cycle was completed with help of γaminobutyric acid shunt and 2 enzymes 2- oxoglutarate decarboxylase and succinic semialdehyde dehydrogenase found in *Synechocystis sp.* PCC 6803 [53] and *Synechococcus sp.* PCC 7002 [54], respectively. The later reported protein-encoding genes are present in most cyanobacteria with variation in their organization.

The PHA polymer biosynthesis is linked with mobilization or depolymerization [5]. The PHA polymers usually undergo a cyclic process of biosynthesis and depolymerization, where the PHA is formed from acyl-CoA precursors via different metabolic routes under nutrient depletion/limitation conditions as the carbon source is stored as polymer granules in the cells. The mobilization of PHB polymers is carried out by intracellular PHB depolymerase generating acetyl-CoA which is used to generate oxidation via the Krebs cycle. Many studies reported the regulatory effect of acetyl phosphate produced by the phosphotransacetylase catalytic activity on the posttranslation of PHB synthase enzyme [55–59]. The exploitation of exogenous carbon sources such as glucose, fructose, and acetate showed decreased mobilization and increased biosynthesis of PHA [60–63].

#### **4.2 PHA production**

The PHA-producing cyanobacterium is classified into two groups—one group requires a limitation of an essential media component for PHA production, and another group does not require any limitation in nutrients for the production of PHA. The cyanobacterium that can be cultivated without nutrient limitation is preferred on an industrial scale. A few studies have been conducted to optimize the nutrients for the production of PHA and PHB on large scale in batch mode. In a study, *Synechocystis* sp. PCC 6803 was cultivated in BG11 media with reduced nitrogen concentration and showed a maximum PHB accumulative of 180 mg.ml<sup>1</sup> [47], *Synechocystis* sp. CCALA192 was cultivated in a 200 L tubular photobioreactor in batch mode and accumulated a maximum of 125 mg.ml<sup>1</sup> of PHB, and a wild-type cyanobacterial strain *Synechocystis* sp. PCC 6714 produced a maximum of 640 mg.L<sup>1</sup> of PHA when cultivated in optimized growth media [64].

Several studies reported that higher PHA accumulation in cyanobacteria occurs under nutritional stress activating the PHA biosynthesis pathway. According to Mendhulkar and Shetye [65], the metabolic pathways are diverted to produce carbonrich compounds for energy storage, such as PHAs, and glycogen, when the cyanobacteria experience nutrient deficiency (nitrogen and/or phosphorus). The study on cyanobacteria *Synechococcus subsalsus* and *Spirulina sp.* LEB18 in nitrogendeficient environment revealed that the carbon source is diverted to other metabolic pathways for biopolymer production which is used as energy storage and reused in favorable conditions [66]. PHA accumulation in *Botryococcus braunii* and *Synechocystis salina* grown in BG-11 medium without any nutritional limitation was reported [67, 68]. Different nutritional conditions are employed to increase the production of PHA such as excess or limited levels of nitrogen and/or phosphorus, acetate, and propionate, and various other conditions like salinity, gas exchange, wastewater as a source, etc., were summarized in **(Table 1).** Apart from culture condition variations,

*Cyanobacteria as a Source of Biodegradable Plastics DOI: http://dx.doi.org/10.5772/intechopen.110376*



#### **Table 1.**

*PHA production in cyanobacteria under different culture conditions.*

highly productive strain selection can also increase the PHA accumulation yields ranging from 5.0% to about 70% (dcw).

Coelho et al., [77] reported higher percentages of PHA accumulation in *Spirulina sp.* using Zarrouk medium with nitrogen and phosphorus limitations of 30.7% and 14.1% (dcw), respectively. Phosphorus and gas exchange limitations along with additional acetate and nitrogen and phosphorus limitations in *Synechocystis sp.* PCC 6803 lead to PHA accumulation of about 38% and 11% (dcw), respectively [69]. Studies conducted by Bhati and Mallick on PHB-PHV co-polymer production in *N. muscorum* under nitrogen and phosphorus deficiency resulted in co-polymer accumulation of about 60% and 69% (dcw), respectively [10, 11]. Samantaray and Mallick reported a maximum of 85% (dcw) PHB and 77% (dcw) PHB-co-PHV in *Alusira fertilisima* CCC444 under nitrogen deficiency with fructose and valerate supplementation and phosphorus deficiency along with additional citrate and acetate, respectively [79, 80].
