Cyanobacteria as a Source of Biodegradable Plastics

*Mohanasundaram Yogeswar, Natarajan Valle and Arumugam Nagarajan*

### **Abstract**

Polyhydroxyalkanoates (PHAs) are a group of biopolymers produced from various microorganisms that attracted many researchers for their use as a substitute for conventional petrochemical plastics. PHA possesses similar material properties to petrochemical plastics with the added benefits of biocompatibility, biodegradability, hydrophobicity, thermoplasticity, piezoelectricity, and stereospecificity. The first discovery of PHA production in cyanobacteria was in 1969, and the commercialization of PHA produced from cyanobacteria is not feasible to date. The difficulty with the commercial production of cyanobacterial PHA is due to the low biomass production and lower PHA accumulation than the heterotrophic bacteria. The biosynthesis of PHA, production of cyanobacterial PHA, and strategies to improve the production of PHA and commercialization are discussed in this chapter.

**Keywords:** cyanobacteria, Polyhydroxyalkanoates, biodegradable polymers, bioplastics, bioprocess, PHB, P(HB-HV), PHA properties

### **1. Introduction**

Bioplastics are a type of plastic that can be produced from natural materials like plant starches and oils. By 2025, it is anticipated that the amount of petroleum used to produce plastic would have decreased by 15–20% due to the use of bioplastics, which are made from plants. Asia and Europe will hold the biggest market share for bioplastics by 2025. Asia will make up 32% of the market, followed by Europe at 31% and the United States at 28%. The market for bioplastics is now growing at a rate of 10% per year, accounting for 10–15% of the entire plastics business in 2016 and increased to 25–30% in 2020 [1]. Synechocystis, Spirulina, Anabaena, and Nostoc muscorum are cyanobacteria that can serve as bio-factories for the production of biofuel and bioplastic. They can produce biopolymers like polyhydroxybutyrate (PHB) and polyhydroxyalkanoates (PHAs), among other copolymers, that are both affordable and sustainable [2].

Recent bioplastics like Bio-PET are only called biobased since their monomers are made from corn, but the polymerization process is chemical, and the final polymer has the same qualities as traditional PET, making it nondegradable [3]. Scytonema geitleri and other cyanobacterial species can store internal poly-hydroxybutyrate granules for energy and carbon reserve when under stress. The environmentally benign and biodegradable PHB can then be collected and utilized to create biocompatible thermoplastics [4]. Polyhydroxyalkanoates (PHAs) are a type of polymer produced by cyanobacteria. PHAs are lipid compounds that a variety of microbes accumulate when there are abundant carbon sources present. They can be used for a variety of purposes, including the creation of bioplastics [5]. Cyanobacteria need only a small amount of nutrients to develop, and they produce PHAs through oxygenated photosynthesis [6].

Biochemical processes can naturally recycle bioplastics manufactured from renewable resources, reducing the need for fossil fuels and preserving the environment. Bioplastics are therefore environmentally friendly, generally biodegradable, and biocompatible. In many industrial applications today, including horticulture, food packaging, hygiene, AND composting bags, bioplastics have become essential. Additionally, bioplastics are utilized in biological, structural, electrical, and other consumer goods. With the demand for plastic usage increasing globally, a lot of research is being done to investigate green materials and novel processing techniques.

Chlorosis is the term for the dormant state that occurs when nutrients are scarce, such as nitrogen. During chlorosis, cyanobacteria deteriorate their photosynthetic machinery. Beyond this breakdown, there is a significant buildup of glycogen for the storage of carbon and energy. The process ends with the cells starting to break down the glycogen and turn it into PHB [7]. The sole PHA synthesized under the described photoautotrophic state out of the several PHAs is PHB. It is possible to add organic carbon precursors like valerate to make the additional short-chain-length PHAs (scl-PHAs), such as P(3HB-co-3 HV). Long-chain-length PHA or mcl-PHA have not yet been found in cyanobacteria. The most effective known catalyst for PHB synthesis in cyanobacteria is nitrogen restriction [8]. It has been noted that elements including culture conditions, such as N, P, light exposure, and CO2 dynamics, have an impact on cyanobacteria's ability to produce PHA. Additionally, it has been noted that additional elements including two-stage (growth and PHA accumulation) processes, metabolic inhibitors for other pathways, and bioengineering have a favorable effect on PHA production [9–13].
