A review on production of poly β hydroxybutyrates from cyanobacteria for the production of bio plastics

Highlights

• The importance and awareness of using cyanobacteria as PHB resource are reported.

• Genetic engineering of microorganisms to introduce pathways in PHB discussed.

• Alternative substrates and novel extraction methods of PHB are discussed.

• Future prospects of cyanobacterial PHB in the industry and research are discussed.

Abstract

The increasing effect of non-degradable plastic wastes is a growing concern. As an alternative, researches are being attempted from living resource to produce bio plastics on the basis of their biodegradability. Due to their cost effect nature, now the scientists are searching an alternative resource like photoautotrophic cyanobacteria. In this review the promising importance and growing awareness of using cyanobacteria as PHB resource are being reported. Many publications evidenced that various cyanobacterial species accumulate intracellular poly-β-hydroxybutyrate granules as energy and carbon reserves inside their cells when they are in stress conditions. PHB is biodegradable, environmental friendly and biocompatible thermoplastics. Varying in toughness and flexibility, depending on their formulation, they can be used in various ways similar to many non-biodegradable petrochemical plastics currently in use. Promising strategies involve genetic engineering of microorganisms to introduce production pathways are being investigated for the past two decades. Such kind of researches focusing on the use of alternative substrates, novel extraction methods, genetically enhanced species and mixed cultures with a view to make PHB from cyanobacteria (blue green algae) more commercially attractive are presented and discussed.

Introduction

Poly-β-hydroxybutyrate is a wide spread intracellular storage compound typically in prokaryotic organisms [1], [2], [3], [4], [5]. The properties of pure poly-β-hydroxybutyrate including thermoplastic process ability, absolute resistance to water and complete biodegradability suggest that PHB could be an attractive to common plastics and would fit well with new waste management strategies [6], [7], [8], [9]. The use of PHB produced by bacterial fermentation as a commodity polymer is limited by its high production cost compared to some widely used petroleum derived plastics. The number as well as the types and potential qualities have greatly increased the production of superior materials such as epoxides, and polysulfones, and have become one of the most widely used products all over the globe [10], [11], [12], [13], [14].

Durability and resistance to degradation are desirable properties when plastics are in use, but they pose problems for disposal when out of use. These non-biodegradable plastics accumulate in the global environment at a rate of 25 × 10⁶ t per year passing serious threats to the solid waste management program [15], [16], [17], [18]. In today's modern era of science and technology plastics have become one of the most widely used materials all over the world [19], [20], [21], [22], [23], and applications are nearly universally important in automobiles, home appliances, computer equipment packages and even medical applications. The quality of plastics and its uses in day today life have long been vilified because they are environmentally unfriendly and they are not biodegradable [24], [25], [26], [27]. So today's demand for biodegradable plastics is one of the most important targets both for basic and applied research.

In the early 1920s, Lemoigne a microbiologist at Pasteur Institute in Paris isolated a polymer from Bacillus megaterium by chloroform extraction and demonstrated that it was a polyester of 3-hydroxybutyric acid [28], [29], [30], [31], [32]. Since Lemoigne discovered PHB, the polymer has presented many challenges to microbiologists and biochemists who are interested in its physiological functions and metabolism. The general knowledge of microbial PHB was first summarized in a comprehensive review by Dawes and Senior in the year 1973. Later, it was found that PHB is only one type in a huge family of polymers collectively known as polyhydroxyalkanoate (PHA). In 1974, PHB was isolated by chloroform extraction of activated sludge [33], [34], [35]. The monomers that were detected in chloroform extracts of activated sewage sludge are 3-hydroxyvalerate (3HV) and 3-hydroxyhexanoate (3HH) as the major and minor constituents respectively. About a decade later following the identification of heteropolymers, the analysis of marine sediments by capillary gas chromatography revealed the presence of 3HB and 3HV as the predominant components among 11 other short chain 3-hydroxyalkanoate monomers [36], [37], [38]. Likewise, research on finding new PHBs is also in the streamline.

Among the 150 different types of polyhydroxyalkanoids identified so far, the homopolymer of hydroxybutyrate like PHB is widespread in different taxonomic group of prokaryotes including cyanobacteria. The properties of pure PHB including thermoplastic processibility, hydrophobicity, complete biodegradability and biocompatibility with optical purity have increasingly become of interest as a raw material for biodegradable plastics [15], [39], [40]. Cyanobacteria can be considered as an alternative host system due to their minimal nutrient requirements and photoautotrophic nature. Cyanobacterial species have the ability to accumulate the homopolymer of PHB under photoautotrophic condition [41], [42], [26] Cyanobacteria are capable of accumulating PHB. Industrial utilization of cyanobacteria as PHB producers has the advantage of converting waste carbon dioxide, a greenhouse gas to environmental friendly plastics using the energy of sunlight. Various species of cyanobacteria accumulate considerable amounts of PHB [43], [44]. Based on the literature available on the cyanobacterial PHB production, this review has been compiled and reported with a clear view on the current status, future prospect and needed improvement in this area.