Bio-plastics: A way towards sustainability

Balaram Mohapatra, Environmental Microbiology Lab, Department of Biotechnology
Indian Institute of Technology(IIT), Kharagpur (West-Bengal) -721302, India

Large scale industrialization and exploitation of land and water resources have culminated in considerable environmental degradation affecting natural homeostasis and flora and fauna. The increasing use of plastics and their accumulation has further contributed to eco-pollution. A large percentage of plastic produced each year is used to make single-use, disposable packaging items or products which will get permanently thrown out within one year. As per the United States Environmental Protection Agency, in 2011 plastics constituted over 12% of municipal solid waste. Plastics may not only release harmful chemicals into surrounding soils but also produce powerful greenhouse gases upon being degraded by microorganisms, contributing to global warming. Switching over to bio-plastics is probably the only way to combat the situation in hand.

What is Bio-plastic?
Plastics produced from biomass sources and are mostly biodegradable and recyclable.

The three types of biodegradable plastics introduced are:
i) Photodegradable: Light sensitive group incorporated directly into the backbone polymer and extensive ultraviolet radiation can disintegrate their polymeric structure and opens up the structure for microbial degradation but if they are land filled then it can't be degraded.

ii) Semi-biodegradable: These are starch linked plastics where starch holds the short chain of polyethylene. The starch can be attacked by microbial enzymes and degraded by other microbes but the polyethylene part remains as non-degradable ones

iii) Complete biodegradable: A new and promising because it is synthesized by bacteria as polymer, which includes Polyhydroxy alknoates (PHA), Polylactides, Aliphatic polyesters, Polysaccharides, Co-polymers and/or blend of above and hence biodegradable.

Production of a specific type of bio-plastic is a strain specific event. Currently, the main limitations for the bulk production of bio-plastics are its high production and recovery costs. However, genetic and metabolic engineering has allowed their biosynthesis in several recombinant organisms improving the yield of production and reducing the cost.

Types of Bio-polymers from living organisms
Bio-plastic- Poly (3-hydroxybutyrate) (PHB) was first obtained from Bacillus megaterium and this was almost unnoticed until late 1970s when, because of petroleum crisis, a scientific movement aimed at discovering the alternative source of fossil fuel reserve was undertaken. Microbes belonging to more than 90 genera- including aerobe, anaerobe, photosynthetic microbes (Microalgae), archae and lower eukaryotes are able to accumulate and catabolize these polymers.

Growing bio-plastics in plants
Plants are becoming factories for production of bio-plastics. Bacteria create the plastic through the conversion of sun light into energy. Researchers have transferred the gene that code for the enzymes into Arabidopsis thaliana plant and is stored in cellular compartments. The plant is harvested and the plastic is extracted using suitable solvent. The liquid resulting from this process is distilled to separate the solvent from the plastic. The enzymatic conversion consist of first condensation of two acetyl- CoA into acetoacetyl CoA, then reduction of acetoacetyl CoA to 3-hydroxybutyryl-CoA by NADPH dependent process and finally hydroxybutyryl-CoA monomers are polymerized into PHB by polymerases. Microbial Production of bio-plastics mainly concerns with fermentation process with genetically engineered microbes but recently transgenic crop plants have been used for commercial cultivation and are becoming competitive with the petroleum based plastics.

Recombinant Bio-plastic?
Gene manipulation has been done for that codes enzyme for production and degradation of PHB. The prime role of metabolism and physiology has become evident by PHB mutants. The PHB biosynthetic genes phbA, phbB and phbC are clustered in one phbCAB operon and it is highly divergent in different bacterial genera with regard to orientation and clustering of genes. Natural PHB producers (microalgae) are having long generation time, low optimal growth rate and difficult to lyse the cell for better extraction. So, E.coli has been promising for cloning and manipulating the genes and can be recovered easily But now people have paid attention to transgenic crops for its better and high product yield. Using gene splicing technology, researchers have been able to introduce the transgene into a plant instead of bacterium so plants acting as a mini-plastic factories. Metabolic engineering thereafter promises to bring a feasible solution for the production of "Green Plastic".

Properties and practical application
• Non toxic, biocompatible and have high degree of polymerization, highly crystalline, optically active, isotactic, piezoelectric and insoluble in water.
• Polyaliphatic esters and alkanoates have emerged as promising biopolymers finding numerous applications as: thermoplastics, elastomers, adhesives, packaging materials, dining utensils, disposable razors, diapers, cosmetic container- shampoo bottles and cups. In addition to this, they are also used as osteosynthetic materials in the stimulation of bone growth owing to their piezoelectric property, in bone plates and surgical sutures etc.

Sustainable Development
However, optimizing the cost of production is still a challenge for the researchers. Plant produced plastics are harvested by chloroform extraction and the cost of extraction is still higher than petroleum based plastics. The Monsanto Team estimated that polymer concentration should be 15% of the plant dry matter to make the extraction economic hence more research is needed. The uses of bio-plastic reduce the dependence on fossil fuels and also limits the environmental impacts of plastic use and manufacture. Fostering new techniques for cost curtailing and quality improvement will definitely lead to a sustainable way of development.

About Author / Additional Info:
The author is working on Micro-organisms associated with As contaminated aquifers of west bengal, their physiological characterstics and genomics-proteomics responses in the same contaminated sites.