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Plastics have become a vital part of our daily lives due to their vast applicability and low manufacturing costs. The high demand and usage have led to a continued escalation in production, which was recorded to be 359 million metric tons worldwide in 2018.
Plastic waste can accumulate in nature for centuries without appropriate treatment, causing serious health hazards and an environmental crisis. Biocatalytic degradation of plastic waste is a cost-effective, eco-friendly approach for tackling plastic pollution without the use of harmful chemicals.
A number of microorganisms are able to degrade a wide-ranging variety of commonly used synthetic plastics, such as polyethylene terephthalate (PET), polyethylene furanoate (PEF), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), polyurethane (PUR), and polyethylene (PE).
These microorganisms are often isolated from the soil of a plastic dumping site, marine water, sewage sludge, landfills, and the guts of plastic-eating worms.
Microbial Degradation of Plastics
Microbial biodegradation of plastic waste involves changing the chemical structure, shape, tensile strength, color, and molecular weight of plastic polymers. This process involves the enzymatic (exoenzymes and endoenzymes) and non-enzymatic hydrolysis or oxidation of microorganisms that results in the splitting of the large compound polymer into small molecular monomers.
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Microorganisms such as bacteria (e.g. Pseudomonas sp., Azotobacter sp., Halomonas sp.), fungi (e.g. Aspergillus sp., Chaetomium sp., Fusarium sp., Penicillium sp.) and actinomycetes (e.g. Amycolatopsis sp., Cryptococcus sp., Actinomadura sp.) are able to degrade both natural and synthetic plastics.
Does Pretreatment Enhance Microbial Degradation of Plastics?
The progression of pretreatments, such as γ-irradiations and mechanical grinding, help disorder the macromolecular aggregate structures of plastics and increase enzymatic degradation.
Ultraviolet (UV) radiation has recently been recognized as a prospective pretreatment technique for the enzymatic degradation of PET. The enzymatic (polyester hydrolase) degradation of PET films pretreated with UV radiation results in appreciable weight losses compared to the untreated PET sample.
The development of stable microbial enzymes plays a vital role in the enzymatic degradation of plastic. The technical evolution in protein engineering has improved the stability and activity of depolymerases, which ultimately enhances the enzymatic degradation efficiency.
The immense advancement in the understanding of the depolymerase and the microbial metabolic pathways of depolymerization products help develop microbial cell factories that could utilize the small depolymerization products to develop chemicals with high commercial value and depolymerize plastic wastes.
Factors that Determine Microbial Degradation of Plastics
In nature, plastics are normally biodegraded aerobically or anaerobically in sediments, landfills, and soil. The key factors that determine the microbial biodegradation are the nature of biodegradable substances and their chemical composition, age, production and processing methods, and the application conditions.
The physical and chemical properties that define the biodegradability of a polymer are:
- The presence of functional groups that increase hydrophobicity: Hydrophilic degradation is typically faster than hydrophobic.
- Molecular weight and density: A smaller molecular weight and lower density polymer can degrade at a faster rate than a higher molecular weight polymer.
- The nature of polymer plastic (crystalline and amorphous): Polymers with a greater number of amorphous regions degrade faster.
- Structural complexity (linearity or presence of branching)
- Presence of easily breakable bonds (ester or amide): These result in faster biodegradation.
- Physical form: The physical form of the polymer such as films, pellets, powder or fibers.
- Hardness: Soft polymers can degrade at a faster rate than hard ones.
In 2016, experts deemed the discovery of the Japanese plastic-eating bacteria as a potential natural solution to plastic pollution. The plastic-eating bacteria was first isolated from a Japanese recycling center.
While verifying such claims, Professor John McGeehan, University of Portsmouth, and his colleagues accidentally developed a novel highly potent version of the plastic-eating enzyme. This enzyme is named as PETase for its ability to degrade PET plastic, which is commonly used to make drinks bottles. This enzyme is also capable of degrading PEF polymers.
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PETase could enhance a degradation process that would typically take hundreds of years – a remarkable achievement. The scientists believe that this plastic-eating enzyme could help recycle millions of tonnes of plastic bottles. A similar view was seconded by Professor Nilay Shah of the Imperial College London. He stated that this is a potentially very useful technology to support the recovery and recycling of plastics.
Professor Shah went on to say that although the improvement over the bacterial enzyme isolated in Japan is modest, there is great potential for further research in this area. Successful future research will propel us closer to a recycling solution for the ever-increasing plastic pollution problem.
This discovery is indeed a great step forward and has been welcomed by other scientists. However, more research is required before these enzymes are widely applied to the recycling industry.
References and Further Reading
Falkenstein, P., Gräsing, D., Bielytskyi, P. et al. (2020). UV Pretreatment Impairs the Enzymatic Degradation of Polyethylene Terephthalate. Frontiers in Microbiology. 11, 689
Ru, J., Huo, Y. and, Yang, Y. (2020). Microbial Degradation and Valorization of Plastic Wastes. Frontiers in Microbiology. 11, 442
Haben, F. and Fasil, A. (2019). Degradation of Plastic Materials Using Microorganisms: A Review. 4(2), 57-63
Gabbatis, J. (2018) Plastic-eating enzyme accidentally created by scientists could help solve pollution crisis. [Online] The Independent. Available from: https://www.independent.co.uk/news/science/plastic-eating-enzyme-pollution-solution-waste-bottles-bacteria-portsmouth-a8307371.html (Accessed on 19 May 2020).