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Introduction
A new enzyme that can degrade PET plastic which is commonly found in disposable plastic bottles is a revolutionary breakthrough for recycling plastics.
However, there are many other nature-inspired methods, ranging from enzymes to worms, that can be used in the degradation and recycling of plastics. In this article, we look at some of these methods, as well as what the recent breakthrough has achieved.
Recycling Plastics Through Nature-Inspired Methods
Nature has been inspiring scientists for many decades. From natural hydrophobic surfaces that brought about the Lotus spray, to new natural products that are now used in many of our pharmaceuticals.
Nature is said to be the best chemist, and it is true, as many humans can’t replicate what goes on in nature (although nature has had longer to perfect it); so, it is only natural that scientists take inspiration from nature and adjust it to fit a certain application.
The recycling of plastics is no different. Plastics are known to take a long time to break down in nature, so their degradation needs a helping hand.
Whilst the recent breakthrough shows such great promise, there have been other attempts to break down plastics using nature-inspired methods. These include enzymes derived from other sources and various types of worms that will eat plastic.
In 2012, a team of researchers in Japan isolated two different yeast strains found in the gut of a stag beetle (Aegus laevicollis) that can break down biodegradable plastic films. Both strains are most closely related to Cryptococcus magnus and have shown a high affinity to the degradation of poly(butylene succinate) (PBS) and poly(butylene succinate-co-adipate) (PBSA). The enzyme also showed the ability to breakdown poly(ε-caprolactone), and poly(lactic acid), but the isolation and purification process was complex.
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In 2014, researchers from China and the USA investigated how bacteria from the gut of waxworms, Plodia interpunctella, could be used for digesting polystyrene – a plastic that is often resistant to biodegradation. The researchers isolated two different strains of this bacteria, and after 28 days, the amount of plastic eaten by these bacteria was noticeable in polystyrene thin films. After 60 days of bacterial incubation, the polystyrene films had decreased in weight by 10-11%.
In 2015, a similar group of researchers from China and the USA used mealworm larvae, Tenebrio molitor Linnaeus, to break down polystyrene. The polystyrene was used in the form of its most common product, Styrofoam, to see how the worms survived on a ‘plastic only’ diet. To much surprise, the mealworms lived just as healthily eating Styrofoam as they did in their normal diet. Most of the polystyrene was found to be egested into carbon dioxide, whilst a small portion was converted into faecal matter and biomass.
The Recent Breakthrough
A group of scientists from Japan have created an enzyme that can break down polyethylene terephthalate (PET) – a well-known plastic that is used to create bottles worldwide. The researchers created the enzyme by accident, and it has been termed a ‘mutant enzyme’.
The enzyme was first discovered by in 2016 when the Japanese team found a bacterium, Ideonella sakaiensis, that naturally ate plastic. Upon isolation of the enzyme, the researchers have further tweaked the enzyme by replacing some of the residues on its surface, and it now ‘eats’ plastic more efficiently than the natural bacterium.
Even though the research is from Japan, the UK has played a small part. The synchrotron beamline at Diamond Light Source, Oxfordshire, was used to create a high definition 3D model of the enzyme. The enzyme has since been termed PETase.
The researchers have suggested that the enzyme isn’t fully optimized yet, and further work could yield a more efficient enzyme. Moreover, the researchers also tested the enzyme on polyethylenefuranoate (PEF) – a plant-based alternative to PET, and PETase showed an even greater affinity to breaking down PEF than it did PET.
Conclusion
Plastic left in landfills, or in the natural environment, can take centuries to biodegrade and fully break-down. By comparison, this new enzyme starts to degrade a PET bottle within a matter of days. The enzyme is still a while off being used commercially, especially on a worldwide scale.
However, with over 1 million plastic bottles being sold every minute around the world, something that can easily remove the huge amounts of plastic waste being produced every year is sure to be welcomed with open arms by many people. Even if it can tackle just one part of the problem, it could be used in conjunction with other methods that break down different types of plastic.
Sources:
- BBC: http://www.bbc.co.uk/news/science-environment-43783631
- The Guardian: https://www.theguardian.com/environment/2018/apr/16/scientists-accidentally-create-mutant-enzyme-that-eats-plastic-bottles
- “Affinity purification and characterization of a biodegradable plastic-degrading enzyme from a yeast isolated from the larval midgut of a stag beetle, Aegus laevicollis”- Suzuki K., et al, Applied Microbiology and Biotechnology, 2013, DOI: 10.1007/s00253-012-4595-x
- “Evidence of Polyethylene Biodegradation by Bacterial Strains from the Guts of Plastic-Eating Waxworms”- Yang J. et al, Environ. Sci. Technol, 2014, DOI: 10.1021/es504038a
- “Biodegradation and Mineralization of Polystyrene by Plastic-Eating Mealworms: Part 1. Chemical and Physical Characterization and Isotopic Tests”- Yung Y., et al, Environ. Sci. Technol., 2015, DOI: 10.1021/acs.est.5b02661
- “Biodegradation and Mineralization of Polystyrene by Plastic-Eating Mealworms: Part 2. Role of Gut Microorganisms”- Yung Y., et al, Environ. Sci. Technol., 2015, DOI: 10.1021/acs.est.5b02663
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