Plastics are made by joining monomers together to form long flexible chains in a process known as polymerisation. The strength of the bonds formed between monomers is what makes the plastics persistent in the environment. Chemical recycling relies on using chemical action at high temperatures or in organic solvents, to break the bonds at the end of the product’s life so that the monomers can be recovered and used to make new plastic.[1]

Biodegradable plastics (e.g. PHAs, described in Section 3.7) are made using bonds that are similar to those found in natural materials and so are able to be broken by microbes in mild conditions. These processes are controlled within the microbe by enzymes, which enable the chemistry to take place in environmental conditions. One of the attractions of PHA is that it is derived from bacteria, which means naturally occurring microbial enzymes are well equipped to break it down efficiently in the environment at the end of its useful life.

​A long-term solution to the decomposition of non-biodegradable plastic might be found by building on exciting new science aimed at engineering enzymes, or selecting microorganisms, that can digest traditionally non-biodegradable plastic in environmentally friendly conditions.

Plastic-eating bacteria have been shown to digest PET, which is commonly used for drink bottles. Image via Medium.

For example, in recent high-profile papers a major step forward was reported in which the authors describe enzymes that can decompose PET (#1) by breaking it down into its constituent molecules by hydrolysis.[2] The enzymes were isolated from a microbe that shows some ability to degrade PET (#1).[3] This represents an excellent first step towards a bioengineering solution for the removal of plastic waste. The work builds on an increasing body of literature in this field (for a review, see Kawai et al.[4]) and is an avenue of research that holds promise for a revolutionary new approach to use microbes or enzymes to address limitations in the chemistry we rely on today by having potential to degrade macro-, micro- or nano-plastics in the environment.

Despite attention grabbing headlines – ‘Are plastic eating enzymes the planet’s only hope?’; ‘Mutant Enzyme Gobbles up Plastic’; and ‘A mutant plastic eating enzyme could help solve the world’s waste problem’ – there is a very long path from this initial exciting result to a practical solution. The significant advance is so far restricted to decomposing PET (#1) – probably the easiest of the plastics to recycle. One of the biggest difficulties with enzyme degradation of plastics is getting the plastic into solution so the enzyme can degrade it. However, with deeper understanding and increased focus, there is long-term hope that organisms might be discovered or engineered to disassemble other plastics.

Researchers in Aotearoa New Zealand are also becoming interested in this new idea, with a team at Auckland winning a recent MBIE Endeavour grant 2019-2022 to explore this field – focused on engineering a heat stable version of the protein – and the MBIE Endeavour Aotearoa Impacts and Mitigation of Microplastics project 2018-2023 including bioprospecting of potential degraders from marine and wastewater environments.

References

[1] Christensen et al., “Closed-Loop Recycling of Plastics Enabled by Dynamic Covalent Diketoenamine Bonds,” Nature Chemistry 11, no. 5 (2019)

[2] Palm et al., “Structure of the Plastic-Degrading Ideonella Sakaiensis Mhetase Bound to a Substrate,” Nature Communications 10, no. 1 (2019)

[3] Yoshida et al., “A Bacterium That Degrades and Assimilates Poly (Ethylene Terephthalate),” Science 351, no. 6278 (2016); Palm et al., “Structure of the Plastic-Degrading Ideonella Sakaiensis Mhetase Bound to a Substrate,”

[4] Kawai et al., “Current Knowledge on Enzymatic Pet Degradation and Its Possible Application to Waste Stream Management and Other Fields,” Applied Microbiology and Biotechnology 103, no. 11 (2019)

More case studies

Reducing the carbon footprint of plastics by using recycled plastic

Can recycling contribute to reducing the carbon footprint of plastic?

How big is the plastic clothing problem for Aotearoa New Zealand?

Not all plastic fibres are captured by Statistics NZ import data. We estimated the weight of synthetic textiles imported into Aotearoa New Zealand as finished products.

A reusable system to replace single-use cups

Reusable cups at festivals in Aotearoa New Zealand can save thousands of plastic cups from going to landfill.

Sustainability through connection, learning and action

With a kaupapa of creating a healthy, peaceful, more sustainable world, Toimata Foundation supports inter-generational learning and action by running Te Aho Tū Roa and Enviroschools.

Empowering brands to make informed packaging decisions

The Sustainable Business Network (SBN), in partnership with the Ministry for Primary Industries, Foodstuffs NZ and New Zealand King Salmon, has run a three-part plastics packaging masterclass series to help empower brands to make informed decisions around their packaging choices.

Operation Clean Sweep

Plastic pellets, or nurdles, are the raw material of the plastics manufacturing industry. They are commonly found in beach and river clean ups. The plastic manufacturing industry in Aotearoa New Zealand identified this as a key issue for their members to address.

What’s stopping the uptake of new materials?

An outline of critical success factors to drive local development and uptake of sustainable new materials as part of a circular economy.

Recyclable shoes

As part of its recent pledge to use only recycled plastics by 2024, Adidas revealed a new sneaker made from 100% recyclable materials.

Controlled plastic decomposition

​A long-term solution to the decomposition of non-biodegradable plastic might be found by building on exciting new science aimed at engineering enzymes, or selecting microorganisms, that can digest traditionally non-biodegradable plastic in environmentally friendly conditions.

Plastic Bag Free Raglan, Pēke Kirihou Kore Whāingaroa

In Raglan, a whole community worked together to engage in significant behaviour change around one troublesome item of waste – single-use plastic carry bags.