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)