Researchers at the University of Portsmouth in the United Kingdom have examined around 100,000 different microorganisms and finally found what they were looking for in a pile of leaves. The mutated bacterial enzyme, nowpresented in the journal "Nature", breaks down polyethylene terephthalate (PET)

into its constituent parts within a short space of time: In just 10 hours, the mutated enzyme decomposed 1 tonne (1.1 US tons) of old plastic bottles into 90% of their components.

Solution to the global waste problem?

Hopes are high that microorganisms will be able to solve our plastic problem in the near future. After all, effective plastics recycling is still only a myth. Worldwide, around 359 million tonnes of plastics are produced annually, of which an estimated 150-200 million tonnes end up in landfills or in the environment.

Polyethylene terephthalate (PET) is the most widespread polyester plastic, with almost 70 million tonnes used each year. In its purest form, PET is used in the food industry, for example, in the production of drinking bottles, films and food packaging. The tear-resistant, weatherproof and wrinkle-free polyester is also used to make textile fibers.

However, PET cannot really be recycled, only downcycled. During this thermomechanical process, the material loses many of its properties and can be used only for inferior products such as fleece textiles or carpets.

Japanese research results pursued

Japanese researchers from the Kyoto Institute of Technology discovered in 2016 that certain enzymes can break down plastics. When examining the wastewater, sediments and active sludge in a recycling plant for PET bottles, they found the bacterium Ideonella sakaisensis 201-F6. Two previously unknown enzymes of the bacterium are responsible for the natural decomposition process. The enzyme ISF6_4831 converts PET into an intermediate product, and the other enzyme, ISF6_0224, further converts this intermediate product so that, in the end, only harmless terephthalic acid and glycol  remain.

However, since this natural decomposition process takes more than a year, scientists from the University of Portsmouth and the US Department of Energy's Renewable Energy Laboratory continued their research and, rather accidentally, created the above-mentioned novel enzyme that decomposes PET much faster. The results were first published in 2018 in the journal "Proceedings of the American Academy of Sciences" (PNAS).

Germans discover polyurethane-degrading bacterium

It has long been known that certain fungi can decompose not only PET but alsopolyurethane. Millions of tonnes of this plastic are also produced annually, mostly as soft foam used for things like foam insulation, kitchen sponges and diapers, or as rigid foam in sports shoes, for example. Polyurethane is usually disposed of in landfills because the material is too tough to be recycled.

When polyurethane degrades, toxic and carcinogenic chemicals can be released that would kill most bacteria. However, the bacterium found on a landfill site and presented to the public in March by the Helmholtz Centre for Environmental Research (UFZ) in Leipzig  is not affected.

It comes from the strain of Pseudomonas bacteria, which are able to survive even under harsh conditions such as high temperatures and acidic environments.

Although it is much easier to use bacteria than fungi for industrial applications, Hermann Heipieper from the Helmholtz research team estimates that it could still take 10 years before the bacterium can be employed on a large scale. In the meantime, it is important to decrease the use of plastics that are difficult to recycle and to reduce the amount of plastics in the environment, says Heipieper.

Profitable prospects

The newly presented enzyme could be of great benefit not only for the environment but also for the French company Carbios, which has been working intensively for years on the large-scale decomposition of PET using enzymes and which has also financed this enzyme development.

Within five years, Carbios intends to bring the new recycling process onto the market on an industrial scale. This might be worthwhile because the cost of the enzyme is only 4% of the cost of the crude oil required to produce a comparable quantity of new plastic bottles.

Although the PET bottles still need to be shredded and heated, the new process is nevertheless profitable, said Martin Stephan, deputy managing director of Carbios. To ensure that the enzyme development can quickly gain momentum, Carbios receives strong financial support from big companies such as Pepsi and L'Oreal.