billion tonnes

of plastic have been produced worldwide over the past 70 years.

Prof. Wierckx and his colleagues have two plans: first of all, they want to tackle the mountain of plastic waste with the help of enzymes and bacteria and break it down in an eco-friendly way. They then want to produce new, biobased plastics from the individual components that are created during decomposition. They were inspired, among other things, by the bacterium that Japanese scientists discovered four years ago. Ideonella sakaiensis was found on half-rotten PET bottles not far from a recycling plant. The bacterium first attaches itself to the plastic and secretes enzymes. These enzymes separate the long chain molecules of PET into their individual links. The bacterium then continues to metabolise these fragments until only water and carbon are left.

Faster and more effective

“Unfortunately, these bacteria found in Japan work extremely slowly. A thin strip of plastic only a few centimetres in size took the bacteria over 60 days to decompose,” says Nick Wierckx. Their performance fluctuates in the process: sometimes the bacteria are very busy, sometimes they are not. Moreover Ideonella reproduces very slowly. Wierckx and his team are therefore focusing on the bacterium Pseudomonas putida. Using optimised enzymes, the plastic is first decomposed within a few days at high temperatures. The bacteria then continue to convert the plastic broth.

“Pseudomonas putida is very robust and an old acquaintance when it comes to cleaning contami­nated soil. It even survives in an environment with high levels of pollutants and toxins,” explains the expert. It is precisely this property that is important, as the researchers have developed a decomposition method that makes things uncomfortable for microorganisms: the researchers do not apply the bacterium to individual plastic bottles, but let it work in a broth of different plastics.

“In this plastic broth, special enzymes have predigested the various plastics, which makes our bacteria’s work easier. We developed the process together with colleagues from RWTH Aachen University and Leipzig University,” explains Nick Wierckx. The advantage of the mixture of different plastics: the plastic waste does not have to be presorted. “In the broth, the bacteria decompose exactly those plastics for which their new enzyme equipment is suitable – in our specific case, the plastic type PET, but also biodegradable plastic such as PLA. What remains will be chemically processed,” says the biotechnologist. In this way, the Jülich laboratory bacteria decompose plastic fragments in four days that, in nature, would still not have finished decomposing after 400 years.

years

is how long it takes for a package, bottle, curd cheese cup or plastic bag to decompose in nature.

The researchers’ aim is to get their bacteria to digest as many different types of plastic as possible so that the recycling process will be as efficient as possible. In addition, the Jülich researchers are also getting Pseudomonas to synthesize valuable chemicals such as aromatic compounds from the colourful plastic broth. These aromatics can then be used to produce new, more environmentally compatible plastics. The circle is thus complete: from plastic waste to new, environmentally friendly materials.

Finding new ways

Together with research groups from Heinrich Heine University Düsseldorf, the Jülich biotechnologists are pursuing a number of paths to achieve this. One path leads through point mutations – tiny, targeted changes in genetic information that cause the bacteria to also produce other, more helpful enzymes. Another path is to replicate already known enzymes on the computer in order to understand their interaction with the plastic molecules in detail. Computer simulations are then used to find a better – that is, more effective – variant that, for example, degrades plastics even faster. The third path is to find previously unknown enzymes in bacteria that, for example, work at lower temperatures so that they could also be used in sewage treatment plants. “In collaboration with our partners in the PlastiSea project, we want to find such candidates, such as in the Atlantic garbage patch,” says doctoral researcher Rebecka Molitor from the Düsseldorf Institute for Molecular Enzyme Technology. “We have already discovered that marine bacteria of the species Pseudomonas aestusnigiri also process plastics,” she adds.

Such brine bacteria are difficult to keep in the laboratory because their requirements in terms of temperature and environment are too complex – but the researchers at Jülich and Düsseldorf are nevertheless examining their enzymes closely. For example, they have transferred the genetic information of the enzymes into comparatively undemanding bacteria. In the laboratory, the modified bacteria can be used to break down poly­ester from textile coatings or PET components. Such plastics wrap synthetic fibres of waterproof or windproof clothing, for example. After each wash, residues are found in the waste water and finally in the environment.

Added value with bacteria

“The potential use of such microorganisms is of particular importance in sewage treatment plants,” says Wierckx, “because up until now, the filters in the plants have not been sufficient enough to completely remove the artificial substances, fibres and microparticles from the wastewater.” It would be even better if the textile fibres were directly biologically produced and biodegradable.