Anna Moldenhauer: Dr Sonnendecker, you have discovered the enzyme PHL7, which is able to decompose plastic much more effectively than the previous favourite from Japan, the enzyme LCC. You found PHL7 in a leaf compost heap in Leipzig. Why were you looking for it there?
Dr Christian Sonnendecker: The discovery and description was a real team effort, many outstanding scientists were involved here. Enzymes that can degrade the synthetic plastic PET are highly unlikely to target it in their natural environment. Instead, they were developed by nature to break down plant polyesters - such as the outer wax layer of a plant, which is very complex. In order to break down this structure, the enzymes are relatively non-specific, meaning they can break down a wider range of natural and synthetic polyesters. And it is these enzymes that we are looking for. A leaf compost heap is full of plant material. In its layers there is a high temperature of about 70 degrees Celsius. This is crucial for the enzyme to efficiently degrade PET, because only at these temperatures does PET become sufficiently flexible for efficient enzymatic attack. For us, the pile of leaves is therefore the ideal source.
You have taken samples from the compost and analysed them. How did you know that the seventh sample contained the right enzyme?
Dr Christian Sonnendecker: To begin with, we extracted the total DNA of the sample, which consists of fungi, plants and bacteria, from a soil sample. The DNA contains the blueprints (genes) that encode the enzymes. We know what the DNA we are searching for must look like, so we already know the rough blueprint. To do this, we use small THEN snippets which bind to the desired gene sequence, mass-propagating the corresponding region. We then insert these copies of the DNA into a bacterium, our "pet" in molecular biology, the E. coli bacterium. This is able to read out the information encoded in the DNA. The bacterium is also controlled to mass reproduce the enzyme found in the DNA. We then grow the engineered bacterium on a nutrient medium - like an agar plate. Polyester particles in the agar will cause turbidity. An active enzyme will cleave the particles, creating a clear zone. We then look at active candidates more closely. Trivially speaking, we build a kind of gene fishing rod to specifically fish out the genes we are looking for from the total DNA. Instead of checking the entire DNA, we look specifically for the desired genes, which greatly reduces the amount of work.
The enzyme PHL7 is able to initiate biological PET recycling at record speed. Can you give me an example of how many hours are meant by this?
Dr. Christian Sonnendecker: We used the enzyme LCC as a comparison, which is also able to degrade PET and is considered the standard for this type of study. Moreover, almost all scientists researching in this field use the same PET as a test material to compare the data. We have found that our enzyme PHL7 is twice as active as LCC and that we can degrade twice the amount of PET in the same time. In detail, this is a complete degradation of PET with a layer thickness of 250 micrometres in less than 18 hours. No enzyme has been described so far that could degrade PET film in such a short reaction time. Therefore, we can assume that it is the most active enzyme to date. In a reactor that holds one litre, we can now degrade even the thickest PET packaging in one day.
The LCC enzyme was discovered in 2012, and research into the biodegradation of plastics has been going on for a good 20 years. Apart from the time advantage, in what way is your enzyme more efficient than all those discovered so far?
Dr Christian Sonnendecker: In order for these enzymes to work at high performance, they must be able to withstand a working temperature of 70°C. Basically, there are only two bacterial enzymes known worldwide so far that can achieve this - LCC and PHL7. These also help us to obtain valuable structural information that we can then use for further optimisation. When we compare LCC with PHL7, we see certain key positions that are different. It is in these key positions that the difference in activity must lie. So with the information from the newly discovered enzyme, we can improve the already known enzymes.
What remains when the decomposition of the PET has taken place via the enzyme PHL7? Is the residual material harmless to nature?
Dr Christian Sonnendecker: Our goal is to recycle the PET completely, the degradation will take place in a reactor, the basic building blocks obtained are valuable resources. Thus, we want to use biological recycling to close the loop in PET recycling. Compared to other plastics, PET is very well suited for recycling. But since the previous approaches are unfortunately hardly economical, the mechanical process is the only established method for PET recycling. Here, the PET is melted and transformed into a new shape. However, melt recycling can only be carried out with a certain number of cycles, because each cycle degrades the material properties. The PET then turns yellowish and can finally be used for fillers or directly thermally recycled. Thus, in PET recycling we have a linear path from petroleum to CO2, or to landfill, which is only interrupted by a few recycling loops. Our approach is to break PET down into its basic components at the point where it can no longer be recycled. These can be easily cleaned and new PET can be created from them again. In this way, we no longer need petroleum-based raw materials, but can keep what we have once produced in a perpetual cycle. This would give us independence from fossil raw materials and reduce CO2 emissions. In parallel, this process must be able to filter out and remove the heavy metals and "dirty additives" introduced into the plastic on the part of the manufacturers in order to produce a pure product.
What does it take to finally transfer this process into an industrial context so that it revolutionises the recycling of plastics to date?
Dr Christian Sonnendecker: I think the most important steps are to consider which types of plastics we really need - in industry as well as on the consumer side. For a long time, the typical development was only aimed at wanting a polymer with certain properties. The question of recycling was not asked. How can we develop a holistic recycling system for plastics when they are chemically completely different? In addition, there are the additives of various plastics, which have the potential to accumulate in the environment and the organisms in it. In other words, they are persistent, mobile in the environment and also accumulate in our food chain. The more we produce of them, the more they come back to us. A newborn baby thus already absorbs these compounds via the umbilical cord. There are already indications that our hormone balance and fertility are negatively affected by this exposure. Some substances have therefore already been banned, others have not yet been conclusively proven. Therefore, they are produced until this is available. The problem is that it is hardly possible to prove the health hazards of the plastic additives in question, as this process requires a control group that is not exposed to them. But we have all grown up with the effects of plastics on our organism, a global experiment with an uncertain outcome. PET, which is basically still considered a harmless plastic, has already been detected in the bloodstream of humans, and we simply don't know what harm it could do in the body. Moreover, even if we stopped producing plastic right now, the amount of microplastics in the environment would continue to increase for decades to come, as there is already a huge amount of plastic waste in the environment.
In other words, there would be a much greater urgency to address the recycling problem of plastics from all sides?
Dr Christian Sonnendecker: Absolutely. In my opinion, a plastic should be designed from the very beginning in such a way that it can be produced from renewable raw materials and is also harmless to health and easy to recycle. A plastic that only works if we mix in highly questionable additives and at the same time there are no recycling options cannot be effective. Instead, we should learn to work with fewer plastics and develop manageable alternatives. PET is a prime example because it is considered to be a safe plastic with great recycling options, and the raw materials can be obtained from sustainable sources. Here, the technology we are developing is a possible key – together with chemical and mechanical recycling. Biological recycling alone will probably not be enough. It is simply too costly and it is also not necessary to completely degrade the polymer after each cycle, but only when the material properties no longer meet the criteria.
Can you tell me what are the next steps?
Dr Christian Sonnendecker: The research is part of the EU project ENZYCLE, which will run until mid-2024 and by then we want to move from the current one-litre reactor to a reactor with over 100 litres. Then we want to check the efficiency of the process, e.g. we want to record the CO2 footprint of the technology in figures and of course see whether money can be made with it. Unfortunately, we can only use this kind of technology on a large scale if the bottom line is in the black. That is why we are developing the process as cost-efficiently as possible in all steps. The second point is to further improve the enzymes through protein engineering, such as in terms of their longevity in the reactor and their activity. We have already managed to halve the reaction time until the PET is degraded in the reactor from 24 hours to almost 12 hours. By pre-treatment with simultaneous surface enlargement, this can even be done in less than an hour. To what extent a new recycling process is economical, however, also depends on the willingness of all of us to adapt our previous waste disposal habits. In Japan, households already sort the different types of plastic. Basically, we are all required to completely rethink how we deal with plastics and to what extent we need them at all. We have to give up our comfort in this issue if we really want to change something. There will not be a general solution on the part of technology, even with PHL7.