More and more often we hear about microorganisms or mealworms that can “eat” plastics. The comment "Possibilities and limitations of biotechnological plastic degradation and recycling", jointly written by scientists from the University of Greifswald, the RWTH Aachen University, the Fraunhofer Institute UMSICHT and the University College Dublin, was published. With this publication, the scientists highlight the current state of research in this field and present strategies for future developments.
Especially in recent years and months, considerable research results have been achieved in the field of biotechnology. The success of research does not remain behind closed doors in the laboratory: it is also becoming increasingly topical among the general public – more often one reads about bacteria or mealworms that are able to digest plastic. It is both fascinating and frightening to hear that these little organisms could help to solve our plastic problem. But how far has science really progressed in this field? In this comment article, the scientists share their knowledge and opinion on biotechnological advances as well as their envisaged bio-based circular plastic economy – a realistic opportunity for our plastic problem.
Let’s start from the very beginning: There are various types of plastics, each with different polymers. There are polyesters like PLA, PHA and PET, polyurethanes (PUR) and polyamides (PA) or vinyl polymers like PE, PS and PP. Many advances have been made in biotechnology, but still limited to polyesters. "While highly efficient enzymes have already been discovered and improved for the widely used plastic polyethylene terephthalate (PET), which enable economic recycling, there has been little significant progress for most other plastics to date," explains Prof. Uwe Bornscheuer from the University of Greifswald in a german press release. This is mainly because chemical bonds determine the bio-recyclability of plastics, which is explained in more detail in the comment.
Modern biotechnology can make an important contribution both to the recycling of plastics at the end of their useful life and to a neutral CO2 balance. But on the one hand, we often hear about new advantages or organisms that are able to degrade plastics and on the other hand, it is not yet being applied in industry like mechanical or chemical recycling. The reason is, that biotechnological recycling does not yet exist commercially. Actually, this is not that surprising, since the history of biotechnological recycling started only a few years ago: The first enzyme reported to degrade PET was reported in 2005, but the degradation was way too slow. In 2016, wild-type bacterium, called Ideionella sakaiensis was identified, also arising a lot of public attention. “This has been a major discovery, especially in light of natural evolution" say the authors of the comment. However, this microorganism was also too slow for industrial use.
Another big milestone followed this year, published in Nature: Tournier et al. have identified and designed enzymes for efficient depolymerization and recycling of PET. Here, PET is enzymatically depolymerized within 10 hours – performed at an industry-relevant scale and thereby able to close the recycling loop. The resulting monomers are then used for the synthesis of virgin polymers for PET. This news also got a lot of attention - for good reason. However, the commercialization itself is not finished yet, "the development is still in its infancy," Dr. Ren Wei, who heads a junior research group at the Institute of Biochemistry on this topic, explains to everwave.
Besides this closed-closed-loop recycling strategy, the open-loop upcycling strategy offers an alternative, as displayed in the comment. Within this strategy, two distinct catalysis steps in separate reactors are established: one for Enzymatic degradation into monomers and one for the assimilation by microbes to synthesize molecules of high-value. This allows PET to be turned into a more environmentally friendly product such as PHA. Tiso et al. could already show this. He cultivated the bacterium Pseudomonas putida directly on the hydrolysate of the degraded PET to produce environmentally friendly PHA. Besides, this strategy is also pursued by us in the MIX-UP project.
How the implementation of biotechnological recycling could look like, displays the envisaged bio-based circular plastic economy of the authors (figure above). “Biotechnological Recycling of plastics is a great opportunity. It is not going to solve all of our problems, but it is certainly part of a wider solution to recycling of plastics.” Says Prof. Kevin O’Connor from the University College of Dublin in an interview with everwave.
But why do we even need such technologies, if microorganisms that are able to degrade our virgin plastics are already present in the environment? Can’t we make it easy and just dump those bacteria into the sea and thus solve our problem? Or can’t we just wait, until evolution will finds its way? This scenario is very, very unlikely, even impossible. It needs controlled conditions such as sufficient substrate, a certain temperature, pH-values and other culture conditions so that the degradation of virgin plastics by enzymes and bacteria can take place. Further points of the comment refer to such myths or reality of plastic-eating bacteria and call for a stronger exchange and coherent explanation by scientists and journalists. "Unfortunately there are also a number of publications that raise false hopes. In some reports on plastic-eating insects, for example, scientifically substantiated evidence is missing,” explains Dr. Ren We in a german press release.
With whichever enzyme or bacterium we finally succeed in making the breakthrough in biotechnological recycling, the following is inevitable: Single-use plastic should be reduced dramatically. Moreover, biodegradable plastics such as PHA and PLA should be used in different sectors, starting now. Prof. Lars Blank of RWTH Aachen University emphasizes: "We have to distinguish between two aspects: Plastics that we deliberately release into nature, such as mulch film for agriculture, must be able to biodegrade very quickly - within weeks or months. For durable plastics we need a medium-term solution. Degradation should be ensured within a few years - instead of hundreds of years as is currently the case".
In all considerations, the scientists are referring to the "6 R" principle to build a better, more sustainable plastic future:
More than ever, it is up to us to decide what the future will look like. We can make sure that only biodegradable plastics are used where plastics are intentionally released. We can use biomass and CO2 for production and gradually reduce fossil resources. We can decide if we are willing to pay extra taxes to help our environment. We can ensure that plastics are incorporated into the envisaged circular bio economy and get the plastics crisis under control.
Finally we like to congratulate Ren Wei, Till Tiso, Jürgen Bertling, Kevin O'Connor, Lars M. Blank and Uwe T. Bornscheuer for this comprehensive and interesting publication. Read the full article here. The authors also released a 'Behind the paper' contribution which was published on the Nature Chemistry Research Community website and can be accessed here.