Residues of plastic can now be found all over the world, even in the snow on the mountains or at the poles. Are there also bacteria or fungi in cold regions such as the Alps or the Arctic that can break down these plastics? That is what the microbiologists Joel Rüthi and Beat Frey from the Swiss Federal Institute for Forest, Snow and Landscape Research WSL wanted to know. The answer to this question is not only interesting for the decomposition of waste in nature, but microorganisms adapted to cold temperatures could also be useful for technological plastic degradation.
For his study, Rüthi buried plastic in soil samples taken from 3,000 metres above sea level in the Engadine: two biodegradable types used for compost bags and a piece of polyethylene (PE), which is used for the common black waste bags. The results are published in the Journal of Hazardous Materials: After five months at 15 degrees Celsius, the two degradable films showed small holes and a growth (biofilm) of bacteria and fungal threads. The PE, on the other hand, was completely unchanged. "The compost bags were partially degraded, but it would take much longer for them to completely decompose," says Rüthi. Therefore, under no circumstances should plastic waste be disposed of in the Alps - or anywhere else in nature - even those made of biodegradable plastic.
What exactly was growing on the films and apparently feeding on the plastic could only be determined using genetic methods. Rüthi chemically chopped up all the DNA in his biofilm samples, multiplied them and put overlapping sequences together on the computer. This is how he discovered millions of genes. Thanks to comparative databases, he was able to filter out those that contain the blueprint for molecular tools, so-called enzymes. These are specialised, for example, in breaking down substances such as long-chain plant molecules like lignin and cutin or so-called esters (the building blocks of polyester).
On the compostable plastic, genes were found for enzymes that can break down such long molecule chains at specific sites. "From this we conclude that microorganisms have settled there that break down chemical substances like those that make up plastic," says Rüthi. Bacteria that bind atmospheric nitrogen and are therefore important for healthy soils also thrived. They apparently used the plastic as an additional source of energy and carbon - the latter is otherwise rare in alpine soils. With the study, Rüthi succeeded in proving that plastic-degrading microbes are present in the Alps and can become active - although it would take years for them to decompose plastic films completely.
New plastic from waste
At the same time, he discovered many new DNA sequences from which plastic-degrading enzymes could be obtained - which, in contrast to most of those known so far, also function optimally at "normal" ambient temperature, without additional heat. The goal: to break down plastic into its building blocks and then use these again to produce new plastics - without the need for fresh fossil oil. There are already concrete attempts to apply this method: the company Carbios in France, for example, wants to have a functional PET recycling system ready for the market by 2025. "This would make a real circular economy for plastic possible," says Rüthi. The researcher's next step is to examine soils he brought back from an Arctic expedition