Discovery of special microbes capable of breaking down plastic precisely at low temperatures
Plastic has only been around for seventy years, but it is now found in nature all over the world. Recycling and breaking it down is still not so easy. Science is therefore looking for microorganisms that can help us. And now they’ve found something.
Many different microbes have been discovered in recent years that can break down plastic. There have also been extensive tests with enzymes that make this process possible, but unfortunately they all only work well above 30 degrees Celsius. The method is therefore not sustainable enough and it is too expensive to heat the enzymes on a large scale. This is why scientists are looking for alternatives. They now target microorganisms adapted to cold temperatures and have learned to eat plastic. In the northern polar regions and in the high mountains of the Alps, they tracked down these plastic-eating bacteria and fungi and took them to the laboratory.
These organisms live in the so-called plastisphere, which consists of ecosystems that have evolved to live in human-made plastic environments. “We found new groups of microorganisms in the plastisphere, not too deep in alpine and arctic soil. They are able to handle biodegradable plastics at 15 degrees,” says lead researcher Joel Rüthi. “With the help of these microbes, it is possible to significantly reduce the costs and carbon footprint of industrial plastic recycling.”
Rüthi and her colleagues encountered nineteen different bacterial species and fifteen different fungi in the cold soil of Switzerland, Spitsbergen and Greenland. The microorganisms lived partly on pieces of plastic that had accidentally ended up in nature. Other samples came from places where all sorts of plastics had been intentionally buried a year earlier.
The researchers grew the isolated microbes in the lab in the dark at 15 degrees Celsius, then examined the type of meat they had in the tub. It turned out that there were thirteen strains of bacteria, belonging to the Actino-bacteria and Proteo-bacteria. Additionally, they identified ten fungal strains that fall under the Ascomycetes And Mucoromycete.
The microbes were no doubt eager to sink their “teeth” into a piece of plastic, so the researchers presented them with a variety of artificial treats: non-biodegradable polyethylene (PE), biodegradable polyester-polyurethane (PUR), and a popular blend. polybutylene (PBAT) and polylactic acid (PLA) which are unfortunately often found in nature. None of the tribes ventured into PE, even after it was hung in a Petri dish in front of their “nose” for 126 days. But PURE tasted much better to them. No less than nineteen strains (eleven fungi and eight bacterial species) digested this plastic at 15 degrees. The mixture of PBAT and PLA also worked well for fourteen fungal species and three bacterial strains. The plastic polymers seemed to have broken down into smaller molecules.
The big winners and therefore the microbes with the greatest potential for further research were two species of fungi that have yet to be named, but are among the Neodevriesia And Genus Lachnellula. Gluttons loved almost everything except non-biodegradable polyethylene. “We found it very surprising that most of the strains we tested were able to break down one or more plastics,” says Rüthi.
Plant polymers look like plastic
How has the ability to break down plastic actually evolved? Plastic has only been in circulation since the 1950s, so plastic decomposition was certainly not a major evolutionary advantage before that. “Previous research has shown that microbes can produce all sorts of polymer-degrading enzymes that cause plant cell walls to break down. In particular, fungi that target plants, making them sick and killing them, seem good at breaking down biodegradable polyesters, because they produce cutinases, which break down plastic polymers because they look like cutin, a plant polymer,” explains researcher Beat Frey.
There is still a long way to go before we know which way the selected enzymes work best. So far Rüthi has only tested at 15 degrees Celsius and the enzymes probably still need some fine tuning. “We know that the strains tested do well between 4 and 20 degrees. It seems that around 15 degrees is an optimum. The next big challenge is to identify the best enzymes and optimize them for the process of growing large amounts of protein. We may still need to modify enzymes to improve molecular stability and other things,” Frey concludes.
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