Finding, cultivating, and bioengineering organisms that can digest plastic not only helps eliminate pollution, it is now big business. A few microorganisms have already been found that can do this, but when the enzymes that make it possible are used on an industrial scale, they usually only work at temperatures above 30 °C.

The required heating means that industrial applications are expensive and not carbon efficient today. But there is a possible solution to this problem: the search for special cold-adapted microbes whose enzymes work at lower temperatures. Scientists from the Swiss Federal Institute WSL knew where to look for such microorganisms: in the high altitudes of their country’s Alps or in the polar regions. Their findings have been published Boundaries in Microbiology.

“Here we show that new microbial taxa derived from the ‘plastisphere’ of alpine and arctic soils can degrade biodegradable plastic at 15°C,” said first author Dr Joel Ruti, now a WSL Visiting Scientist. “These organisms can help reduce the costs and environmental burden of the enzymatic plastic recycling process.”

Rüti and colleagues took samples of 19 bacterial species and 15 fungi growing on plastic that was either free-standing or intentionally buried in the ground (stored in the ground for one year) in Greenland, Svalbard, and Switzerland. Most of the plastic waste from Svalbard was collected during the Swiss Arctic Project 2018, where students did fieldwork to see the effects of climate change firsthand. Soil from Switzerland was collected at the Muot da Barba Peider peak (2979 m) and the Val Lavirun valley, both located in the canton of Graubünden.

The scientists allowed the isolated microbes to grow as single-line cultures at 15 °C in the dark in the laboratory and used molecular techniques to identify them. The results showed that bacterial strains belonged to 13 genera in Actinobacteria and Proteobacteria phyla, and fungi belonged to 10 genera in Ascomycota and Mucoromycota phyla.

great results

They then used a series of assays to screen each strain for its ability to digest sterile samples of non-biodegradable polyethylene (PE) and biodegradable polyester-polyurethane (PUR), as well as two commercially available mixtures of biodegradable polybutylene dibutyl terephthalate (PBAT). . and polylactic acid (PLA).

Even after 126 days of incubation on these plastics, none of the strains were able to digest PE. However, 19 (56%) strains, including 11 fungi and eight bacteria, were able to digest PUR at 15°C, while 14 fungi and three bacteria were able to digest PBAT and PLA plastic blends. Nuclear magnetic resonance (NMR) and fluorescence-based analysis confirmed that these strains were able to break down PBAT and PLA polymers into smaller molecules.

“It was very surprising to us that we found that a large proportion of the strains tested were able to degrade at least one of the plastics tested,” Ruti said.

Two uncharacterized fungal species from the genera Neodevriesia and Lachnellula performed best: they were able to digest all the plastics tested except PE. The results also showed that the ability to digest plastic was dependent on the culture medium for most strains, with each strain responding differently to each of the four media tested.

A side effect of the ability to digest plant polymers

How did the ability to digest plastic evolve? Because plastic has only existed since the 1950s, the ability to degrade plastic was almost certainly not a trait that natural selection originally aimed for.

“Microbes have been shown to produce a wide variety of enzymes that break down polymers involved in breaking down plant cell walls. In particular, plant pathogenic fungi are often reported to biodegrade polyesters because of their ability to produce cutinases that target plastics. Polymers because of their similarity to plant polymer cut,” explained recent author Dr. . Beat Frey, Senior Research Fellow and WSL Group Leader.

Challenges continue

Ruthi et al. they have only been tested for digestion at 15°C, they do not yet know the optimum temperature at which the enzymes of successful strains work.

“However, we do know that most strains tested can grow between 4°C and 20°C, around 15°C optimum,” Frey said. Said.

“The next major challenge will be to identify the plastic-degrading enzymes produced by microbial species and to optimize the process to produce large amounts of protein. In addition, further enzyme modification may be required to optimize properties such as protein stability.”