A new step in the fight against pollution: they discover in the Alps and the Arctic microbes that digest plastic at low temperatures

Swiss scientists have identified 19 new strains of specialized cold-adapted bacteria and fungi from the Alps and the Arctic region that can digest biodegradable plastics at 15°C. This capacity, if expanded to an industrial scale, will save money and energy during recycling, as published in the journal 'Frontiers in Microbiology'.

Finding, cultivating, and bioengineering organisms capable of digesting plastic not only helps eliminate pollution, but is now big business as well.. Several microorganisms capable of doing so have already been found, but when the enzymes that make it possible are applied on an industrial scale, they usually only work at temperatures above 30ºC.

The necessary heating makes industrial applications still expensive to date and not carbon neutral, but there is a possible solution to this problem: finding specialized cold-adapted microbes whose enzymes work at lower temperatures.

Scientists from the Swiss Federal Institute WSL searched for such microorganisms at high altitudes in their country's Alps, or in the polar regions.

“Here we demonstrate that novel microbial taxa obtained from the 'plastisphere' of alpine and arctic soils are capable of decomposing biodegradable plastics at 15°C,” says Dr. Joel Rüthi, first author and currently visiting scientist at the WSL-. These organisms could help reduce the costs and environmental burden of an enzymatic plastic recycling process.”

Rüthi and colleagues sampled 19 strains of bacteria and 15 fungi growing on free or intentionally buried plastic (kept in the ground for a year) in Greenland, Svalbard, and Switzerland.. Most of Svalbard's plastic litter had been collected during the 2018 Swiss Arctic Project, in which students conducted fieldwork to witness first-hand the effects of climate change.. The soil from Switzerland had been collected at the summit of Muot da Barba Peider (2,979 m) and in the Val Lavirun valley, both in the canton of Grisons.

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

They then used a series of assays to test each strain's ability to digest sterile samples of non-biodegradable polyethylene (PE) and biodegradable polyester-polyurethane (PUR), as well as two commercially available biodegradable blends of polybutylene adipate terephthalate. (PBAT) and polylactic acid (PLA).

None of the strains was able to digest the PE, not even after 126 days of incubation in these plastics.. But 19 (56%) of the 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 mixtures.

Nuclear magnetic resonance (NMR) and a fluorescence-based assay confirmed that these strains were capable of cleaving PBAT and PLA polymers into smaller molecules.

“We were very surprised to find that a large part of the strains analyzed was capable of degrading at least one of the plastics tested,” says Rüthi.

The best results were obtained by two uncharacterized fungal species from the genera 'Neodevriesia' and 'Lachnellula', which were able to digest all plastics except PE. The results also showed that the ability to digest plastic was dependent on the culture medium for most of the strains, with each strain reacting differently to each of the four media tested.

Since plastics have only been around since the 1950s, it is almost certain that the ability to degrade plastic was not a trait originally sought by natural selection.

“Microbes have been shown to produce a wide variety of polymer-degrading enzymes that are involved in the breakdown of plant cell walls.. In particular, phytopathogenic fungi often biodegrade polyesters by their ability to produce cutinases, which target plastic polymers because of their resemblance to plant cutin,” explains Dr Beat Frey, study author and WSL group leader.

Since Rüthi and his colleagues only tested digestion at 15 °C, they do not yet know what is the optimal temperature at which the enzymes of the selected strains function.. “But we know that most of the strains tested can grow well between 4°C and 20°C, with an optimum around 15°C,” Frey stresses.

“The next big challenge will be to identify the plastic-degrading enzymes produced by the microbial strains and optimize the process to obtain large amounts of protein -he announces-. Furthermore, further modification of the enzymes might be required to optimize properties such as protein stability.”