Scientists discover plastic-degrading microbial strains at low temperatures

SWITZERLAND – Scientists in the Alps and the Arctic have discovered microbes that can digest plastics at low temperatures, a valuable tool in recycling.

Scientists from the Swiss Federal Institute WSL have found microbes that can do this at 15^oC, which could lead to a breakthrough in microbial recycling. Their findings have been published in the journal Frontiers in Microbiology.

Dr. Joel Rüthi from WSL and colleagues sampled 19 strains of bacteria and 15 of fungi growing on free-lying or intentionally buried plastic kept in the ground for one year in Greenland, Svalbard and Switzerland.

They let the microbes grow as single-strain cultures in the laboratory in darkness at 15^oC and tested them to see if they could digest different types of plastic.

The results showed that the bacterial strains belonged to 13 genera in the phyla Actinobacteria and Proteobacteria, and the fungi to 10 genera in the phyla Ascomycota and Mucoromycota.

The plastics tested included non-biodegradable polyethylene (PE) and the biodegradable polyester-polyurethane (PUR) as well as two commercially available biodegradable mixtures of polybutylene adipate terephthalate (PBAT) and polylactic acid (PLA).

None of the strains were able to digest PE, even after 126 days of incubation on these plastics. But 19 strains (56%), including 11 fungi and eight bacteria, were able to digest PUR at 15^oC, while 14 fungi and three bacteria were able to digest the plastic mixtures of PBAT and PLA.

Rüthi said: “Here we show that novel microbial taxa obtained from the ‘plastisphere’ of alpine and arctic soils were able to break down biodegradable plastics at 15^oC.

“These organisms could help to reduce the costs and environmental burden of an enzymatic recycling process for plastic.”

The scientists also tested for the best performers and found that they were two uncharacterized fungal species in the genera Neodevriesia and Lachnellula, which could digest all of the tested plastics except PE.

While plastics have only been in wide use since the 1950s, microbes can degrade polymers because they resemble some structures found in plant cells.

Dr. Beat Frey, one of the study authors, explained: “Microbes have been shown to produce a wide variety of polymer-degrading enzymes involved in the breakdown of plant cell walls.

“Plant-pathogenic fungi are often reported to biodegrade polyesters, because of their ability to produce cutinases, which target plastic polymers due to their resemblance to the plant polymer cutin.”

The scientists only tested the microbes at one temperature, so have not yet found the best one to use. Nevertheless, they say it works well between 4^oC and 20^oC.

Last month, Sustainable Packaging Africa reported that researchers from The University of Sydney, Australia harnessed two strains of fungi found in soils to break down lab samples of polypropylene (PP) in just 140 days.

The two fungi, Aspergillus terreus and Engyodontium album, made a meal of the plastic in the lab experiments: Between 25 and 27 percent of samples were devoured after 90 days, and the plastic was completely broken down after 140 days, the researchers report.

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