Cold-Specialized Microbes Discovered That Digest Plastic at Low Temperatures
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A new study has discovered 19 novel strains of bacteria and fungi, adapted to the cold climates of the Alps and the Arctic, that can digest plastics at temperatures as low as 15°C/59°F. The study is published in Frontiers in Microbiology.
Tackling plastic pollution with microbes
Several microorganisms that digest biodegradable plastics have already been discovered, and methods to find, culture and bioengineer these organisms to tackle plastic pollution are now a thriving industry.
However, many of the enzymes that digest these plastics will only work in industrial settings at temperatures above 30°C/86°F. Generating the heat required for these methods is costly and, importantly, isn’t carbon neutral.
Scientists from the Swiss Federal Institute WSL took an alternative approach to solve this problem – finding microbes adapted to cold environments, such as the Alps and the Arctic, whose enzymes can operate effectively at lower temperatures.
Could cold-adapted bacteria be the key?
The researchers sampled microbes from both free-lying and intentionally buried plastic (which had been in the ground for one year) sourced from Greenland, Svalbard and Switzerland. Plastic litter from Svalbard had been collected during the Swiss Arctic Project 2018, and Swiss soil was collected from the summit of the 2,979-meter peak Muot da Barba Peider and from the Val Lavirun valley.
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From these samples, the researchers identified 19 strains of bacteria as well as 15 fungal strains that they grew in darkness in the laboratory at 15°C. The cultured bacterial strains belonged to 13 genera in the phyla Actinobacteria and Proteobacteria, and the fungi to 10 genera in the phyla Ascomycota and Mucoromycota.
Next, they assessed the ability of these strains to digest plastics, including non-biodegradable polyethylene (PE), biodegradable polyester-polyurethane (PUR) and two mixtures of biodegradable polybutylene adipate terephthalate (PBAT) and polylactic acid (PLA).
The results revealed that PE was not digested by any of the identified strains. Meanwhile, 19 strains (11 fungi and 8 bacteria) digested PUR at 15°C/59°F, and 17 strains (14 fungi and 3 bacteria) digested the PBAT and PLA mixtures, also at 15°C/59°F.
Chemical analysis of these plastics using nuclear magnetic resonance (NMR) and fluorescence assays confirmed that PBAT and PLA polymers were degraded into smaller molecules.
The most successful strains were two uncharacterized fungal species in the genera Neodevriesia and Lachnellula, which were able to digest all tested plastics with the exception of PE. The strains also reacted differently to the four culture media tested, which influenced their plastic-digesting abilities.
“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°C,” said Dr. Joel Rüthi, lead author of the study and visiting scientist at WSL. “These organisms could help to reduce the costs and environmental burden of an enzymatic recycling process for plastic.”
Nevertheless, there are still questions that remain to be answered surrounding how these microbes acquired the ability to digest plastics, which have only been around since the 1950s. “Microbes have been shown to produce a wide variety of polymer-degrading enzymes involved in the breakdown of plant cell walls. In particular, 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,” explained Dr. Beat Frey, senior author of the study and group leader at WSL.
The authors also note that digestion was tested only at 15°C, and the optimum temperature at which the enzymes of the successful strains work has not yet been determined. “But we know that most of the tested strains can grow well between 4°C and 20°C with an optimum at around 15°C,” said Frey. “The next big challenge will be to identify the plastic-degrading enzymes produced by the microbial strains and to optimize the process to obtain large amounts of proteins. In addition, further modification of the enzymes might be needed to optimize properties such as protein stability.”
Reference: Rüthi J, Cerri M, Brunner I, et al. Discovery of plastic-degrading microbial strains isolated from the alpine and Arctic terrestrial plastisphere. Front. Microbiol. 2023;14. doi: 10.3389/fmicb.2023.1178474
This article is a rework of a press release issued by Frontiers Science News. Material has been edited for length and content.