Finding, culturing, and bioengineering organisms that can digest plastic not only help remove pollution, but it’s now also big business. Several microorganisms that can do this have already been found, but when their enzymes that make this possible are applied on an industrial scale, they only work at temperatures above 30°C. The heating required means that industrial applications remain costly yet and are 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 knew where to look for these microorganisms: at high altitudes in the Alps of their country, or in the polar regions. Their findings have been published in Frontiers in Microbiology.
“Here we show that new microbial taxa obtained from the ‘plastic ball’ of alpine and arctic soils were able to degrade biodegradable plastics at 15 °C,” said first author Dr Joel Rothy, a current visiting scientist at WSL. . “These organisms can help reduce the costs and environmental burden of the enzymatic recycling of plastics.”
Rüthi and colleagues sampled 19 strains of bacteria and 15 fungi growing on loose or intentionally buried plastic (kept in the ground for one year) in Greenland, Svalbard and Switzerland. Most of the plastic rubbish was collected from Svalbard during the 2018 Swiss Arctic Project, where students did fieldwork to witness the effects of climate change firsthand. Soils were collected from Switzerland on the Muot da Barba Peider (2,979 m) and in the Val Lavirun valley, both in the canton of Graubünden.
The scientists allowed the isolated microbes to grow as mono-strain cultures in the laboratory in the dark and at 15°C and used molecular techniques to identify them. The results showed that the bacterial strains belong to 13 genera in the phyla Actinobacteria and Proteobacteria, and the fungi belong to 10 genera in the phyla Ascomycota and Mucoromycota.
They then used a set of assays to check each strain for its ability to digest sterile samples of non-biodegradable polyethylene (PE) and biodegradable polyester-polyurethane (PUR) as well as two biodegradable blends of polybutylene adaptate (PBAT) and polyacid Lactic (PLA).
None of the strains were able to digest polyethylene, even after 126 days of incubation on these polyethylenes. But 19 (56%) strains, including 11 fungi and eight bacteria, were able to digest polyurethane at 15 °C, while 14 fungi and three bacteria were able to digest plastic mixtures of PBAT and PLA. Nuclear magnetic resonance (NMR) and fluorescence-based assay confirmed that these strains were able to cleave PBAT and PLA polymers into smaller particles.
“It was very surprising to us that we found that a significant fraction of the tested strains were able to degrade at least one type of plastic material tested,” said Rothe.
The best performers were two uncharacteristic fungi in the genera neofreesia And Luckilola: These were able to digest all plastics tested except polyethylene. The results also show that the ability to digest plastic is dependent on the culture medium for most strains, with each strain reacting differently to each of the four media tested.
Side effects of the ability to digest plant polymers
How did the ability to digest plastic evolve? Since plastic has been around since the 1950s, the ability to decompose plastic was certainly not a trait originally targeted by natural selection.
Microbes have been shown to produce a variety of polymer-degrading enzymes that are involved in breaking down plant cell walls. In particular, plant pathogenic fungi are often reported to degrade polyesters, due to their ability to produce chitinases that target plastic polymers due to their similarity to the plant polymer cutin. “
Dr. Pete Frey, another author and chief scientist and group leader, WSL
Since Rüthi et al. Tested for digestion only at 15°C, they don’t yet know the optimal temperature at which the enzymes of the successful strains work.
“But we know that most of the strains tested can grow well between 4°C and 20°C with the optimum at around 15°C,” Fry said.
The next big challenge will be identifying the plastic-degrading enzymes produced by microbial strains and optimizing the process to obtain large amounts of proteins. In addition, further modification of the enzymes may be required to improve properties such as protein stability.”
De Freitas, A.; et al. (2023). Dark Amazonian soils promote tree species establishment in forest ecological restoration. Frontiers in soil science. doi.org/10.3389/fsoil.2023.1161627.