Finding Microorganisms That Can Digest Plastics at Low Temperatures

Credit: Photo by Nick Fewings on Unsplash

Some microorganisms produce enzymes that can digest plastic, but putting these enzymes to work at industrial scale typically requires temperatures above 30°C, which is costly. Scientists from the Swiss Federal Institute WSL have now discovered a number of bacterial and fungal strains that can break down some biodegradable plastics at much lower temperatures. Collected from soils at high altitudes in the Swiss Alps, and in polar regions, these types of cold-adapted microorganisms could feasibly make industrial-scale enzymatic processes for plastic recycling cost-effective.

"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 Joel Rüthi, PhD, currently a guest scientist at WSL. "These organisms could help to reduce the costs and environmental burden of an enzymatic recycling process for plastic." Rüthi is first author of the team's published paper in Frontiers in Microbiology, which is titled "Discovery of plastic-degrading microbial strains isolated from the alpine and Arctic terrestrial plastisphere." In their paper the team concluded, "Our results suggest that microorganisms from high-alpine and polar regions are efficient producers of plastic-degrading enzymes and thereby may contribute to future efforts for an environment-friendly circular plastic economy."

Annual global plastic production is still rapidly rising, having reached 367 megatons in 2020, the authors wrote. "The persistence of conventional plastics in the environment, the excessive usage of single-use plastics and waste mismanagement are causing a significant environmental problem." Conventional mechanical and chemical approaches to reuse and recycling have "some considerable downsides," the researchers continued. Alternative approaches for a more sustainable plastic economy include the use of bio-based and biodegradable plastics, as well as "novel recycling strategies using microbial plastic-degrading enzymes," they suggested.

Finding, cultivating, and bioengineering organisms that can digest plastics and help to address pollution, is now also big business. But while several microorganisms that can do this have already been found, the heating required for their enzymes to work means that industrial applications remain costly and aren't carbon neutral.

One potential solution is to identify specialist cold-adapted microbes whose enzymes work at lower temperatures. "However, the plastic degradation potential of cold-adapted microorganisms has rarely been studied so far," the scientists stated. For their reported work Rüthi and colleagues sampled 19 strains of bacteria and 15 fungal strains growing on free-lying or intentionally buried plastic (kept in the ground for one year) in Greenland, Svalbard, and Switzerland. Most of the plastic litter from Svalbard had been collected during the Swiss Arctic Project 2018, where students did fieldwork to witness the effects of climate change at first hand. The soil from Switzerland had been collected on the summit of the Muot da Barba Peider (2,979 m) and in the valley Val Lavirun, both in the canton Graubünden.

The scientists let the isolated microorganisms grow as single-strain cultures in the laboratory in darkness and at 15°C, and identified them using molecular techniques. The results showed that the bacterial strains belonged to 13 genera in the phyla Actinobacteria and Proteobacteria, and the fungal strains to 10 genera in the phyla Ascomycota and Mucoromycota.

The researchers then used a suite of assays to screen each strain for its ability to digest sterile samples of 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 microbial strains were able to digest PE, even after 126 days of incubation on 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 the plastic mixtures of PBAT and PLA. Nuclear magnetic resonance (NMR) and a fluorescence-based assays confirmed that these strains were able to chop up the PBAT and PLA polymers into smaller molecules.

"It was very surprising to us that we found that a large fraction of the tested strains was able to degrade at least one of the tested plastics," said Rüthi. "Several taxa (e.g., genera Collimonas, Kribbella, Lachnellula and Thelebolus) were shown, for the first time, to degrade plastics," the scientists stated. "Most notably, the tested strains degraded dispersed PUR and the polyester films ecovio® and BI-OPL at lower temperatures (15°C) than previously reported microbial strains."

The best performers were two uncharacterized fungal species in the genera Neodevriesia and Lachnellula. These microorganisms were able to digest all of the tested plastics except PE. "The fungal strains 800 (Neodevriesia sp.) and 943 (Lachnellula sp.) are promising candidates for further studies, as they degraded all the tested biodegradable products, were shown to reduce the masses of the PBAT and PLA components in the plastic films, and efficiently hydrolyzed the pure PBAT polymer," the team noted.

The results also showed that the ability to digest plastic depended on the culture medium for most strains, with each strain reacting differently to each of four media tested. "… we demonstrated that culturing conditions have a strong influence on plastic degradation. This finding might help to optimize the degradation rates achieved by the microbial strains and may also have consequences for plastic degradation in natural environments where carbon and nutrient contents are limited, in particular in oligotrophic Arctic and high-mountain soils." The team further pointed out it is likely that screening tests for plastic-degrading microorganisms may only detect a subset of the potential plastic-degrading strains because only a few conditions are tested, "whereas some strains may require very specific conditions to express plastic-degrading enzymes."

Since plastics have only been around since the 1950s, the ability to degrade plastic almost certainly wasn't a trait originally targeted by natural selection. So how did the ability to digest plastic evolve? "Microbes have been shown to produce a wide variety of polymer-degrading enzymes involved in the break-down of plant cell walls," said co-author Beat Frey, PhD, a senior scientist and group leader at WSL." In particular, plant-pathogenic fungi are often reported to biodegrade polyesters, because of their ability to produce cutinases which target plastic polymers due their resemblance to the plant polymer cutin."

Rüthi and colleagues only tested for plastic digestion at 15°C, so they don't yet know the optimum temperature at which the enzymes of these successful strains work. "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. And as the authors concluded in their paper, "This study expands our knowledge about microbial plastic degradation and provides a basis for future discovery of cold-active plastic-degrading enzymes. The identified microbial strains could serve as a valuable resource for the development of efficient and sustainable plastic-waste recycling at lower temperatures."

Added 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." The authors further commented "A promising method to optimize plastic degradation might involve the identification of the genes encoding the responsible enzymes and the heterologous expression of these genes in a suitable host."