Cold-loving microbes could eat away our plastic crisis

Interesting Engineering (IE) connected with the study's first author Dr. Joel Rüthi, a guest scientist at WSL, for a deeper understanding of the study's findings and its implications for addressing plastic pollution.

A remarkable diversity

"We were interested in finding cold-adapted plastic-degrading microorganisms because of the potential to save energy due to their enzymes being active at lower temperatures," Rüthi told IE. 

He explained that in their previous studies, his team observed a remarkable diversity of microorganisms in soils from alpine and Arctic regions. 

"In addition, we found that many of the organisms found in these regions are still completely unknown, and we, therefore, don't know a lot about their metabolic abilities," he added. 

The team made another significant discovery: the microbial community that colonized the biodegradable plastics buried in these soils differed from those in the plastic-free soil.  

"From this finding, we hypothesized that biodegradable plastics might select for microorganisms able to use the plastics as energy and nutrient source," he said. "To test this hypothesis, we isolated microbes from the buried plastic surfaces."

Collecting plastic litter from cold zones

Essentially, Dr. Rüthi's team employed classical microbiological techniques to isolate and cultivate microbes within the controlled environment of their laboratory. 

They collected a total of 19 bacterial strains and 15 fungal strains. These microorganisms were sourced from both free-lying plastics and intentionally-buried plastics left in the ground for one year. The sampling sites spanned diverse locations, including Greenland, Svalbard, and Switzerland. 

Most plastic litter from Svalbard was obtained during the Swiss Arctic Project in 2018. This initiative was undertaken by a group of Swiss students and researchers who explored the environmental changes occurring in the Arctic due to global warming. They collected plastic litter from various locations in Svalbard during their research.

Soil samples from Switzerland were gathered from two distinct sites: the summit of Muot da Barba Peider, standing at an elevation of 2,979 meters, and Val Lavirun, located in the canton of Graubünden.

The isolated microbes were then cultivated separately in the laboratory under controlled conditions, including darkness and relatively cool temperatures of 15°C.

"We used genetic markers to find out whether the isolated microbes belong to already known species or if they represent so far unknown species," Dr. Rüthi said.

To test plastic degradation, he explained that the microbes were grown alongside a piece of plastic, in a liquid medium that contained nutrients. His team then monitored the weight loss of the plastic piece over a certain period. This allowed them to measure how effectively the microbes could break down the plastic, determining the extent of plastic degradation.

Discovering the 15°C sweet spot

The results revealed a diverse array of microorganisms. The bacterial strains belonged to 13 different genera within the Actinobacteria and Proteobacteria phyla; the fungi belonged to 10 genera within the Ascomycota and Mucoromycota phyla.

Dr. Rüthi's team then employed a range of tests to assess the capacity of each strain to break down different types of plastics. These included non-biodegradable polyethylene (PE), biodegradable polyester-polyurethane (PUR), and two commercially available, biodegradable mixtures of polybutylene adipate terephthalate (PBAT) and polylactic acid (PLA).

"It was very surprising to us that we found that a large fraction of the tested strains were able to degrade at least one of the tested plastics," said Rüthi in a previous press release.

Despite none of the strains showing the ability to digest polyethylene (PE), even after a 126-day incubation period, 19 strains (56 percent) could digest the biodegradable polyester-polyurethane (PUR) at a temperature of 15°C. This group comprised 11 fungi and eight bacteria. 

Furthermore, 14 fungi and three bacteria could digest the plastic mixtures of polybutylene adipate terephthalate (PBAT) and polylactic acid (PLA). 

"To further confirm the degradation and analyze which components of the plastics were degraded, we used more advanced technologies such as nuclear magnetic resonance spectroscopy (NMR) and a fluorescence-based method," Dr. Rüthi said to IE.

The most impressive performers in the study were two types of fungi, called Neodevriesia and Lachnellula. These fungi had the ability to break down all the plastics tested, except for polyethylene (PE). 

Interestingly, the results indicated that the type of culture medium used had a significant impact on the plastic-digesting capabilities of each strain. In other words, each strain reacted differently to the specific conditions provided by the different culture media.

Unlocking enzymes for industrial-scale use

"The novelty in our research is that our found microorganisms can efficiently degrade the tested biodegradable plastics at lower temperatures than most previously known organisms," highlighted Dr. Rüthi.

"From this, we hope we can find enzymes working at lower temperatures which might be used in enzymatic recycling applications in the future."

When asked about the timeframe for these microbes to biodegrade a substantial quantity of plastic, Dr. Rüthi clarified that it largely depends on the feasibility of scaling up the process.

"Currently, we only did small-scale experiments where our most successful microbes degraded around half of a 4x4 cm plastic piece in two months," he said. 

He recognized that scaling up might require a different approach., adding, "We also hope that the process can be significantly accelerated when using isolated enzymes rather than the microbes. Such enzymes may be produced in large amounts."

Dr. Rüthi further discussed with IE that his team's current work primarily focuses on basic research. It is still too early to determine the suitability of the discovered microbes for industrial applications, as more extensive studies and evaluations are needed. 

"However, enzymatic recycling can, in principle, be used to recover the building blocks of plastics, which can then be used to produce new plastic," he said.

"We currently don't have any industrial partners, but I know about one company called Carbios which is currently working on enzymatic recycling of PET."

One of the main limitations of Dr. Rüthi's research is that it primarily focused on the degradation of biodegradable plastics. "These plastics are only a very minor portion of all produced plastic," he emphasized.

They also only conducted digestion tests at a temperature of 15°C. This means they have yet to discover the ideal temperature at which the enzymes of the successful strains operate optimally. 

"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," expressed co-author Dr. Beat Frey, a senior scientist and group leader at WSL, in a press statement. Further tests will need to be carried out to confirm this.

"In addition, we have so far not observed the complete degradation of a whole piece of plastic. Further experiments are needed to see whether the whole plastic can be degraded."

To conclude, Dr. Rüthi emphasized that the next crucial step for the team is to identify and characterize the specific enzymes produced by the discovered microorganisms responsible for plastic degradation.