Newswise — Discovering, nurturing, and genetically modifying creatures that can decompose plastic not only facilitates the elimination of contamination but is presently a significant industry. A few microorganisms with this capability have been identified, however, their enzymes, which enable the process, commonly operate only at temperatures exceeding 30°C when employed at an industrial level. The necessary warming increases the expense of industrial usage and is not environmentally friendly. Nevertheless, locating specialized microbes adapted to colder temperatures with enzymes that function at reduced temperatures could potentially solve this issue.
The researchers from the Swiss Federal Institute WSL were aware of the possible location of such microorganisms: either at elevated elevations in the Alps within their nation or in polar regions. Their discoveries have been made public in Frontiers in Microbiology.
Dr. Joel Rüthi, the primary author of the study and currently a visiting scientist at WSL, stated, "Our research demonstrates that newly discovered microbial categories from the 'plastisphere' in alpine and arctic soils could decompose biodegradable plastics at a temperature of 15°C." He added, "These organisms have the potential to decrease the expenses and ecological impact of an enzymatic plastic recycling process."
Dr. Rüthi and his team examined a total of 19 bacterial and 15 fungal strains that were present on plastic waste, either left on the ground or intentionally buried for a year, in Greenland, Svalbard, and Switzerland. The majority of the plastic waste from Svalbard had been gathered during the Swiss Arctic Project 2018, where students undertook fieldwork to observe the impacts of climate change firsthand. The soil samples from Switzerland were obtained from the summit of Muot da Barba Peider (2,979 m) and the Val Lavirun valley in the canton Graubünden.
The scientists cultivated the isolated microorganisms as single-strain cultures in the laboratory, in the absence of light and at a temperature of 15°C, and utilized molecular methods to classify them. The findings revealed that the bacterial strains were part of 13 genera in the Actinobacteria and Proteobacteria phyla, while the fungi belonged to 10 genera in the Ascomycota and Mucoromycota phyla.
Surprising results
The team employed a range of assessments to evaluate each strain's ability to decompose sterile samples of non-biodegradable polyethylene (PE) and biodegradable polyester-polyurethane (PUR), as well as two commercially available biodegradable blends of polybutylene adipate terephthalate (PBAT) and polylactic acid (PLA).
Although none of the strains were able to digest PE, even after being incubated for 126 days on this type of plastic, 19 strains (56%), including 11 fungi and eight bacteria, demonstrated the ability to decompose PUR at 15°C. Additionally, 14 fungi and three bacteria could break down the plastic mixtures of PBAT and PLA. Nuclear Magnetic Resonance (NMR) and a fluorescence-based assay verified that these strains could fragment the PBAT and PLA polymers into smaller molecules.
Rüthi commented, "We were astonished to discover that a significant portion of the examined strains had the ability to degrade at least one of the plastic types tested."
The most effective strains were two uncharacterized fungal species in the Neodevriesia and Lachnellula genera. These fungi were capable of digesting all the tested plastics, except for PE. The findings also indicated that, for most strains, the ability to digest plastic was influenced by the culture medium, with each strain reacting differently to each of the four media tested.
Side-effect of ability to digest plant polymers
What caused the development of plastic digestion? Given that plastics emerged only in the 1950s, it is unlikely that plastic decomposition was initially selected for by natural selection.
Dr. Beat Frey, a senior scientist and group leader at WSL, explained that microbes are known to produce various enzymes that break down polymers, including those found in plant cell walls. Certain plant-pathogenic fungi are especially adept at breaking down polyesters due to their production of cutinases, which can also target plastic polymers due to their structural similarity to cutin, a plant polymer.
Challenges remain
Since Rüthi et al. only tested for digestion at 15°C, they don’t yet know the optimum temperature at which the enzymes of the 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.
“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”.
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