Plastic is everywhere, from the center of the ocean to Arctic glaciers. Plastics can break down over time due to physical damage from rain, wind, heat, and sunlight. However, traditional plastics like polyethylene (PE) can take hundreds or even thousands of years to degrade on their own. To help reduce plastic pollution, many companies have switched to plant-based materials like biodegradable plastics that break down more quickly. Because biodegradable plastics are new materials, scientists don’t yet know how they affect soil ecosystems and microbes.
Like all living organisms, microbes need a source of carbon to build their cells. Scientists have discovered microbes that can consume the carbon within plastics. These microbes produce proteins that break down different types of p…
Plastic is everywhere, from the center of the ocean to Arctic glaciers. Plastics can break down over time due to physical damage from rain, wind, heat, and sunlight. However, traditional plastics like polyethylene (PE) can take hundreds or even thousands of years to degrade on their own. To help reduce plastic pollution, many companies have switched to plant-based materials like biodegradable plastics that break down more quickly. Because biodegradable plastics are new materials, scientists don’t yet know how they affect soil ecosystems and microbes.
Like all living organisms, microbes need a source of carbon to build their cells. Scientists have discovered microbes that can consume the carbon within plastics. These microbes produce proteins that break down different types of plastics, called enzymes. A group of researchers from Switzerland tested whether microbial communities that live on biodegradable plastics are different from those that live on traditional plastics or in plastic-free soil.
The researchers created an experiment with 3 types of plastics. Two of the plastics, Ecovio and BI-OPL, were made from blends with plant-based materials, while the third plastic was a traditional PE plastic. They constructed 12 small glass containers the size of flower pots and filled them with soil from the Swiss Alps. Then they cut small squares from the 3 plastic types and buried squares of each plastic in 3 separate soil pots, so each type of plastic had 3 replicates. They kept 3 pots as control replicates with plastic-free soil. They covered the pots with lids and kept them at a constant temperature of 15°C (around 60°F) in the dark for 5 months.
After 5 months, the team unearthed the plastic pieces for testing. First, they looked for signs of degradation on the plastic surfaces using a light microscope, which magnifies the surfaces up to 200 times larger than what we can see with the naked eye. This allowed the researchers to see holes and cracks on the surfaces of the biodegradable plastics. They were even able to see traces of fungi living on the Ecovio plastic. However, they saw no holes or cracks on the surfaces of traditional PE samples.
The team then took small soil particles from the plastics to investigate the microbial communities that had formed. They identified the microbes living on the plastic surfaces by extracting and sequencing their DNA. They found more fungi on the biodegradable plastics than on the PE plastic or the plastic-free soil. In particular, the Ecovio plastic had about 6 times more Ascomycota fungi than the PE plastic or plastic-free soil, and the BI-OPL plastic had about 3 times more. They also found more Proteobacteria on both biodegradable plastics than on the PE plastic or plastic-free soil.
From previous studies, the team knew that 2 types of Proteobacteria, Rhizobiales and Burkholderiales, form mat-like colonies called *biofilms *on the surface of particles. Humans can have biofilms of different microbes form on the surfaces of our teeth, causing our enamel to break down and leading to tooth decay. The researchers hypothesized that biofilm-forming microbes could similarly be important in breaking down plastic.
After identifying the microbes, the team used another sequencing technique, called metagenome sequencing, to “read” the microbes’ DNA and learn about their genetic abilities. They found that the microbial communities on the Ecovio and BI-OPL plastics had a higher number of genes for the enzymes that break down plastic than the microbial communities on the PE plastic or plastic-free soil. These enzymes act like scissors to cut the long carbon chains that make up the plastics. One of these gene families, called alpha/beta hydrolases, was 250 to 500 times more common in microbes on the biodegradable plastics than in the other groups.
The researchers concluded that biodegradable plastics select for specific microbes that have more genes for plastic breakdown than traditional plastics. This means that microbes should be able to degrade biodegradable plastic faster and easier than traditional plastic. The researchers recommended that people continue to study plastic pollution in alpine regions like the Swiss Alps because scientists are likely to discover new enzymes for plastic degradation in these microbial communities. They suggested these enzymes could be used for new methods of plastic recycling in the future.
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