Plastic waste is especially a nuisance for us. You have to hand in the soft drink bottles for a deposit or they end up in the roadside or the sea and thus pollute nature. For some tiny microbes, however, plastic is not a burden but a treat. take Ideonella Sakaiensis, discovered in 2016 at a Japanese factory. This micro-organism eats plastic. He breaks the long molecules that make up plastic PET, which is used in soft drink bottles, into smaller pieces. It takes the bacteria about six weeks to make a wafer-thin piece of PET disappear. Converted, this bacterial species would take half a year to a year for an entire bottle.
It turns out Ideonella isn’t the only microbe eating plastic. The plant bacteria Thermobifida Beetle also like it, a handful of fungi from European lakes break down the insulation material PUR and a fungus from the Amazon region does the same. Bacteria that break down plastic bags live in the intestines of the wax moth caterpillar and the intestines of the mealworm contain bacteria that feast on Styrofoam.
Just like in nature
The plant bacterium Thermobifida fusca is used to break down the fat-like layer of cutin.
Pixabay, FotoRieth via CC0
They’re not that well known, but plastic-degrading bacteria aren’t rare. “Relatively many of these bacteria live in the soil, although most have probably not yet been discovered,” says Jo-Anne Verschoor, PhD student at Leiden University. Her research includes the Streptomyces family. If you go outside after a rain shower you can smell the streptomycetes: the smell of wet earth is produced by these bacteria. Some of them are able to – very slowly – break down certain types of plastic.
The fact that these microbes break down plastic is not unusual for those who look closely. “Organisms were able to adapt so quickly to consuming some plastics, because those plastics are very similar to natural polymers,” explains Verschoor.
As an example she mentions the plant bacteria Thermobifida Beetle. It devours cutin, a waxy protective coating on leaves. “Thermobifida breaks chemical bonds in cutin molecules. Those compounds are the same as the compounds in a PET molecule. That is why this plant bacterium can also break down PET.” A relatively large number of microbes have been discovered that eat the plastics PET and PUR. Precisely because they contain compounds that are frequently found in nature. Other plastics seem to be less edible.
Break down or digest
The carbon isotope C13 reveals which bacteria digest carbon, not just break it down. It’s like marking a banana with a red color, and after you eat it you would exhale red CO2, Niemann says.
Microbes eating our garbage mountain sounds like excellent news, but only if they digest it completely. Otherwise you will still be left with junk. “It is a major concern that some microorganisms make pieces of plastic unstable, causing microplastics and nanoplastics to come loose,” says Helge Niemann. He conducts research into plastic-eating micro-organisms in the sea at the Royal Netherlands Institute for Sea Research. “At the moment no one knows if that is the case.”
Researchers are testing whether microbes digest plastic by feeding it a special kind of carbon as a building block, the carbon isotope 13C. If they find this in the proteins and waste products of the microbe, he not only breaks down the plastic, but uses the plastic to live on. This turned out to be the case for the PET bacterium from Japan and also for the PUR-eating fungus from the Amazon.
Micro- and nanoplastics are harmful to nature. Microplastics clog the intestines of fish and seabirds. Nanoplastics are so small that they pass the blood-brain barrier, the ‘dam’ that filters the blood that flows into our brains. There they influence hormonal communication, also in humans. “That is of course not good news, especially if you as a human being are exposed to high doses of nanoplastics,” says Niemann. “But: which is better? That the plastic remains in the ocean, or that it is broken down, whereby nanoplastics are created as an intermediate step until they are also completely broken down?”
PET recycling in bioreactors
Can plastic-eating microbes help clean up plastic waste? “It depends on what you do exactly,” says Verschoor. “There are relatively many plastic-degrading bacteria that live in the soil. We can take a few out of the ground and put them somewhere else to see if they do it there too.” Unfortunately, these microbes are not very efficient. Breaking down a small piece of plastic takes weeks or even years. A vision of the future of microbes that massively consume the plastic waste mountain is too rosy.
But there are still other possibilities. Researchers are trying to improve the enzymes in the lab. This has been achieved with the enzymes of the Japanese PET bacteria. The improved enzyme breaks down PET much faster than the natural version. Unfortunately, a microbe that produces this improved enzyme cannot simply be released into nature. It is a genetically modified organism, which is subject to strict safety regulations within Europe.
Recycling collected soda bottles in an isolated location may be an option. Verschoor is thinking of a bioreactor with plastic-eating bacteria. When a PET-eating microbe digests bottles, they are left with ethylene glycol and terephthalic acid. These are the building blocks from which PET is made. No new plastic needs to be added to the mix, as is currently required for the recycling of PET. It will take a few years before such a thing is possible. “We are still at the beginning,” says Verschoor. “Which microbe breaks down which plastic and with which enzymes? How do we find it? Can we improve this process?”
Although plastic has only been around for about seventy years, microbes are able to break down plastic, albeit slowly. Improving the enzymes of the soda bottle eater Ideonella Sakaiensis is a big step in the right direction to increase the pace. If we want to use these plastic-eating microbes to eat through our waste mountain, we will have to be patient for a while.
Source: Kennislink by www.nemokennislink.nl.
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