By some estimates there are more than 8.3 billion tons of plastic on the planet – more than 6.3 billion tons of that is waste. Recycling isn’t an option for all of it. But scientists around the world are looking at organic solutions in the form of hungry bugs and the enzymes and bacteria they produce.
Among them: Dr. Chris Rinke and a team of researchers at Australia’s University of Queensland in 2022 published a study in Microbial Genomics about their work with the larvae of the darkling beetle Zophobas morio.
It found that the so-called “superworms,” which normally feed on such decaying material as dead leaves and animal carcasses, could survive on polystyrene alone. Most are able to complete their transition to adult beetles on just a diet of the synthetic resin commonly used for such items as disposable cups and surfboards.
“Our understanding is that superworms mechanically shred the polystyrene, ingest it, and then the bacteria in the worm’s gut further degrade the plastic. We found several encoded enzymes associated with polystyrene degradation in the gut bacteria,” Rinke tells KUST Review, adding that the team is also looking into the degradation of such other thermoplastics as polyethylene and polypropylene.
And, sure, your local waste-reclamation facility might set up a giant worm farm to decompose unwanted polystyrene, but Rinke tells NPR it would be cheaper and easier to reproduce the enzymes that allow the larvae to digest, say, old dishwasher parts and packing material. A synthetic “enzyme cocktail” could be sprinkled over shredded waste. Add microbes to the material and you could create useful and more sustainable bioplastics.
Rinke cautions that it will take a while before the enzymes are available for industrial use.
“It will take sufficient research funding and several years of research to characterize the enzymes involved in polystyrene degradation, but once we have found the most efficient enzymes, we can offer a biological solution to degrade plastic waste,” he says.
In the meantime, he encourages consumers to avoid plastic, “especially single-use plastic packaging, whenever possible,” he tells KUST Review.
“If plastic needs to be used and eventually becomes waste, then one should recycle plastic waste as much as possible. Last but not least, it’s also important to ask local councils to increase the amount of plastic recycling,” he says.
ANOTHER HUNGRY, HUNGRY CATERPILLER
But the Zophobos morio isn’t the only insect bellying up to the plastics buffet.
Researchers in Poland published their results on a study of Tenebrio molitor in the journal Polymers.
The researchers fed the insect – commonly called a yellow mealworm and another species of darkling beetle – a diet of polystyrene foam (PS), two types of polyurethane (PU1 and PU2, like kitchen sponges and commercial insulation foam) and polyethylene foam (PE, commonly used in packing materials).
The researchers concluded that genetic variances among mealworm populations could account for different rates of consumption, but say 1 kilogram of PS, PU1, PU2 and PE could be consumed over 58 days by 40.5 kg, 46.0 kg, 36.5 kg and 30.9 kg of Z. morio, respectively.
FROM PEST TO PROMISE
The Polish researchers mention other plastivore species, including Galleria mellonella, a wax moth whose palate for plastics was discovered accidentally when a researcher put the caterpillars in a plastic bag and found later that they had eaten holes in it. The information that resulted was featured in a recent study from Brandon University in Canada.
The moth caterpillar larvae, which normally invade beehives and eat wax, can digest polyethylene – the kind of plastic found in shopping bags – and excrete ethylene glycol, a form of alcohol that can be used as antifreeze.
In the study, 60 waxworms consumed 30 square centimeters of the plastic in less than a week. The researchers published their results in Current Biology.
Although the waxworms can consume the plastics on their own, researchers also isolated an intestinal bacteria from the larvae that was able to survive on polyethylene as its sole source of nutrition for a year. Working together, the waxworms and the bacteria accelerate plastic biodegradation. Researchers caution, however, that the waxworms and their bacteria aren’t a solution to the plastics problem but point to possible future directions for waste management.
ENTER MICROBES
Different kinds of bugs – not insects but microbes – are also emerging as potential solutions to the world’s plastics-waste problem.
Researchers in 2016 discovered a bacterium in a Japanese garbage dump that had evolved naturally to eat plastic, and when they tweaked a promising enzyme to see how it evolved, they accidentally made the molecule even better at breaking down polyethylene terephthalate, the plastic used in soft-drink bottles.
But more recently, a group of scientists in Sweden has found that microbes around the world are evolving to eat the plastic trash that has found its way into mountain peaks, ocean depths and remote tropical beaches. They published the results of their study, the first to assess the global potential of plastic-eating microbes in mBio.
Scanning 200 million genes, the researchers found 30,000 enzymes that could degrade 10 kinds of plastics.
The number and type of enzymes they found corresponded to the amount and type of plastics in their locations. One in four organisms examined carried an enzyme that could break down plastics.
“We did not expect to find such a large number of enzymes across so many different microbes and environmental habitats. This is a surprising discovery that really illustrates the scale of the issue,” Chalmers University researcher Jan Zrimec says in the Guardian.
The remarkable thing about these microbes and insects is that plastics are man-made and, in evolutionary terms, quite recent, says Khalifa University’s David Sheehan. “Yet microbes clearly have evolved enzymes that can degrade them in a short period of evolutionary time. If we can identify a panel of these enzymes, we could use enzyme engineering approaches to improve their activity and substrate range and produce these commercially much as we do with biological detergents.”
