Outlawed in some U.S. cities as unrecyclable waste and controversial almost everywhere, the plastic foam carryout food container could become a new part of the mealworm's diet — and solve a major garbage problem in the process.
Larvae of the darkling beetle will not only feed on expanded polystyrene, but microorganisms in their guts biodegrade it internally. And then, they poop out a seemingly safe product that may be suitable as soil for crops. Talk about waste disposal.
The promising trash-to-food chain is unfolding in a laboratory at Stanford University, where a research team is the first to provide detailed evidence of bacterial degradation of plastic in an animal's gut.
“It's actually astonishing to see the worms eating their way through the polystyrene,” Craig Criddle, a Stanford professor of civil and environmental engineering, said in a telephone interview. “It is surprising.”
Another surprise is that the PS doesn't seem to be toxic to the insects, which also are getting potatoes and food scraps for a nutritious balanced diet.
“They're fine with a little bit of the good stuff on the side,” Criddle said.
He is supervising the study at Stanford, which is being led by senior researcher Wei-Min Wu in collaboration with colleagues in China. The study is still in its early stages, but Wu described in an email the high hopes he has for its implications: “to find a way to remediate current plastic pollution.”
Wu is building on research initiated at the Beihang University in China, where researchers observed waxworms, the larvae of Indian mealmoths, break down polyethylene in the form of plastic bags thanks to microorganisms in their guts. Wu said he decided to study EPS because it is commonly used for coffee cups and Americans throw away some 2.5 billion of those each year. The findings to date also are significant because EPS “has been considered basically non-biodegradable and it causes pollution problems in soil, rivers, lakes and oceans,” Wu said.
In the Stanford study, 100 mealworms ate 34-39 milligrams of EPS — the equivalent weight of a small pill — each day. Microbes in the guts of the baby bugs broke down the plastic and converted some of it into carbon dioxide and some of it into biodegradable fragments, which were excreted like tiny rabbit droppings within 24 hours.
So far, that excreted waste appears safe to use as soil, according to the researchers.
“Our findings have opened a new door to solve the global plastic pollution problem,” Wu said in a Sept. 29 news release.
But first, many other questions need to be answered about the microbes and their role in the processing of the plastic.
“The insects do some of the work because they're chewing up the material,” Criddle said. “Then the microbes take it to an even smaller scale by breaking it down with enzymes and that effectively is recycling the material at a molecular scale. In the near term we'd like to find out what the enzymes are and understand the conditions under which they are operating.”
When the microbes are grown outside the insect, they still break down the polystyrene but not at as high of a rate, Criddle said.
“The rate of degradation seems to happen faster inside these worms,” he said. “Something is occurring inside the insect.”
A lot of questions about how the mealworms process plastics and what that could mean for the polluted planet are eating away at the researchers.
“If we learn more about new enzymes or new processes, how can we engineer them in way that helps get rid of the problem,” Criddle asked. “…There are many issues with [EPS] and these other plastics so we need to find solutions to these problems for many reasons. Landfills are filling up. There's ocean debris.”
Scientists could be at the forefront of engineering more powerful enzymes to degrade plastic or guiding manufacturers to design polymers that don't accumulate in the environment or food chains.
“This is early stage research,” Criddle said. “We don't know where it will go.”
The researchers at Stanford and in China plan to study whether the microorganisms in mealworms and other insects could biodegrade other plastics, such as polypropylene, mircobeads and bioplastics. Wu said they also begin looking for a marine equivalent of the mealworm.
The long-term ripple effect is exciting, Criddle said, but he doesn't want to raise hopes too high.
“We're human beings. We have those hopes, too,” he said. “We don't want to raise expectations to a level where we can't meet them. We want to be realistic about what's possible this science. We just don't know; there's a lot we don't know right now.”