The plump, glossy larvae of the darkling beetle, nicknamed “superworms” perhaps because of their size, are usually content to munch on wheat bran. But a number of the two-inch-long critters recently found themselves dining on much stranger fare in the service of science: polystyrene, the long-lived plastic packing material known sometimes by the brand name Styrofoam.
What’s more, the larvae that managed to choke down this peculiar feedstock did not, as you might expect, expire. As scientists documented in a paper published on Thursday in the journal Microbial Genomics, they even gained a bit of weight and were able to metamorphose into beetles most of the time, prompting the researchers to check their digestive systems for microbes that could break down the polystyrene. If scientists can understand such microbes’ tool kits, they can devise a better way to recycle this tenacious substance, which, if left on its own, may persist in the environment for hundreds of years or more.
These are not the first insects that have been fed polystyrene in a lab. Mealworms are known for their ability to eat the substance that makes up packing peanuts, among other plastics, said Christian Rinke, a microbiologist at the University of Queensland in Australia and an author of the new paper. Mealworms and superworms alike have been observed consuming polystyrene, and they lose this ability when they’re fed antibiotics. So researchers have concluded that their gut microbiomes are likely to be behind this unusual talent.
The question was, what was in those microbiomes, exactly? To find out, Dr. Rinke and his colleagues grew three groups of superworms in the lab. One group ate bran, one ate blocks of polystyrene and the third ate nothing. (The experiments were temporarily halted by hungry superworms’ tendency to turn cannibal; giving each unfed superworm its own private space allowed the study to continue.)
While bran was obviously much more attractive to the superworms, they were willing to give polystyrene a go. Within 48 hours, the polystyrene group’s feces turned from light brown to white, and their weight crept up very slowly over the course of three weeks.
When the time came for the insects to metamorphose into beetles, those that ate bran completed the transition successfully nearly 93 percent of the time; those that had starved mustered only 10 percent. Strikingly, 66.7 percent of the polystyrene-eating larvae that were given the chance to pupate were successful. They managed to get enough energy from the notoriously indigestible substance to transform.
“Polystyrene is definitely a poor diet,” Dr. Rinke said. But “the worms can survive it — they don’t look sick or anything.”
The researchers sequenced all the DNA they could extract from the guts of the larvae. They were less interested in which specific microbes were present than in what enzymes were being made as the microbes worked to break down polystyrene. They pinpointed a handful of likely candidates — all types of enzymes known for their slicing-and-dicing abilities — that were possibly shearing polystyrene down into smaller pieces.
“The next step will be to express those enzymes in the lab and experimentally verify that they are doing what we think they are,” Dr. Rinke said.
With more details about the conditions these enzymes require and the precise nature of their abilities, Dr. Rinke hopes that an industrial process to recycle packing foam can someday be designed. At the moment, used polystyrene can be processed into certain kinds of building materials to try to keep it out of landfills. However, a much better solution would be a way to break down its components and then build them back into something new, perhaps using microbes that could spin them into fresh bioplastics.
“It would make the whole thing more interesting economically,” he said. “It would create something sought after,.”
