Plastic waste is so ubiquitous that particles of it have been detected in the air we breathe, the food we eat, and even our blood. Human consumption and disposal of plastic products has also polluted habitats across the planet, including practically every corner of the oceans, which is especially pernicious because plastics can take centuries of even millennia to biodegrade.
Given that plastic pollution is expected to increase in the coming decades, there is a dire need to develop sustainable recycling and upcycling processes for this waste, which includes the common material polystyrene that is used to make Styrofoam. Enter: superworms.
Now, scientists have demonstrated that the larvae of the darkling beetle species Zophobas morio, known as “superworms,” can “survive on polystyrene feed” thanks to microbes in their guts, a finding that “will provide a base for future investigations into microbial upcycling of plastic waste,” according to a study published on Thursday in Microbial Genomics.
“Insect larvae actually have a good track record of damaging and eating plastics,” said Chris Rinke, a senior lecturer at the Australian Center for Ecogenomics (ACE) at the University of Queensland and senior author of the study, in an email. “Initial studies, by other authors, have reported that waxworms and common mealworms can eat plastic, so we thought if these rather small larvae can do it, then the large superworms (up to 5.5 cm) might be even more efficient in munching plastic.”
“It turned out that superworms have a great appetite for polystyrene,” he added. “So, we didn’t know if superworms could survive on plastic when we started our experiments, but we had high hopes.”
To reach this conclusion, Rinke and his colleagues divided 171 superworms into three groups with different diets: One ate only polystyrene, another ate bran, and a third was put on a strict fast. In a macabre twist, the team notes that instances of cannibalism among the fasting superworms “led to our modified experimental design housing the starving control group animals in isolation, whereas animals in the other two groups were housed together during the feeding trial,” according to the study.
In addition to observing the worms throughout the trial, the team also used gene sequencing to identify many of the genetic pathways associated with plastic-eating powers. As a result, the experiment provided “the first metagenomic analysis of a plastic-associated insect microbiome,” according to the study.
Superworms are hardy creatures, and over 95 percent of each group survived their respective three-week diets. The bran-fed worms gained the most weight, but the larvae on the polystyrene diet also got marginally heavier and displayed more activity compared to the starved worms, suggesting that they were able to derive nutrition from the plastic waste—though it came at a cost to their health.
“The superworms reared on polystyrene gained only a small amount of weight and the diversity of their gut microbiomes decreased, both signs that polystyrene is, as expected, a rather poor diet,” Rinke said. “We also found evidence of potential pathogenic bacteria, indicating that the polystyrene diet has negative impacts on the worm's health. Providing food waste or agricultural bioproducts with the polystyrene could be a way to improve the health of the worms.”
While superworms may play a role in mitigating plastic waste, Rinke noted the real focus is on the larvae’s gut microbes, because their digestive secrets could be artificially mimicked and harnessed on large scales for use in bioreactors and other plastic-processing facilities.
“We have now a catalogue of all the bacterial enzymes encoded in the superworm gut, and plan to further investigate the enzymes with polystyrene degrading capabilities,” Rinke said. “We will characterize them in more detail over the next years to find the most efficient enzymes, which can then be even further improved with enzyme engineering.”
“Ultimately, we want to take the superworms out of the equation, and mechanically shred the plastic waste, followed by microbial degradation in bioreactors, and subsequent microbial production of higher value compounds such as bioplastic,” he concluded. “This upcycling approach will make plastic recycling more economically feasible and should incentivise plastic recycling.”