Polystyrene—most familiarly produced as foams, including Styrofoam—has given the world cheap, lightweight insulation and containers for taking food to go. But the material biodegrades so slowly that it can sit in a landfill for hundreds of years. A new finding points to a potential solution: Mealworms will dine on polystyrene foam when they can’t get a better meal, converting up to 48% of what they eat into carbon dioxide (Environ. Sci. Technol. 2015, DOI: 10.1021/acs.est.5b02661).
To stem the tide of plastic waste, researchers have been searching for microbes or invertebrates that can biodegrade hydrocarbon-based polymers into inorganic compounds such as CO2 and water. In an effort to find creatures that might digest stubborn polystyrene, Jun Yang and Lei Jiang of Beihang University, in Beijing, along with their colleagues, tested mealworms—the tan, segmented larvae of the mealworm beetle. Mealworms are raised for pet food, and have even been proposed as a dietary protein supplement for people.
The larvae will eat polystyrene, the team found, though perhaps understandably, they prefer not to. “When we supply additional food like potatoes, mealworms will eat potatoes first,” Yang says. To see if the worms were biodegrading the plastic, the team set up three incubators: sealed glass jars each holding 40 worms and a 6 g block of polystyrene foam. The team then monitored the CO2 the mealworms produced. After 16 days, the researchers quantified the carbon in the remaining polystyrene, the mealworms, and the critters’ fecal matter using elemental analysis; they also analyzed the compounds in the worms’ feces with nuclear magnetic resonance.
The mealworms ate about a quarter of the foam, converting some of it to CO2 at a rate that increased throughout the experiment. At 16 days, they had converted 48% of the carbon they had eaten into CO2 and excreted 49% in their feces. Only 0.5% was incorporated into the worms’ bodies—comparable to the small amount of carbon termites absorb when eating wood. The compounds in the mealworms’ fecal matter had 20% lower molecular weight on average than the original polymers, indicating the polystyrene was degraded in their guts.
Yang and his team also showed that microbes in the mealworms’ guts were responsible for breaking down the polystyrene. When they gave the worms the antibiotic gentamicin, the polystyrene in their fecal matter no longer showed degradation (Environ. Sci. Technol. 2015, DOI: 10.1021/acs.est.5b02663).
To zero in on these polystyrene-splitting microbes, the team then isolated 13 bacterial cultures from the guts of the mealworms and grew them on polystyrene film. The most abundant was a strain of Exiguobacterium. Over two months, the bacteria made holes and cavities in the film, although the degradation was less efficient than in the mealworms’ guts.
Ramani Narayan, an expert in plastic biodegradation at Michigan State University, says the researchers have made an interesting discovery, but that it is not yet ready for practical application. He notes that about half of the polystyrene the mealworms eat is excreted back into the environment in fragments that may not be biodegradable and could carry toxins up the food chain.
Yang agrees that applying the discovery to manage waste—perhaps by engineering bacteria that could boost the reported biodegradation—may take time. Next, he and his colleagues hope to understand the mechanism and environmental factors responsible for degradation in the mealworms’ guts, and identify the bacterial enzymes and genes involved.