You are tasked with the care of a clew of mealworms. You swap out the bran they are happily munching on with – wait for it – Styrofoam. The worms keep eating as usual, metamorphose their way into beetlehood, and go happily on with their lives.
What just happened?
In 2015, researchers Yang et al. took a look at mealworms capable of digesting polystyrene, the main component of Styrofoam. Granted, the larval beetles didn’t do great on it. Styrofoam isn’t exactly rich in vitamins and minerals. But – especially when substituted with a little bran for nutrition – they are capable of digesting the plastic and breaking it all the way down into carbon dioxide.
The secret to the mealworm’s superpower is its microbiome. There are bacteria in its gut that, using unknown enzymatic pathways, are capable of breaking those polystyrene bonds. With their research, we can learn about a cool new possibility for plastic biodegradation.
A Study in Mealworms
The researchers used to methods for testing Styrofoam biodegradation: grow them on a Styrofoam block, or feed them powdered polystyrene. The Styrofoam block was measured throughout the experiment: all you needed to do was measure weight. A lighter block meant it was getting eaten by the worms.
The block of Styrofoam certainly dropped in weight over the course of the experiment. To confirm that the Styrofoam was, in fact, entering the mealworm digesting track by virtue of being a meal, they got chemical profiles of mealworm fecula. NMR spectroscopy can trace the carbon atoms inside of organic compounds; an NMR spectra of Styrofoam blocks was dominated by two major peaks, but the fecula spectra had reduced peaks at those two locations, in addition to many more peaks denoting many more components. This suggests that not only are the mealworms consuming the polystyrene – they appear to be breaking it down into other components.
The second test, with powder, had an isotopic trick: polystyrene powder was labeled with C-13. This isotope is very rare; only 1.1% of naturally occurring carbon is C-13. By feeding it to the mealworms at a higher concentration, they could track where that carbon was going. The split the mealworms into groups fed only bran, or bran mixed with powdered, isotopically labeled polystyrene. They then collected the carbon dioxide gas exhaled by the mealworms. Sure enough, the carbon dioxide produced by plastic-eating mealworms had a higher C-13 fraction than the control. The worms were digesting polystyrene, and mineralizing it all the way into carbon dioxide gas.
These discoveries were completely new when the paper was published. The next question is, how exactly can mealworms do this? It all depends on their gut microbiome.
And Now: Featuring Their Microbes
Microbiomes are found on surfaces everywhere. There’s a consortium of bacterial species unique to your hands, face, mouth, digestive tract. Other animals have them too, including mealworms. It appears that the gut microbiome of these beetle larvae are the secret superpower enabling Styrofoam biodegradation.
Now, if we hypothesize that this is the case – that gut microbes are doing the real work of breaking down Styrofoam – we can test it easily. Feed the mealworms an antibiotic, killing their gut bacteria. Were these sanitized mealworms able to digest Styrofoam? Not anymore.
That’s just a first step, however. The next step is actually collecting the gut microbes and running tests on them. The researchers were able to isolate various species from the mealworms. They grew cultures of bacteria directly onto polystyrene film and took pictures (using a microscope) to assess how well they were doing. In doing so, they found a strain of bacteria able to grow and live happily while fed nothing but that polystyrene film. They also grew that species up in a liquid culture with tiny Styrofoam pieces in it. Those bacteria did just fine too, and sixty days later, chemical measurements of the food source confirmed their activity. The Styrofoam had been degraded, and the liquid media contained by-products of biodegradation.
What this Means
Styrofoam is a major pollutant piling up in landfills. While the biodegradation here is slow, it is much faster than the natural degradation of polystyrene exposed to the elements.
Mealworms are also a big part of the animal feed industry. Raising mealworms on bran is more expensive than raising them on Styrofoam (or perhaps, on Styrofoam with bran on the side). Thus, farmers in this field can reduce costs and save the environment, killing two birds with one stone.
Yang, Y., Yang, J., Wu, W. M., Zhao, J., Song, Y., Gao, L., Yang, R., & Jiang, L. (2015a). Biodegradation and Mineralization of Polystyrene by Plastic-Eating Mealworms: Part 1. Chemical and Physical Characterization and Isotopic Tests. Environmental Science and Technology, 49(20), 12080–12086. https://doi.org/10.1021/acs.est.5b02661
Yang, Y., Yang, J., Wu, W. M., Zhao, J., Song, Y., Gao, L., Yang, R., & Jiang, L. (2015b). Biodegradation and Mineralization of Polystyrene by Plastic-Eating Mealworms: Part 2. Role of Gut Microorganisms. Environmental Science and Technology, 49(20), 12087–12093. https://doi.org/10.1021/acs.est.5b02663