The US discards well over 33 million tons of polymer waste annually, but less than 10 percent of that plastic is recycled. If only it were all biodegradable it would be less of a problem, which is where the work of Wei-Min Wu of Stanford University comes in. He and his colleagues have discovered that microorganisms in the gut of the mealworm can biodegrade one of the most intractable and unrecyclable plastics - polystyrene and its chemical cousins. Wu suggests that this discovery has "opened a new door to solve the global plastic pollution problem." [Wu et al, 2015, Environ Sci Technol; DOI: 10.1021/acs.est.5b02661 and 10.1021/acs.est.5b02663]
This is the first detailed evidence of bacterial degradation of plastic in an animal's gut and could open up an entirely novel way to manage plastic waste. The by-products of the digestion process, other than carbon dioxide, perhaps even offering a new source of soil filler for agriculture or perhaps even as a semi-sustainable biofuels. The team found that 100 mealworms, the larvae of the mealworm beetle, Tenebrio molitor, could munch their way through up to 40 milligrams of expanded polystyrene each day. The mealworms excreted the bulk of the digested plastic as fecal pellets containing biodegraded fragments from the polymer, about half of the carbon from the source was respired as carbon dioxide just as it would be if the larvae were eating their natural diet. The researchers said that that mealworms fed a steady diet of expanded polystyrene were as healthy as controls eating a normal diet.
In their earlier work, Wu and colleagues had demonstrated that waxworms, the larvae of Indian mealmoths, Plodia interpunctella, could biodegrade polyethylene in their guts, but no one suspected that a microbe would exist to degrade polystyrene. The scientists at Stanford are also collaborating with Jun Yang of Beihang University in China and others to find microorganisms that might biodegrade plastics such as polypropylene and other materials. The research will also look at how the ingestion and digestion of such materials worms its way up the food chain through the animals that eat such insects and those that dine on them.
In parallel work, researchers will investigate what conditions are more favorable to plastic biodegradation and the specific enzymes present in the microbes capable of breaking down polymers. This could lead to a more powerful approach in which the microbes or even just their enzymes are fermented to break down waste plastics into useful materials as potential feedstock for chemical manufacturing, for instance. Conversely, such understanding might simply lead to better biodegradable plastics that don't accumulate in the environment or on unkempt city streets.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".