Another way to recycle plastic

PLASTIC production has tripled over the past 25 years, and the mess it causes has risen commensurately. Recycling is one option. Another is biology, and with that in mind researchers have been hunting for creatures that can digest plastics. Several species of fungi and bacteria can do the job, but only slowly. Now Anja Brandon, a student at Stanford University, and her research supervisor, Craig Criddle, have found that bacteria in the guts of mealworms can break down polymers much more quickly.

Other researchers had already found that mealworms can digest a particular plastic called polystyrene. Ms Brandon and Dr Criddle wondered whether polystyrene was uniquely palatable, or whether the bacteria in the worms’ guts might be able to eat other sorts of plastic, too. To check, they turned to polyethylene, which is both more common than polystyrene and very different in chemical terms. If the worms found it nutritious as well, that would suggest their tastes might be usefully wide-ranging.

As they describe in Environmental Science & Technology, the researchers divided their worms into groups. Some were given 1.8 grams of either polyethylene or polystyrene. Some were given both. Others had their plastic meals supplemented with wheat bran. (Wheat bran had been found to increase the rate at which mealworms could digest polystyrene). A control group of worms was fed only bran.

More than 90% of the worms survived the 32-day experiment. Those fed only polyethylene found it very agreeable, polishing off 0.87 of their 1.8-gram helping. That was significantly more than the worms eating polystyrene, who managed just 0.57 grams of the stuff. Best of all were the worms that were given bran with their plastic. They chewed through 1.1 grams of polyethylene and 0.98 grams of polystyrene.

Nor were the insects merely chewing up the plastics and then passing them in their faeces. Instead, chemical reactions in their guts were converting them into carbon dioxide. The conversion rate was low at first, but by the end of the experiment the worms fed polyethylene were converting 50% of it into gas and those fed polystyrene were converting 45%.

Ms Brandon and Dr Criddle theorised that the bacterial ecosystems inside the insects’ guts were changing to fit their unusual diets. They dissected the worms at the end of the experiment and compared the gut fauna of those that had been eating plastics with the fauna found in the control group. They found big differences, with several types of bacteria being more common in the guts of mealworms that had been fed plastic.

The researchers argue that not only are mealworms probably capable of digesting a wide range of plastics, but that the protean nature of their gut bacteria should allow them to specialise in a particular sort relatively quickly. A small population of a thousand worms, they reckon, might manage to devour 0.32 grams of polyethylene or 0.28 grams of polystyrene in a day. That is still not lightning fast. But it is quicker than waiting for it to break down in a landfill.