Another bacteria that digests plastic could be the key to solving an old mystery

Researchers have identified another microbe that can digest plastic – and probably is already breaking down some of the plastic in our oceans.

According to a study published in Marine Pollution Bulletin, the plastic-munching bacteria Rhodococcus ruber could be responsible for removing roughly 1% of the plastic in our oceans per year, digesting it into CO₂.

“This is certainly not a solution to the problem of the plastic soup in our oceans,” says lead author Maaike Goudriaan, a PhD student at the Royal Netherlands Institute for Sea Research (NIOZ).

“It is, however, another part of the answer to the question of where all the ‘missing plastic’ in the oceans has gone.”

Rhodococcus ruber, which grows in films on plastic, isn’t the first bacterium known to digest it: there have been several other discoveries in the past decade of microbes that can break down different types of plastics.

Read more: Nature fights back with plastic-eating bacteria

But Goudriaan and colleagues have used a novel method to show what Rhodococcus ruber is up to.

They did a lab-based experiment with the bacteria, feeding it polyethylene they’d tagged with special isotopes: slightly heavier carbon atoms, called carbon-13.

Before throwing it to the hungry bacteria, the researchers treated the plastic with UV light.

“The treatment with UV light was necessary because we already know that sunlight partially breaks down plastic into bite-sized chunks for bacteria,” says Goudriaan.

Then, after marinating the plastic with bacteria and seawater, the researchers were able to spot the tagged carbon-13 atoms appear above the water in carbon dioxide molecules.

This means that Rhodococcus ruber must have been digesting the polyethylene and converting it into energy and CO₂.

Goudriaan could also figure out the total amount of plastic the bacteria was converting with this trick – coming up with 1.2% of the plastic in our oceans per year.

“That’s probably an underestimate,” she says.

“We only measured the amount of carbon-13 in CO₂, so not in the other breakdown products of the plastic.

“There will certainly be carbon-13 in several other molecules, but it’s hard to say what part of that was broken down by the UV light and what part was digested by the bacteria.”

Unfortunately, this bacterium isn’t going to be the solution to our plastic pollution problems. Only a massive reduction in plastic making will achieve that.

“These experiments are mainly a proof of principle. I see it as one piece of the jigsaw, in the issue of where all the plastic that disappears into the oceans stays,” says Goudriaan.

“If you try to trace all our waste, a lot of plastic is lost. Digestion by bacteria could possibly provide part of the explanation.”