Plastic pollution poses a planetary boundary threat. This planetary-scale exposure to novel entities and chemical pollution is not readily reversible and threatens to disrupt vital Earth system processes. In addition to mechanical and chemical recycling, biodegradation is widely considered a promising strategy for disposing of plastic residues.
Now, a new study has revealed that the bacterium Rhodococcus ruber eats and actually digests plastic. This has been shown in laboratory experiments by Ph.D. student Maaike Goudriaan at Royal Netherlands Institute for Sea Research (NIOZ).
Based on a model study with plastic in artificial seawater in the lab, Goudriaan discovered that bacteria could break down about one percent of the plastic they ate each year into CO2 and other harmless substances.
“But this is certainly not a solution to the problem of the plastic soup in our oceans. It is, however, another part of the answer to the question of where all the ‘missing plastic’ in the oceans has gone,” Goudriaan emphasizes.
For these experiments, Goudriaan had a special plastic made with a distinct form of carbon (13C) in it. When she fed that plastic to bacteria after treating it with “sunlight” (a UV lamp) in a tank of saltwater, she saw that special version of carbon appear as CO2 above the water. “The treatment with UV light was necessary because we already know that sunlight partially breaks down plastic into bite-sized chunks for bacteria,” the researcher explains.
“This is the first time we have proven in this way that bacteria actually digest plastic into CO2 and other molecules,” Goudriaan states. It was already known that the bacterium Rhodococcus ruber could form a so-called biofilm on plastic in nature. It has also been measured that plastic disappears under that biofilm. “But now we have really demonstrated that the bacteria actually digest the plastic.”
According to Goudriaan’s calculations of the total breakdown of plastic into CO2, the bacteria can break down about one percent of the available plastic per year. “That’s probably an underestimate,” she adds. “We only measured the amount of carbon-13 in CO2, so not in the other breakdown products of the plastic. There will certainly be 13C 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.”
Marine microbiologist Goudriaan is very excited about the plastic-eating bacteria; however, she stresses that microbial digestion is not a solution to the huge problem of all the plastic floating on and in our oceans.
“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. If you try to trace all our waste, a lot of plastic is lost. Digestion by bacteria could possibly provide part of the explanation.”
More research needs to be done to discover whether ‘wild’ bacteria also eat plastic ‘in the wild,’ Goudriaan said. She already did some pilot experiments with real sea water and some sediment that she had collected from the Wadden Sea floor.
“The first results of these experiments hint at plastic being degraded, even in nature,” she says. “A new Ph.D. student will have to continue that work. Ultimately, of course, you hope to calculate how much plastic in the oceans really is degraded by bacteria. But much better than cleaning up is prevention. And only we humans can do that.”
- Maaike Goudriaan, Victor Hernando Morales, Marcel T.J.van der Meer, Anchelique Mets, Rachel T. Ndhlovu, Johanvan Heerwaarden, Sina Simon, Verena B. Heuer, Kai-Uwe Hinrichs, Helge Niemann. A stable isotope assay with 13C-labeled polyethylene to investigate plastic mineralization mediated by Rhodococcus ruber. Marine Pollution Bulletin, 2023; DOI: 10.1016/j.marpolbul.2022.114369