A new study confirms that some bacteria love to eat plastic
A new study has revealed that plastic is consumed and digested by the bacterium Rhodococcus ruber. This was shown experimentally under laboratory conditions by research by Ph.D. candidate Maaike Goudriaan at the Royal Netherlands Institute for Sea Research (NIOZ).
Based on a model study with plastic in saltwater made in a lab, Goudriaan discovered that bacteria could turn about one percent of the plastic they ate each year into CO and other harmless compounds.
"But," Goudriaan emphasizes, "this is certainly not a solution to the plastic soup problem in our oceans. It is, however, another part of the answer to where all the 'missing plastic' in the oceans has gone."
To conduct the experiment, Goudriaan had a special plastic made with a specific type of carbon (C) in it for these trials. She saw that kind of carbon show up as CO above the water when she gave the plastic to bacteria after treating it with "sunlight" (a UV lamp) in a tank of saltwater that looked like the ocean.
The researcher says, "The treatment with UV light was necessary because we already know that sunlight partially breaks down plastic into bite-sized chunks for bacteria."
"This is the first time we have proven in this way that bacteria digest plastic into CO and other molecules," Goudriaan explains.
It was already known that the bacterium Rhodococcus ruber could form a so-called biofilm on plastic in nature. Plastic has also been measured to vanish beneath that biofilm. "But now we have demonstrated that the bacteria digest the plastic," she said.
According to Goudriaan's calculations of the total amount of plastic converted into carbon dioxide, the bacteria can break down around one percent of the available plastic annually. "That's probably an underestimate," she adds.
Rhodococcus ruber could digest more than one percent, but more research is needed
"We only measured the amount of carbon-13 in CO, so not in the other breakdown products of the plastic. There will certainly be C 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," she said.
Even though she is excited about bacteria that eat plastic, Goudriaan says 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 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 provide part of the explanation," she said.
She also said that more research needs to be done to determine if "wild" microorganisms could eat plastic in the same way in the real world.
Goudriaan 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, you hope to calculate how bacteria degrade much plastic in the oceans. But much better than cleaning up is prevention. And only we humans can do that," Goudriaan says.
Annalisa Delre, who works with Goudriaan, recently published research on how sunlight breaks down plastic on the ocean's surface. Microplastic floating in the water is broken down into chemicals bacteria can completely break down.
About two percent of the visible floating plastic could be removed from the ocean surface annually, according to calculations made by Ph.D. student Annalisa Delre and colleagues for the most recent issue of the Marine Pollution Bulletin.
"This may seem small, but year after year, this adds up. Our data show that sunlight could thus have degraded a substantial amount of all the floating plastic that has been littered into the oceans since the 1950s," says Delre.
You can read the study for yourself in the journal Marine Pollution Bulletin.
Methods that unambiguously prove microbial plastic degradation and allow for quantification of degradation rates are necessary to constrain the influence of microbial degradation on the marine plastic budget. We developed an assay based on stable isotope tracer techniques to determine microbial plastic mineralization rates in liquid medium on a lab scale. For the experiments, 13C-labeled polyethylene (13C-PE) particles (irradiated with UV-light to mimic exposure of floating plastic to sunlight) were incubated in liquid medium with Rhodococcus ruber as a model organism for proof of principle. The transfer of 13C from 13C-PE into the gaseous and dissolved CO2 pools translated to microbially mediated mineralization rates of up to 1.2 % yr−1 of the added PE. After incubation, we also found highly 13C-enriched membrane fatty acids of R. ruber including compounds involved in cellular stress responses. We demonstrated that isotope tracer techniques are a valuable tool to detect and quantify microbial plastic degradation.
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