The bacterium Rhodococcus ruber eats and actually digests plastic. This has been shown in laboratory experiments by PhD student Maaike Goudriaan of the Royal Netherlands Institute for Marine Research (NIOZ). Based on a model study with plastic in artificial seawater in the lab, Goudriaan estimates that bacteria can break down about one percent of the fed plastic annually into CO2 and other harmless substances. “But,” Gudrian emphasizes, “this is certainly not a solution to the problem of plastic soup in our oceans. However, this is another part of the answer to the question of where all the ‘missing plastic’ in the oceans has gone.”
Special plastic
Goudriaan had a special plastic made especially for these experiments with a different form of carbon (13C) in it. When she fed this plastic with bacteria after pretreatment with “sunlight”—a UV lamp—in a bottle of simulated seawater, she saw the special version of carbon appear as CO2 above the water. “Treatment with UV light was necessary because we already know that sunlight partially breaks down plastic into bite-sized pieces for bacteria,” explains the researcher.
Proof of principle
“This is the first time we’ve demonstrated in this way that bacteria actually break down plastic into CO2 and other molecules,” says Goudriaan. It was already known that the bacterium Rhodococcus ruber can form a so-called biofilm on plastic in nature. Plastic was also measured to disappear under this biofilm. “But now we’ve actually shown that the bacteria actually digest the plastic.”
I underestimate
When Gudrian calculates the total degradation of plastic to CO2, she estimates that bacteria can degrade about one percent of available plastic per year. “That’s probably an understatement,” 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 a few other molecules, but it’s hard to tell how much of that is broken down by the UV light and how much is taken up by the bacteria.”
No solution
Although marine microbiologist Goudriaan is very excited about the bacteria that feed on plastic, she stresses that microbial digestion is not the solution to the huge problem of all the plastic floating on and in our oceans. “These experiments are a major proof of principle. I see it as one piece of the puzzle of where all the plastic that disappears into the oceans ends up. If you try to track all our waste, a lot of plastic goes to waste. Digestion by bacteria could provide part of the explanation.”
From laboratory to mudflats
Further research needs to be done to determine whether “wild” bacteria also eat plastic “in the wild”. Goudriaan had already done some pilot experiments with real seawater and some sediment she had collected from the bottom of the Wadden Sea. “The first results of these experiments show that plastic even degrades in nature,” she says. “A new PhD student will have to continue this work. Ultimately, of course, you’re hoping to calculate how much plastic in the oceans is actually being degraded by bacteria. But much better than cleaning is prevention. And only we humans can do that,” Gudrian says.
Sunlight breaks down the plastic soup
Recently, Goudriaan’s colleague Annalisa Delre published a paper on sunlight breaking down plastics on the ocean’s surface. Floating microplastics break down into smaller and smaller, invisible nanoplastic particles that spread throughout the water column, but also into compounds that can then be completely broken down by bacteria. This is shown by experiments in the NIOZ laboratory on Texel.
In the latest issue of Marine Pollution Bulletin, PhD student Annalisa Delre and colleagues estimate that about two percent of visibly floating plastic may disappear from the ocean’s surface this way each year. “This may seem small, but year after year it increases. Our data show that sunlight could degrade a significant amount of all the floating plastic that has been thrown into the oceans since the 1950s,” says Delre.
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