Plastic pollution has gotten out of hand. More than 8 million tons of synthetic polymers end up in the ocean each year, and while some sink to the bottom, return to shore, or collect in the middle of nowhere, a significant portion is not so easy to account for.
All that missing plastic is a mystery, but some researchers suspect hungry microbes are partly responsible.
Experiments in the laboratory have now shown that a type of marine bacteria known as Rhodococcus rubercan slowly break down and decompose plastic made from polyethylene (PE).
Largely used in packaging, PE is the most widely produced plastic in the world, and although it is not clear if R. ruber chews on this detritus in the wild, the new research confirms it is at least capable of doing so.
Previous studies have found strains of R. ruber floating in dense cellular films on marine plastic. In addition, an initial study in 2006 suggested the plastic underneath R. ruber broke faster than usual.
The new study confirms that this is the case.
“This is the first time that we have proven in this way that bacteria actually digest plastic into CO2 and other molecules,” says microbial ecologist Maaike Goudriaan of the Royal Netherlands Institute for Sea Research (NIOZ).
To mimic the natural ways plastic decomposes on the ocean surface, Goudriaan and her colleagues exposed their plastic samples to UV light and placed them in artificial seawater.
“The treatment with UV light was necessary because we already know that sunlight partially breaks down plastic into bite-sized chunks for bacteria,” explains Goudrian.
Then the team introduced a strain of R. ruber to the scene.
By measuring levels of an isotope of carbon released from decomposing plastic called carbon-13, the authors estimated that the polymers in their experiments broke down at a rate of about 1.2 percent per year.
The team isn’t sure how much the UV lamp did to the plastic compared to the activity of the microbes, but the bacteria clearly played a role. Post-experiment bacterial samples showed fatty acid membranes enriched in carbon-13.
The rate of plastic decay identified in the current study is far too slow to completely solve the problem of plastic pollution in our oceans, but it does indicate where some of our planet’s missing plastic may have gone.
“Our data shows that sunlight may thus have broken down a significant portion of all floating plastic that has entered the oceans since the 1950s,” says microbiologist Annalisa Delre.
Microbes may then have come in and consumed some of the sun’s remnants.
Since 2013, researchers have been warning that microbes likely thrive in plastic patches in the ocean, forming a synthetic ecosystem that has come to be known as a “plastisphere.”
There is even evidence that some of these microbial communities are adapting to eating different types of plastic.
Previous studies have identified specific bacteria and fungi, on land and in the sea, that appear to eat plastic. But while that knowledge could help us better recycle our waste before it ends up in the wild, its other uses are controversial.
Some scientists have proposed that we release plastic-chewing equivalents at pollution hotspots like the Great Pacific Garbage Patch.
Others aren’t so sure that’s a good idea. Engineered enzymes and bacteria that break down plastic may sound like a great way to make our waste disappear, but some experts worry about unintended side effects to natural ecosystems and food webs.
After all, breaking down plastic is not necessarily a good thing. Microplastics are much harder to clean up than larger pieces, and these tiny remnants can enter food webs. For example, filter feeders can accidentally grab small pieces of plastic before microbes do.
In a 2020 study, every seafood sample tested in an Australian market contained microplastics.
What that does to human or animal health is completely unknown.
“Much better than cleaning up is prevention,” argues Goudriaan.
“And only we humans can do that.”
The study is published in Marine Pollution Bulletin.