Bacteria Can Use Plastic Waste as a Food Source, Which Isn't as Good as It Sounds

Plastic pollution is out of control. Each year, more than 8 million tonnes of synthetic polymers enter the ocean, and while some sink to the floor, return to the shore, or collect in the middle of nowhere, a significant portion isn't so easily accounted for.

All that missing plastic is a mystery, but some researchers suspect hungry microbes are partly responsible.

Experiments in the lab have shown that a species of marine bacterium known as Rhodococcus ruber can slowly break down and digest plastic made from polyethene (PE).

PE is the most commonly produced plastic in the world, used largely in packaging. While it's not clear if R. rubber munches on this waste 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 2006, initial research suggested the plastic underneath R. rubber was breaking down faster than normal.

The new study confirms that to be the case.

"This is the first time we have proven in this way that bacteria digest plastic into CO2 and other molecules," says microbial ecologist Maaike Goudriaan from the Royal Netherlands Institute for Sea Research (NIOZ).

To emulate the natural ways that plastic disintegrates 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.

Next, the team introduced a strain of R. ruber to the scene.

By measuring levels of carbon-13, an isotope of carbon released from disintegrating plastic, the authors estimated that the polymers in their experiments broke down at about 1.2 percent a year.

The team can't be sure how much the UV lamp decayed the plastic compared to the activity of the microbes, but the bacteria were playing a role. Bacterial samples after the experiment showed fatty acid membranes enriched with 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. Still, it does indicate where some of our planet's missing plastic might have gone.

"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 microbiologist Annalisa Delre.

Microbes could have then come in and digested some of the Sun's leftovers.

Since 2013, researchers have warned that microbes are likely thriving on plastic patches in the ocean, forming a synthetic ecosystem known as a 'plastisphere'.

Evidence suggests 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. 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 we unleash plastic-munching equivalents on pollution hotspots, like the Great Pacific Garbage Patch.

Others are not so sure that's a good idea. Engineered enzymes and bacteria that break down plastic might sound like a great way to make our waste disappear. Still, some experts are worried about unintended side effects on natural ecosystems and food webs.

After all, breaking down plastic isn't necessarily a good thing. Microplastics are much harder to clean up than larger pieces, and these tiny remnants could infiltrate food webs. Filter feeders, for instance, might mistakenly grab small pieces of plastic before microbes do.

In a 2020 study, every seafood sample tested at a market in Australia contained microplastics.

What that is doing to human or animal health is entirely unknown.

"Much better than cleaning up is prevention," argues Goudriaan.

"And only we humans can do that."

The study was published in the Marine Pollution Bulletin.