The mystery of where plastic goes after it is dumped in the ocean has long puzzled scientists.
At least 14 million tons find its way into marine environments each year, yet only about one per cent is ever detected in sampling surveys.
Now scientists believe they have solved at least part of the riddle. Bacteria are eating it.
A new study by the Royal Netherlands Institute for Sea Research (NIOZ) has proven that the widespread bug bacterium Rhodococcus ruber digests plastic, turning it into carbon dioxide and other harmless substances.
“This is the first time we have proven in this way that bacteria actually digest plastic into CO2 and other molecules,” said Maaike Goudriaan, a doctoral student of NIOZ.
“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.”
Bacteria could potentially help out more
For the research, the team carried out laboratory experiments, feeding plastic to the bacteria in seawater after it had been treated with UV light to mimic sunlight.
Sunlight is known to break down plastic into tiny chunks which are easier for bacteria to absorb.
The team estimates that the Rhodococcus ruber bacteria alone can break down at least one per cent of available plastic per year.
Researchers said it could technically be possible to use the bacteria to clean up more plastic in the ocean, but warned it would require growing “stupendous amounts”.
Such a scheme could also end up producing alarming amounts of carbon dioxide, which would be damaging for the planet.
Previous studies have suggested that large amounts of plastic in oceans and seas fall below the surface, where it is difficult to detect.
A 2017 paper from Utrecht University in the Netherlands estimated that around 196 million tons of plastic may have settled into the deep ocean since 1950.
A deep dive in 2019 even found a plastic bag inside the Mariana Trench, at a depth of 36,000ft (10,975m).
But the new research shows a significant amount may be being digested by widespread bacteria.
Rhodococcus ruber is found across the globe, and is abundant in soil, water and marine environments. The species was chosen for testing because it is known to transform a number of harmful pollutants, including industrial chemicals and pesticides, into harmless molecules.
After proving it in the lab, the team now wants to find out whether wild bacteria also eat plastic and have started pilot experiments with sediment collected from the Wadden Sea floor.
Sunlight may also be playing a role
“The first results of these experiments hint at plastic being degraded, even in nature,” added Miss Goudriaan.
“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. 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.”
The team also believes that sunlight is playing a major part in breaking down microplastics in the ocean.
Researchers estimate that about two per cent of visibly floating plastic may disappear from the ocean surface in this way each year.
“This may seem small, but year after year, this adds up,” said Annalisa Delre, another doctoral student at NIOZ.
“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.
The research was published in the Marine Pollution Bulletin.