Marine microbes digest plastic

Posted March 28th in Nature News by Gwyneth Dickey Zaikab

A 'little world' eating ocean garbage might be a mixed blessing.

Electron microscopy reveals the inhabitants of a plastic bag fished from the Sargasso Sea.T. Mincer/G. Proskurowski

Specialist bacteria seem to be eating the plastic garbage we throw into the ocean. But whether they're cleaning up our poisons or just passing them back up the food chain remains to be seen.

The ocean contains vast amounts of plastic, mostly as tiny shards floating just beneath the surface. Under an electron microscope, each scrap of "plastic confetti" becomes "an oasis, a reef of biological activity," says marine microbiologist Tracy Mincer of the Woods Hole Oceanographic Institution in Massachusetts.
Mincer and his colleagues examined bits of fishing line, a plastic bag and a plastic nurdle (a pre-production plastic pellet) fished out of the Sargasso Sea, an area of the North Atlantic where currents cause debris to accumulate. The region as a whole contains more than 1,100 tonnes of plastic¹.

Scanning electron microscopy revealed bacteria-like cells living in pits in the plastic, as if they were eating the surface away.

"They look like you took a hot barbecue briquette and threw it into snow," says Mincer. "You see this melting bit all around the outside of the cells, and they're just burrowing into the plastic."

Microbes have been found digesting plastic in landfills, he says, but this is the first evidence of marine bacteria breaking down plastic in the ocean. The work was presented the 5th International Marine Debris Conference in Honolulu, Hawaii, on 24 and 25 March.

Disappearing debris

Plastic-eating bacteria might help explain why the amount of debris in the ocean has levelled off, despite continued pollution. But researchers don't yet know whether the digestion produces harmless by-products, or whether it might introduce toxins into the food chain.

"To understand if it's a good thing or not, we have to understand the entire system," says Mincer.

Plastics contain toxins such as phthalates, and also absorb additional toxic chemicals such as persistent organic pollutants from the ocean, says Mark Browne, an ecologist at University College Dublin in Ireland, who was not involved with the project. Those chemicals could leach out into the microscopic animals that eat the bacteria, or broken down microscopic plastic particles could enter cells and release their chemicals there, he says.

"Whether or not that material then passes up the food chain is something of critical importance," he says. "It's yet another mechanism for the particles of plastic that we throw away to potentially come back to haunt us."

Plastic lovers

Genetic analysis shows that the bacteria on the plastic differ from those in the surrounding seawater or on nearby seaweed, says microbiologist Linda Amaral-Zettler of the Marine Biological Laboratory, Woods Hole. So far, the DNA sequences obtained by her lab show that almost 25% of the bacteria on one polyethylene surface were vibrios, bacteria from the same group as the cholera bacterium.

"That was a surprise, because normally in sea water they would be present in much lower concentrations," says Amaral-Zettler, who adds that she can't yet tell if these strains are pathogenic. Wind and ocean currents carry plastic all over the world, so no part of the sea will escape the effects of this bacterial activity.
Amaral-Zettler and Mincer also found genetic and microscope evidence of eukaryotes — organisms with more complicated cells than bacteria — on the plastic. What she calls the "plastisphere" might contain complex living communities. "It may be a little world that we've created, for better or worse."
The Wood's Hole scientists aim to sample more ocean plastic and to isolate, culture and identify the microbes found on it. Then they can determine if and how they're digesting the plastic and discover what the by-products are.