Buoys of polystyrene (EPS), often used for the cultivation of oysters and muscles in open sea in Korea and other countries, slowly disintegrate under the influence of sunlight. However, a recent study showed that sunlight is not the only cause of the disintegration. Bristle worms (polychaetes) work their way into the buoy, eat the polystyrene and then excrete microplastics. This is an alarming find.
Microplastic particles, measuring less than five millimetres in size, have been accumulating in the oceans since the 1960s and are now the most abundant form of solid-waste pollution on earth.
On land, earthworms chow down on dead leaves and fungi and poop out tiny bits of organic matter that enrich the soil. In the sea, it turns out that some marine worms chew on floating plastic and poop out microplastics—a troubling discovery brought to light in new research by scientists in South Korea.
The process of industrial oyster farming, however, can pollute the oceans. According to one research study (Shim Wonjun et al, 2015), the marine debris pollution level of the Korea coast is very high compared to other countries. In this region specifically, microplastic (<2 mm) is present in almost all areas, occurring in over 90% of sampled ocean sites including farming seas.
The team also found that most of the detected microplastics were styrofoam particles and that microplastic-styrofoam was found inside oysters and mussels. This shows that the high microplastics density in this region directly relates to and affects the oyster farming itself. The total surface area of oyster farming in this region is about 3,466 ha, and the average number of styrofoam buoys per ha is about 1,800, leading to an estimate of six million styrofoam buoys in use. As the styrofoam and plastic rope used by these farms degrades or is lost in this densely farmed area, microplastics are accumulating in the ocean and adversely impacting on the marine ecosystem and food web (Mi jang, Shim Wonjoun et al, 2016). Finally, human health is also threatened by this pollution via consumption of shellfish containing microplastics.
Concerned about the effects of plastic pollution on marine ecosystems, scientists from the Korea Institute of Ocean Science and Technology collected eight discarded buoys that were adrift off the Korean coast. The plastic buoys were made out of expanded polystyrene, better known as styrofoam, and the researchers wanted to see if anything was living on them.
The Korean study, published in Marine Pollution Bulletin, found on average six to seven worms per buoy. And a single bristle worm can produce hundreds of thousands particles of microplastics in a single year.
Led by environmental chemist Sang Hee Hong, the team found all sorts of organisms on the surface of the buoys, from seaweed and sea squirts to crabs. But when they took the buoys apart, they were surprised to find marine worms living inside. Buried deep within the buoys, polychaetes were chowing down on plastic.
Tamara Galloway, an ecotoxicologist at the University of Exeter in the United Kingdom who has studied polychaetes but was not involved in the new research, says the finding is bad news for the worms and for the wider marine ecosystem. “Our own research has shown that sediments contaminated with plastic particles take longer to pass through the gut of polychaete worms than normal food, leading to a reduction in growth.”
She found that lugworms feeding in sediment that was highly contaminated with microplastics put on less weight than worms in clean sediment and had less energy to invest in key processes such as growth and reproduction. These effects could cause populations to decline.
She adds that since marine worms are an important food source for many fish and wading birds, this could have consequences across the food chain.
These polychaetes, which usually live in muddy sediment in shallow water, were using their strong teeth to crush and burrow into the plastic buoys. White styrofoam particles were scattered throughout their burrows and were clearly visible in their digestive tracts through their transparent bodies.
In the lab, Hee Hong and the other scientists put 10 of the worms in seawater-filled beakers and collected their feces. Over the next three days, the polychaetes passed a total of 1,306 pieces of styrofoam, ranging in size from 0.2 to 3.8 millimetres. One particularly prodigious worm pooped out 482 pieces of microplastic.
To get a better sense of how effective these worms are at rendering plastic into microplastic, the scientists placed an adult and a juvenile in separate containers with styrofoam blocks. Within a few days, the worms started to burrow. Over the next week, the adult pooped out 11,200 microplastic particles, while the juvenile produced 2,740—an average of 1,600 and 390 particles per day respectively.
Extrapolating from these results, a single marine worm can create hundreds of thousands of microplastic particles annually. A single buoy containing seven to eight polychaetes—the average number of worms the researchers found per buoy—would produce around a million particles every year.
Microplastics carry a complex mix of chemicals which have the potential to harm the worms, the research showed. Many plastics contain chemical additives, such as plasticisers, dyes and antimicrobials, which can leach out into sediments and seawater. Microplastics also concentrate water-borne chemicals on their surfaces, such as pesticides.
There has been much campaigning around the impact of larger marine plastic pollution, with widely documented instances of fish and bird entanglement, ingestion and suffocation. But particles of this microscopic size are available to a much broader range of marine organisms, which ingest and retain these tiny plastic particles and act as prey for larger species.
Marine biologist Richard Thompson, an expert on plastic pollution at the University of Plymouth in the United Kingdom, notes that while this research describes yet another effect of plastic pollution, it says nothing about a possible solution. He says society needs “to stem the flow of plastic to the environment and work towards a more circular economy that considers end of life at the design stage.” The only way to stop marine worms from eating and pooping out microplastic is to stop putting plastic in the ocean for them to eat.
Two years ago, another study discovered that the growth of the land-living earthworms is slowed down, and their live span is shortened, if they are exposed certain concentrations of microplastics. These organisms also spread microplastics by excreting them into the ground at greater depths.