Plastics on the Half Shell

Abstract

People who often eat oysters, mussels, or other bivalves could also be swallowing thousands of plastic particles each year. Recent studies by Belgian scientists offer the first direct evidence that tiny plastic fragments are entering the human diet through cultured and wild seafood. As filter feeders, bivalves draw in an estuary’s rich organic soup of water and plankton through their open shells and gills. They also draw in tiny plastic fragments, many of which can get trapped in the animals’ guts, tissues, and circulatory systems, according to a study in the October 2014 issue of Environmental Pollution by environmental toxicologists Lisbeth Van Cauwenberghe and Colin R. Janssen, of Ghent University, in Belgium. The authors reported on the numbers of very small plastic particles (less than 25 microns in diameter) found in two commonly farmed and commercially important species: the mussel (Mytilus edulis) and the oyster (Crassostrea gigas). Previous studies have exposed animals to microplastics (less than 5 millimeters in diameter) at very high concentrations in laboratory settings to test the physical and chemical impacts on the animals. The 2014 study, however, was the first to look inside organisms for microplastics that animals had ingested in the sea. The researchers faced technical challenges in extracting microscopic plastic bits from fleshy bivalves. “It’s difficult to figure out a method to extract plastic particles from the tissue,” says Cauwenberghe, “because the particles are so small and there’s a lot of tissue.” The researchers acquired farmraised mussels reared in the North Sea of Germany. Oysters, sealed in an airtight package, were acquired from a supermarket and reared in the Atlantic waters of Brittany, France. They put half of the oysters in filtered seawater for three days to clear their guts. The rest of the oysters were immediately prepared for analysis. The mussels were similarly prepared. The researchers then removed each animal from its shell and weighed its soft wet tissues. Next, the tissues were digested in an overnight solution of 69 percent nitric acid, followed by two hours of boiling and then by dilution with filtered water. The remaining material was filtered, dried, and examined for microplastics. The plastic pieces were counted and characterized for size. One surprising result was that gut clearances over three days did not remove the majority of plastic items in either animal. The average microplastic load was 0.36 particles per gram of soft tissue in the mussels prepared for analysis on arrival at the lab, whereas an average of 0.24 particles remained in the gut-cleared mussels. A similar trend was observed in the oysters. So where did plastic particles become stuck in the organisms? Plastic particles in the size range of 10–20 microns are small enough to slip through a bivalve’s gut wall into its circulatory system or tissues and become lodged there. Larger particles could have become lodged in an animal’s digestive system, says Cauwenberghe. In an average portion of cultured oysters, a diner would consume about 50 particles. In an average portion of cultured mussels, a person would consume 90 plastic particles. For a heavy consumer of bivalves, this might amount to dietary exposure of 11,000 microplastics per year. It is not only cultured bivalves that are accumulating microplastics. In an April 2015 study in Environmental Pollution, Cauwenberghe and her colleagues found an average of 0.2 microplastics per gram of soft tissue after gut clearance in wild M. edulis harvested at six sites along the French–Belgian–Dutch shoreline. The wild mussels, then, had slightly lower concentrations than cultured ones had in the 2014 study, but their different locations made comparisons difficult. Wild mussels are intertidal—meaning they are out of the water during part of each day— whereas cultured mussels are subtidal and exposed longer to microplastics in the water column. Now, teams of scientists are working on new technologies that could help identify microplastics lodged in marine animals without the time and expense of laboratory studies, according to Mark Anthony Browne, senior research associate at the University of New South Wales, in Australia. “A lot of people are working quite hard to develop techniques that would allow us for the first time to scan tissues and find bits of plastic accumulating in the protein and the things that we eat every day,” he says. Browne is part of an Australian team developing vibrational spectroscopy techniques that could be used in the field to take samples and find the unique fingerprints of various plastic chemical bonds: “We could take a blood or tissue sample of a killer whale, a mussel, fish, or a human, and then we could see the plastic inside.”