ZETTLER ET AL, 2013. Many whales, albatrosses, and turtles have been found with plastic junk in their stomachs, while others have become entangled in packaging. These large animals have come to symbolize the growing problem of oceanic pollution. But a number of new studies suggest that floating plastics are also providing a new habitat—the “plastisphere”—for smaller marine residents, from insects to microbes.
Plastics have become the most common type of debris in the seas. Contrary to popular depictions of floating garbage islands, most of this junk consists of tiny fragments no bigger than a fingernail. Although they are small, these pieces can become trapped in large concentrations within circular ocean currents called gyres. Parts of the North Atlantic Gyre, for example, hold more than 50,000 plastic pieces per square kilometre.
“The open ocean is low in nutrients and often likened to a desert. Now, we have these microbial reefs—pieces of confetti-like plastic that are their own ecosystem,” said Linda Amaral-Zettler from the Marine Biological Laboratory in Woods Hole. “We’re concerned about how these pieces are changing the nature of this large body of water.”
Along with Erik Zettler from the Sea Education Association and Tracy Mincer from the Woods Hole Oceanographic Institution, Amaral-Zettler has been trawling the North Atlantic gyre with nets for 25 years, to collect samples of this debris. Focusing on microbes, they found that these fragments harbor a very different community of bacteria than the surrounding water. “There’s evidence for predation, symbiosis... everything you see in a normal ecosystem but shrunk down to this tiny sphere of life,” she said.
Earlier studies have shown that microbe communities can vary considerably between different parts of the ocean, depending on environmental factors like temperature and salinity. Nevertheless, Amaral-Zettler said, “it was still really striking how different the community we found in plastic was.” For example, the genus Vibrio accounted for a quarter of the microbes on the plastics, but less than 1 percent of seawater communities.
“If there’s anything floating in the ocean, it attracts life,” said Zettler. “Plastics are just another substrate.” However, compared to other types of debris, plastics are longer-lasting and can carry microbes over much longer distances.
For now, it’s unclear what impact these microbes would have on the ocean ecosystems as a whole. Many Vibrio species infect fish while others cause seafood-related food poisoning in humans; their dominance in the plastisphere could spell health problems for us and other animals.
Some of the microbes also seem to form little pits on the plastic fragments, so it is possible that they could be degrading the plastic. “If they’re completely breaking it down, that would be good news,” said Amaral-Zettler. “But if not, the plastic is becoming even smaller and available to an even larger range of organisms. We’re still very early in terms of our understanding of what the microbes are doing.”
“This opens the door to really interesting and disturbing research directions, such as how these microbes use nutrients and what the effects may be on the surrounding plankton,” said marine biologist Miriam Goldstein, formerly of the Scripps Institution of Oceanography and now assigned to the US House of Representatives Natural Resources Committee. The big concern, she said, is that “plastic may be altering the way nutrients and energy move through the ocean, and encouraging weedy, low-diversity organisms to grow instead of the diverse, open-ocean-adapted organisms that would naturally be there.”
Last year, Goldstein showed that plastic debris could also provide new habitats for insects. Sea skaters—ocean-going relatives of the water striders that skate over ponds and lakes—can only lay their eggs upon hard surfaces, and plastics provide these in abundance. By checking water samples that had been collected from the North Pacific Gyre in the 1970s and 2000s, Goldstein’s team showed that as plastic has accumulated in these waters, sea skater eggs have become more common.
As with the microbes, it’s not clear what broader effects these skater eggs might have. The insects could provide more food for fish, crabs, and other predators, but they could also devour plankton and fish eggs. These studies are only scratching the surface of the ecological changes wrought by the plastics.
Such research is challenging, said Amaral-Zettler, because “every piece of plastic can have a different source and age.” Her team is trying to deal with this problem by mooring fresh pieces of plastic in coastal environments in Caribbean and the north Atlantic, and watching how the communities upon them change over time.