A pair of coccoliths found living on marine plastic. Image: PLOS One
The world's oceans are turning into hellish plastic-filled soups, and organisms large, small, and even way down deep are finding themselves inundated with pollution. But in the game of life, opportunities are rarely squandered. As it turns out, ocean plastic is now home to a surprisingly wide variety of life.
In a survey of just 68 small (1.7–24.3 mm) pieces of floating ocean plastic harvested from across Australia's coasts, researchers at the University of Western Australia found a whopping 19 genera of microorganisms that had previously not been described as living on marine plastic. Combined with dozens of other genera previously described, it's becoming ever-clearer that ocean plastic has become its own ecosystem.
Of course, there's a natural question to address: Knowing the incredible scale of plastic pollution, is the finding that marine plastic can harbor a wide variety of life good or bad? The authors of the latest study, published in PLOS One, put it clearly: It's too early to know.
"The ecological ramifications of this phenomenon for marine organism dispersal, ocean productivity, and biotransfer of plastic-associated pollutants, remains to be elucidated," the team of nine authors, led by Julia Reisser, write.
The major concern with ocean plastic is that while plastic can break down into ever-smaller molecules, it doesn't really appear to ever disappear. It has been shown, however, that microbes can digest plastic waste, and the latest study lends new evidence.
Examples of ocean plastic studied by the team. Click to enlarge.
Using a scanning electron microscope, the team observed "a variety of plastic surface microtextures, including pits and grooves conforming to the shape of microorganisms, suggesting that biota may play an important role in plastic degradation."
The thing is, breaking down plastic further doesn't necessarily solve the major concern with ocean plastic: As plastic pervades the ocean, it's also saturating the food chain. In fact, as a 2011 Nature article points out, breaking down plastic to microscopic sizes could actually make things worse. As author Gwyneth Dickey Zalkab wrote:
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."
The range of organisms found in the latest study offers insight into how that works. Along with bacteria, "by far" the most abundant organisms were diatoms, the tiny phytoplankton that are a major part of the bedrock of the marine food chain. The Australian team found that diatoms grew well on floating plastic, which provided them with "a light and nutrient-filled environment that is stable and beneficial to these organisms."
But therein lies the rub: If diatoms find a happy home in plastic, organisms that eat diatoms—and so on up the food chain—are likely to find more plastic in their diet.
"Because of their rapid growth and production of extracellular substances, epiplastic diatoms may provide an important food source for invertebrate grazers," the authors write. "As plastic debris can contain harmful substances, it remains unclear if such grazer-plastic relationships would have a positive or negative impact on the populations involved in this new type of food web."
The new paper more than doubles the number of diatom genera known to inhabit marine plastic, and it's inevitable that further study will find more organisms that call millimeter-sized plastic bits home. (A full list of known genera, with newly discovered ones in bold, is available at PLOS One.)
The end result is pretty simple: The ocean is filled with plastic, especially small pieces, and all manner of life is taking advantage of the new niche. In many cases, it's a direct positive—floating plastic offers a lot of attractive qualities for photosynthetic creatures needed to stay near light.
But how those potential changes—a surge in phytoplankton growth, the spread of plastic-based toxins throughout the food chain, or the "rafting" of microorganisms across vast swathes of the ocean—will affect marine ecosystems as a whole has yet to be seen. The authors put it simply: "This phenomenon has considerable ecological ramifications and deserves further research."