Any floating object in the ocean tends to attract life; fishermen know this and deploy floating buoys to concentrate fish for harvesting. Plastic marine debris is no different and, at microscopic scales, microbes such as bacteria, algae and other single-celled organisms gather around and colonize plastic and other objects floating in water. Even small pieces of plastic marine debris the size of your pinky nail can act as microbe aggregating devices. We call this community of microbes growing as a thin layer of life (a biofilm) on the outside of plastic the “plastisphere,” analogous to the layer of life on the outside of planet Earth called the “biosphere." Using plastic samples collected during Sea Education Association student research cruises, we are studying what kinds of microbes live in the plastisphere, how they colonize the surfaces of plastic, and how they might affect marine ecosystems.
Scanning electron micrographs reveal a complex geography of microbial life on the cracked and pitted surfaces of plastic pieces that have been aging and weathering in the ocean. Tracy Mincer, a scientist at Woods Hole Oceanographic Institution studying this new community, refers to it as a “microbial reef” because it is a complete ecosystem with primary producers (like plants), grazers, predators, and decomposers, just like the community of larger organisms found on the complex surface of a coral reef. One of our most interesting discoveries is a type of cell that we call “pit formers,” spherical cells that appear to be embedded in the surface of the plastic pieces. These may somehow contribute to the breakdown of plastic marine debris, which would have implications for what happens to plastic in the ocean over the long term.
Linda Amaral-Zettler at the Marine Biological Lab used genetic techniques that allow us to look at the microbes' DNA to reveal surprisingly high biodiversity, with over 1,000 kinds of microbes on a single small piece of plastic only 5mm or less across. What's even more remarkable is that some of the organisms are not normally encountered in the open ocean, but are able to survive there by clinging to the plastic bits. The genetic work also turned up unexpectedly large numbers of the common marine bacterial genus Vibrio; most Vibrio are not harmful but some species can be associated with diseases in humans and animals. We are isolating and studying Vibrio cultures from marine plastic to see if any of them cause disease.
Because plastic persists for so long, microbes in the plastisphere can be transported long distances, making them a potential source of invasive species. If microbes are being moved around in the ocean from a variety of differing ecosystems, they could be impacting the native microbial populations and the larger organisms that depend on those microbes. The plastisphere could also modify plastic debris to make the plastic more, or less, harmful to marine ecosystems.
As a society we recognize that plastic marine debris is a problem but we don’t have answers to even simple questions: How much and what kind of plastic is in the ocean? Where does it come from and what happens to it? The microbes that make up the plastisphere may play a role in the fate and impact of plastic on marine ecosystems, and studying them may help answer some of the questions that remain.