Scientists find a diverse and distinct community of microorganisms that live on plastic trash at the surface of the North Atlantic Ocean. Is the “plastisphere” only a buzzword? Or can plastic waste be considered a new, man-made ecological habitat in the open ocean?
When you picture plastic pollution in our seas you may think of it as huge garbage patch or a mess of bags, duckies, and water bottles. What may not come to mind are the millions of tiny plastic pieces that are teeming with life.
Plastic that is thrown away can escape landfills and creep into our oceans. Plastic waste found in the ocean’s surface is called Plastic Marine Debris (PMD). PMD can wash up on the shores of beaches and also drift into the most remote parts of the ocean. In 60 days PMD can migrate 1000 km from the Eastern Seaboard to locations such as the interior of the North Atlantic Subtropical Gyre. Gyres are large wind-driven movements of water that occur at the ocean’s surface. These surface currents move in a circular pattern along the edges of major ocean basins. Since these surface currents push water and floating plastic debris around like a merry-go round, PMD can accumulate and persist for decades. Unless there is a change in the way we handle plastic waste, the export of PMD from land to sea is expected to increase as more people produce and use plastic.
The amount of PMD in the North Pacific Gyre has increased. In contrast, there has been no significant increase in the
amount of PMD in the North Atlantic Subtropical Gyre (NASG) since the 1980’s despite increased plastic use. Proposed explanations for this difference essentially say that a) the amount of PMD is underestimated or b) PMD is sinking from the surface the deeper ocean depths and this value is underestimated.
Microorganisms can either freely float in the ocean or latch onto a particle. This particle can be natural or man-made. PMD is peculiar because it can not only function as an artificial “microbial reef” but its surface makes it ideal for bacterial growth.
Erik Zettler and his colleagues at Woods Hole Oceanographic Institution hypothesized that the microbes on the plastic were specifically interacting with it and this growth was not a coincidence. They also hypothesized that a microbial community on plastic pieces would be different from a microbial community found in the ocean. How does one determine the difference between microbes?
If you want to describe the community of animals at a zoo your first instinct would be to group them by species. You also know that animals of different species have different DNA. DNA carries the information for making all of the cell’s proteins, and these proteins implement the functions of a living organism and determine the organism’s characteristics — the dimples in a child’s smile, the liver enzymes of the koala, and the long neck of a giraffe.
The quest for defining ‘different’ is more complicated for microbiologists. They use organized information obtained from DNA, called sequences, to group similar microorganisms. Two commonly used approaches are 1) to put sequences into bins based on their similarity to a reference sequence or 2) group them by their similarity to other sequences in the community. In either case, these bins or groups are called operational taxonomic units, or OTUs. This allows the microbiologist to set a threshold of similarity (2 sequences have to be 98% alike to be considered to ‘same’) or difference (2 sequences can be up to 2% different to be considered the ‘same’). The more picky a microbiologist is when determining similarities (higher % alike), the more bins hence ‘different’ microorganisms.
We use a similar sorting approach when we do our laundry. If you like to separate your light and dark clothes, you would do two loads of laundry. If you like to wash your red clothes separately, you would do three loads of laundry. The amount of clothes you have doesn’t change when you change how you sort your laundry; only your number of loads. When microbiologists change their similarity criteria the number of OTUs change; the number of microbes sequenced does not change.
Sample Collection and Analysis
Erik Zettler, his colleagues at Woods Hole Oceanographic Institution, and the crew on a Semester Education Association (SEA) vessel SSV Corwith Cramer collected plastic at the ocean’s surface using a very fine net in the North Atlantic Subtropical Gyre. In a clean environment and using sterile tools they processed and preserved the samples for DNA analysis and for the scanning electron microscope (SEM). For the analysis of the microbial community composition the sequences were assigned OTUs (96 % similarity). A microscope-based Raman spectrometer and a spectra of known standards were used to identify PMD as polyethylene (PE) or polypropylene (PP), which are both buoyant plastics that float on seawater.
The diameter of PMD ranged from nearly zero to several millimeters. All pieces of plastic were degraded to some degree; they were cracked. With the microscope-based Raman spectrometer, most fragments collected were positively identified as polyethylene and polypropylene.
The microscopic images show the diversity in the shape and sizes of bacteria that live on the plastic. The pits in Figure 1 are round like bacterial shapes, which suggest that the bacteria are actively breaking off hydrocarbon polymers using water molecules (plastics are made up of hydrocarbon polymers). Rows or patches with a similar formation as shown above frequently appeared on the surfaces of PMD, which may suggest active cell growth. Using the microscopic images and OTU data, Miller found evidence of microbial phototrophy, symbiosis, heterotrophy on both the PP and PE samples.
The community, described using OTUs
The Venn diagram highlights the number of OTUs found in seawater and the plastic (PP and PE) samples (Figure 3). Overlapping sections show OTUs that have been found in more than one environment, such as seawater and PP (shown as purple in Figure 3). The Venn diagram shows a large number of OTUs not shared between plastic and ocean (meaning that these OTUs were unique to the ocean or the plastic samples). Most bacteria in seawater samples were Pelagibacter and other free-floating bacteria but the abundance of these bacteria was vastly different from the plastic samples. The dominant genus found on the plastics were Vibrio. Some strains in this genus can cause disease, including cholera. They had an abundance of 24% on the plastic samples. Interestingly, this percent abundance value is higher than 1%, the typical percent abundance of Vibrio found in the ocean!
Zettler found that the bacterial community of seawater is more diverse than the community in plastic, but it’s mostly made up of a few ‘species’. The bacterial community in plastic appeared not to be dominated by a few bacteria; their community is more evenly distributed.
PMD: a selective environment?
Zettler found many bacteria that only live in plastic. Some OTUs (‘species’ of bacteria) found in plastic are closely ‘related’ to those able to degrade hydrocarbons: Phormidium sp. (a filamentous cyanobacterium), Psuedoalteromonas (a genus frequently associated with marine algae), and many more. Zettler has images of pits that may suggest the presence of bacteria that can degrade hydrocarbon; he could not say for sure if they lived in plastic.
Scientists have extensively studied and documented the negative effects plastic pollution can have on large marine animals such as birds, sea turtles, and mammals. The findings by Zettler and his colleagues show how PMD is affected by microbes. The SEM images of the plastic’s surface show a diverse and active microbial community and marks that are possibly made by hydrocarbon-degrading bacteria.
Microbiologists who study the distribution of microbes in the ocean frequently encounter this tenant in their field: everything is everywhere, and the environment selects. Zettler has shown that ‘everywhere’ includes plastic bits that drift in the sea… and not ‘everything’ grows on it.
Cat Turner is a Masters Candidate at the University of Rhode Island. Her research topic is on pH and dissolved inorganic carbon (DIC) fluctuations of Narragansett Bay, R.I. In her spare time she draws cartoons, reads horror stories, and collects wine corks. She likes to sail in fair weather.