Welcome to the plastisphere


We are a one-time-use kind of world. I often see labmates grab a plastic plate in our lab’s kitchen rather than a “real” plate because, well, who has time to wash a dish? In our “grab and go” state of mind we legitimize our rampant use of plastic with the palliative thought that we’ll recycle these items and so, it’s like we never even used them, right?

The worldwide use of plastic is increasing, and today about 78 pounds of plastic are produced per person per year to meet an insatiable demand for some of the most annoying things on the planet, like individually wrapped bananas, clam-shell packaging, and the very worst of them all, microbead-containing cosmetics (more on these below). While some of these plastic items make it to the recycling plant –perhaps more aptly named the down-cycling plant when it comes to plastics– there is a percentage of all plastics that simply get washed away from landfills or beaches and into the wide-open sea. Due to the sheer amount of it and its impressive longevity, plastic is now the dominant debris in the ocean. This debris largely accumulates in five gyres, or areas of circulating water pushed together by larger ocean currents. The Great Pacific Garbage Patch, the gyre off of the coast of the northwest US, is the size of Texas. A study from late 2014 makes an estimate from extensive sampling and modeling that there are 5.25 trillion plastic particles that cumulatively weigh about 270 thousand tons floating in the ocean. That’s a lot of plastic.

Various ideas have been put forth to deal with this garbage. One plan, which may soon be deployed off the coast of Japan, involves the use of giant net to gather the debris floating on the water, followed by centrifugation of this water to pellet the plastics, and finally disposal and recycling of this material back on land. However, there’s one major catch (ha!) that this proposal and others like it fail to address: 92% of all marine plastic trash is less than 5 mm in size, which is way smaller than the plastic trash we toss away. These so-called microplastics come from various sources, including the tiny pellets used as raw materials in plastic manufacturing as well as physical fragmentation of larger pieces of trash. Oh, and the majorly-evil microbeads in exfoliating facewash or sparkly toothpaste or a whole cadre of gimmicky products (see this list here), which aren’t caught in sewage systems and are released into the environment to float away and persist forever. Bye-bye, see you in a million years.

The most interesting consequence (at least in the eyes of yours truly at Chemical Intuition) is the ecosystem that has sprung up in and around the ocean’s microplastics. This ecosystem has been coined the plastisphere, and we like that name a lot.

 So, what’s shakin’ in the plastisphere?

First, there is the obvious consumption of microplastics by animals. Birds ingest plastics both accidentally and because some plastic items look like their food. Smaller organisms like zooplankton passively ingest microplastics from filtering seawater (see the image below). While plastics are essentially inert, and plastic ingestion may not be so harmful in and of itself, plastic is a chemical magnet for toxins. Many pollutants, like polychlorinated biphenyls (PCBs), are hydrophobic. Plastic is also hydrophobic and so these chemicals stick to plastic much more readily than they are soluble in water. A recent report found that plastics concentrate PCBs from the surrounding seawater by up to six orders of magnitude. This means that organisms ingesting microplastics may have much higher exposures to toxins than we might think given those compounds’ concentrations in the seawater. This higher exposure could lead to various deleterious consequences for the organisms, like endocrine disruption or other sublethal effects, which can disrupt ecosystem functioning.

Images from a recent study examining ingestion of microplastics by zooplankton. The plastic pieces are visualized using fluorescence microscropy in iii) a bivalve larvae and iv) a Brachyuran larvae. Reprinted with permission from Cole, et al. Environ. Sci. Technol., 2013, 47 (12), pp 6646–66. Copyright (2013) American Chemical Society

Images from a recent study examining ingestion of microplastics by zooplankton. The plastic pieces are visualized using fluorescence microscropy in iii) a bivalve larvae and iv) a Brachyuran larvae. Reprinted with permission from Cole, et al. Environ. Sci. Technol., 2013, 47 (12), pp 6646–66. Copyright (2013) American Chemical Society

The second big thing happening in the plastisphere is the microbial ecosystem supported by plastics. Microplastic is an excellent substrate for microbial growth: its high surface area and hydrophobicity promote the formation of bacterial biofilms. A paper from 2013 from labs at the Woods Hole Oceanographic institute (WHOI) focuses on the bacterial communities present on marine microplastic. The authors hypothesized that due to the distinct physical and chemical environment presented by the microplastics, that these debris could harbor a microbial community distinct from that in the surrounding ocean water. They examined pieces of polyethylene and polypropylene plastic from several areas in the North Atlantic Subtropical Gyre. Using DNA sequencing and scanning electron-microscopy (SEM) techniques, they compared the bacterial communities present on the collected plastic bits to the surrounding seawater. From these genetic and phenotypic data, the authors could begin to construct the ecological relationships and roles of the plastisphere microbial community, which included both microbe-microbe and microbe-plastic interactions. They discovered that the microbes on both types of plastic were distinct from those in the water; interestingly, the plastic widely differed from each other, too. The SEM images in the paper show various eukaryotic microbes, like diatoms and ciliates, coexisting with bacteria. The image below shows an ectosymbiotic relationship, in which the symbiont lives on the surface of its host. Here, rod-shaped bacteria completely cover a protozoan that attaches itself to surfaces, i.e. a suctorian (the unofficial name of the lowest ranked student at graduation). 

This scanning electron microscopy image from the  2013 study out of WHOI shows a stalked predatory suctorian ciliate covered with symbiotic bacteria (magnified in the inset). Diatoms, bacteria and other microbes are seen in the background of this image. Reprinted with permission from Zettler, et al. Environ. Sci. Technol., 2013, 47 (13), pp 7137–7146. Copyright (2013) American Chemical Society

This scanning electron microscopy image from the  2013 study out of WHOI shows a stalked predatory suctorian ciliate covered with symbiotic bacteria (magnified in the inset). Diatoms, bacteria and other microbes are seen in the background of this image. Reprinted with permission from Zettler, et al. Environ. Sci. Technol., 2013, 47 (13), pp 7137–7146. Copyright (2013) American Chemical Society

When I see these images of microplastics teeming with microbes all I can think of is that amazing moment in Jurassic Park when Jeff Goldblum's character half-mumbles the prophetic words, "Life, uh, finds a way." Even though we may dislike the fact that our oceans are polluted, our trash is now the happy home of microbial communities. If we dismiss these communities as uninteresting because they're found on unnatural substrates, we may miss opportunities to understand how human activity impacts other organisms and ecosystems. For instance, can we find microbes on these plastics that could serve as effective bioremediation agents? Do certain types of plastic degrade faster than others due to microbial catabolism? Are the chemicals found in plastics mediating any ecological interactions happening on the plastic? Do marine microplastics harbor any novel symbioses unique to that synthetic surface? 

Finally, we tend to prioritize the conservation and study of ecosystems that we perceive as pure, or "natural."  What does "natural" mean on our tiny planet that hosts more than 7 billion people? Can we truly find any ecosystem free from some mark of humankind? If we effectively remove the trillions of pieces of microplastics from the ocean in a dramatic gesture to clean up our trash, what remaining ecosystems will be affected by the sudden loss of the plastisphere? In a thousand years, will scientists regard the ecology of the plastisphere as compelling an ecosystem to conserve as, say, that which is built around the wolves of Yellowstone?  We're all for avoiding pollution, but we should be wary of practicing ecological relativism.

-CAB