Plastic Pollution Toxicology

Microplastics demystified: a review examining how these tiny plastics act in pollutant transfer

Paper: Lohmann, R. Microplastics Are Not Important for the Cycling and Bioaccumulation of Organic Pollutants in the Oceans—but Should Microplastics Be Considered POPs Themselves? 2017. Integrated Environmental Assessment and Management 13, 460-465. DOI: 10.1002/ieam.1914

Not so fantastic plastic

The development of plastic revolutionized daily life. We owe our convenient modern lifestyle to these polymers in a big way; they are used routinely in medicine, manufacturing, transportation, everyday household items, and essentially every other modern accoutrament. But poor disposal of plastic waste and the proliferation of single-use plastic products has led to a huge pollution problem in the global ocean. A 2014 study based on cruise observations across five ocean basins estimated the sea contains over 269,000 tons of floating plastic pollution. More recent estimates push this estimate even higher; suggesting that ocean microplastic alone weighs 102-260 tons. Microplastics are defined as tiny pieces of plastic, typically less than five millimeters long, that turn up in the ocean and the Great Lakes.

Microplastics are small but significant; these are the most abundant form of plastic pollution in aquatic systems. Here, microplastics are pictured mixed in with sand grains in the Northwestern Hawaiian Islands. Image credit: NOAA

Microplastics can be found as exfoliants in some personal care products like toothpaste or face wash, and can enter the aquatic world via wastewater streams following the use of these products. Or these tiny plastic bits can form in situ, as larger plastic pieces break down into progressivally smaller pieces when exposed to the elements in coastal and open ocean environments.

Ocean creatures that eat these tiny plastic bits can experience direct physical effects such as inhibited digestion, excretion, or organismal bodily function; larger pieces of plastic often ensnare or tangle ocean dwellers, limiting their movement or even drowning them. Yet the chemical impact of these plastic bits is less clear. Researchers have hypothesized that once in the environment, these pieces of plastic attract persistent organic pollutants (POPs) like pesticides or polychlorinated biphenyls (PCBs) and subsequently deliver a high dose of these contaminants into any critter that eats them, leading to increased concentrations of POPs in plastic-eating creatures. It is unclear, however, if this hypothesis holds up in realistic situations since research on microplastics in the ocean is still new.

What’s the story about microplastics and POPs?

Rainer Lohmann, a professor at the University of Rhode Island Graduate School of Oceanography, set out to review the existing evidence related to how microplastics carry POPs in marine food webs. Lohmann collected literature evidence from experimental and observational studies and pieced the results into a more cohesive picture describing how microplastics interact with POPs and what this means for marine food webs. Lohmann sought to address a few specific questions:

  1. Do microplastics take up POPs in marine settings?
  2. Do microplastics play a role in global POP cycling?
  3. How much do microplastics actually contribute to POP introduction to marine food webs compared to other POP introduction pathways?
  4. Should microplastics be considered POPs themselves?

Setting the record straight with science

Lohmann suggests that research supports the idea that microplastics accumulate high concentrations of POPs; this makes sense given the chemical properties of many POPs. Most POP molecules are hydrophobic, meaning they do not like to be dissolved in water and would much rather be associated with solid material like animal tissue or sediment. Thus when in an ocean setting, many POPs readily choose to accumulate in plastics and microplastics rather than remaining in water.

Lohmann also points out that microplastics probably do not play a significant role in the cycling of POPs in ocean waters, mostly because there is so much more water and living matter taking up POPs compared to plastic, even if plastic pollution is a problem.

POPs travel into coastal and open ocean environments, differentially transitioning between air, water, and living matter as the above suggests. The mass of microplastics is tiny compared to water and living matter, meaning that although it attracts POPs, it does not play a large role in the overall POP cycle, pictured above. Image:

Lohmann cites modeling efforts that suggest microplastics take up only 0.0001% of the total mass of POPs contained within the ocean. Those POPs that are taken up by microplastics are also released back into POP cycling slowly, meaning microplastics are not a big contender in the overall POP cycle.

Perhaps most importantly, Lohmann determined that microplastics do not actually play a significant role in adding POPs into marine food webs. Evidence suggests that fish and plastics in a given environment have about the same potential to take up POPs , meaning microplastic has roughly similar POP concentrations compared to a fish from the same environment. In other words, microplastics do not contain extra POPs to transfer to the fish and the overall marine food web. Lohmann also points out that laboratory experiments demonstrating efficient transfer of POPs into animals often involves plastic bits dosed with unreasonably high concentrations of POPs; this does not mean POPs move from microplastics into animals in realistic settings though. Evidence does suggest that microplastics could transfer POPs into animals that are not otherwise in the environment, namely compounds like specific flame-retardants that are directly added to plastics in the producton process.

Lohmann wraps up by suggesting that managing microplastics could be made a lot easier by listing microplastics as POPs themselves under the Stockholm Convention. The Stockholm Convention is an international treaty that regulates the use and production of chemical substances that persist in the environment and can hurt living creatures. The treaty usually governs traditional POPs that can be identified as particular molecular structures. But if the terms were expanded to include microplastics, they could more readily be regulated, monitored, and their impact ameliorated across the world ocean.

So do microplastics not matter?

Nope, microplastics still matter. Lohmann clarifies the way microplastics and POPs interact by stating that microplastics do not act as an important vector, or pathway, for POPs to enter or disproportionately accumulate in marine food webs. Marine animals and microplastics in marine environments are both about equally contaminated with POPs, so POPs aren’t soaking up large loads of POPs and then transferring them into otherwise uncontaminanted animals. Animals gain the bulk of their POP burden from their diet or the surrounding environment like traditionally believed.

Marine creatures, ranging from tiny invertebrates to large marine mammals, ingest plastic and experience physical and specific chemical effects as a result. Here microplastics are pictured in blue within a copepod, a type of zooplankton Image: Matthew Cole, ICES CIEM

But Lohmann makes it clear microplastics are not a non-issue; they can still contribute plastic additive chemicals not found in the ambient environment into marine food webs, while also causing significant (but uncertain) physical effects in animals that eat them. Lohmann’s suggestion to regulate microplastics under the Stockholm Convention could be an excellent idea. Currently, plastic management in the world’s oceans is akin to the Wild West, with little existing regulation preventing or working to combat microplastic pollution. Concerted international action could go a long way to keep microplastics out of the marine environment, benefiting humans and marine animals alike.

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