Plastics themselves are considered biologically inert since most aquatic organisms lack the enzymes to break down synthetic polymers. However, plastics contain additives, many of which are known hazardous substances and can leach out of the plastic during degradation (or digestion). Additionally, unlike the naturally occurring inorganic fine particles present in seawater, microplastics concentrate persistent organic pollutants (POPs), such as polycyclic aromatic hydrocarbons (PAHs), polybrominated diphenylethers (PBDEs), polychlorinated biphenyls (PCBs), and dichlorodiphenyltrichloroethane (DDT). It is the dissolved POPs that yield toxic outcomes. Surveys of plastic particles collected from beaches determined that microplastics have the potential to act as vectors for the transport and release of absorbed contaminants and additives. Though the role of microplastics as transportation for POPs has been suggested to be relatively small compared to other pathways, concentrations of PCBs on polypropylene pellets have been found to be over 100 times higher than the surrounding seawater.
Both microplastics and POPs are typically hydrophobic and attract each other for that reason. Organic pollutants will also preferentially bind to microplastics over larger plastic fragments, due to their comparatively large surface area to volume ratio. In fact, this binding capacity has led to the increasing use of plastics as passive sampling devices to estimate POP concentrations in water, sediment, and flora/fauna. This absorption even ‘cleans’ the seawater of the dissolved pollutant chemicals. However, this seems to be the case only with virgin plastics that haven’t degraded much. The situation is reversed in the case of recycled plastics or partially degraded products. These debris will slowly leach back out a small fraction of the POPs into the sea water. It is a dynamic equilibrium with the POPs diffusing in an out of the plastic fragment, depending on changes in the concentration of the POP in seawater. Recent work has also demonstrated that plastic debris can accumulate metals in greater concentrations than the surrounding water. The critical ecological risk is not the low levels of POPs (or metals) in water, or even the high levels concentrated in microplastics; it is the bioavailability of those highly concentrated microplastics that, when ingested by marine species, becomes a credible route through which the POPs can enter the marine food web. The ability of ingested POPs to transfer across trophic levels and the potential damage posed by these to the marine ecosystem are not fully known, but given the increasing levels of plastic pollution of the oceans, it is an area of increasing interest.
Though ingestion is the most likely interaction between marine organisms and microplastics, they can also enter the very base of the marine food web through absorption. Studies have indicated that nano-sized polystyrene particles may permeate into the lipid membranes of organisms, altering the cellular function. Such was the case when nano-polystyrene beads were absorbed into the cellulose of a marine alga (Scenedesmus spp.), inhibiting photosynthesis and causing oxidative stress (i.e., making it age faster – an extremely simplified explanation).
Microplastics’ small size makes them appealing to a wide range of organisms in benthic and pelagic ecosystems. In some cases, a creature’s feeding mechanisms can’t discriminate between prey and plastic, especially if there is a predominance of microplastic particles mixed with planktonic prey items. And when you’re filtering 20 to 50 gallons of water a day, who’s to say whether that one-millimeter speck is a copepod or a pellet? Clean plastic microbeads have been commonly used in zooplankton feeding research, so there is no doubt they are ingested, and that they can affect the habits of zooplankton. However, no one knows if any warning signals exist that might discourage the ingestion of ‘dirty’ beads by at least some of the species likely to ingest them.
Some species are capable of rapid excretion/egestion of microplastics. Two species of mudsnails allowed to graze on fluorescent microplastics for one week excreted over 80% of the particles. One copepod species also ingested microplastics within a twelve-hour exposure period and egested the majority of the particles within an additional twelve hours. Some species are not so efficient. One study observed polystyrene particles in blue mussels forty-eight days past ingestion.
Exposure studies have demonstrated that benthic invertebrates such as lugworms, amphipods, and blue mussels feed directly on microplastics, and deposit-feeding sea cucumbers even selectively ingest microplastic particles. Gut surfactants in some of these species possibly enhance the bioavailability of POPs. Weight loss, reduced feeding activity, and significantly decreased energy reserves were positively correlated with ingestion of spiked sediments in lugworms. Following a two-generation toxicity test, adult females and larvae of the Tigriopus japonicus copepod suffered increased mortality rates. Chronic exposure to nano-polystyrene caused reduced clutch and neonate size in the copepod, Tigriopus japonicus, and deformations in the offspring of small planktonic crustacean, Daphnia magna. Another copepod, Calanus helgolandicus, exposed to polystyrene for nine days showed reduced appetite and significantly smaller eggs with decreased hatching success. Shore crabs (Carcinus maenas) will not only ingest microplastics but also ‘inhale’ plastics into the gill cavity, just another exposure route to consider.
Fibers of monofilament plastics (sourced to fibers of trawls and fragments of plastic bags) have been found in the intestines of the commercially valuable Norway lobster (Nephrops norvegicus). Normal digestive processes apparently can’t eliminate all of the filaments. Field-caught brown shrimps and farmed bivalves had microplastics in their digestive system as well. The identification of microplastics in commercially harvested organisms that are consumed whole (guts and all) highlights the potential human health issues. Based on estimates of the average consumption of mollusks by European consumers, the average person could ingest between 1,800 and 11,000 microplastic particles per year.
Filter feeders do seem to have greater exposure to microplastics than organisms employing other feeding strategies. The baleen whale, Balaenoptera physalus, was reported to contain microplastics in blubber samples at levels of about one piece every square foot. A third of gooseneck barnacle stomachs examined contained microplastics. In a Canadian study, higher amounts of microplastics were found in farmed mussels than in wild mussels, possibly a result of farming practices that use polypropylene lines to anchor the mussels.
Some of the earliest studies noting ingestion of microplastics by wild-caught fish include coastal species from the USA and the UK. Estuarine fish affected include catfish (Ariidae, 23% of individuals examined) and drums (Scianenidae, 7.9% of individuals examined). Slightly lower amounts of ingestion were found in freshwater and marine fish collected from watersheds of the Gulf of Mexico. A total of 51 fish species from 17 families were examined. Ingestion of microplastics was widespread, with individuals from 65% of species (herbivores, invertivores, and omnivores) showing ingested microplastics.
The delivery of toxins across trophic levels is still being researched, but seems likely. In a study examining effects of trophic transfer, algae were mixed with nanoparticles of polystyrene and fed to Daphnia magna. The daphnids were then fed to crucian carp, Carassius carassius. Over the course of 61 days, reduced feeding activity was observed in the carp, along with changes in behavior. They also had heavier, swollen brains, with greater water content. These results indicate that microplastic particles may accumulate in lipid-rich organs, such as the brain, disrupting biological membranes and inducing the effects observed in fish.
To add insult to injury, in addition to posing as food, microplastics can alter bacterial communities. Some species of heterotrophic bacteria, like Vibrio, have been found to colonize microplastic debris. Colonized microplastics may be more attractive as food items than 'pure' plastic particles, and it is possible that microplastics could act as vectors for pathogens and/or exotic species as a result of these opportunistic colonizers, though these interactions are poorly understood thus far.
A Response to Microplastics
In general, the public and private sector awareness of the potential negative ecological, social, and economic impacts of microplastics is much less developed than for more visible litter. However, in response to growing concerns from the scientific community, Austria, Luxembourg, Belgium, Sweden, and the Netherlands issued a joint statement to the European Union Environment Ministers, calling for a ban on microplastics in personal care products. Bans on microplastics in cosmetic products were also enacted in Illinois, California, and New York, and the Microbead-Free Waters Act of 2015 was finally signed into US federal law in January 2016. Engagement and education at all levels of society (public, government, and private sector) is necessary to effect positive change. With our current resources, it is virtually impossible to remove microplastics from the sea without simultaneously removing similarly sized organisms, like plankton, and subsequently disrupting the ecosystem further.
Wastewater treatment systems fail at filtering out microplastics because of their small size. The best plan, for now, is tackling the multitude of microplastic origins, and that requires addressing the problem at its source – newly engineered materials and smart design, for example, such as clothes that shed fewer fibers or washing machines equipped with filters. And to be successful, these efforts must be supported by legislation and actionable policies that force real change. As pro-plastic consumers, we are responsible for adapting our behaviors and increasing our waste management efficiency. Turning our plastic soup back into the bountiful sea is a challenge we must accept.
We do not inherit the earth from our ancestors; we borrow it from our children.
~Native American proverb, often attributed to Chief Seattle
Where I learned about microplastics, and you can too!
International Union for Conservation of Nature
Sustainable Development Knowledge Platform
Pennsylvania State University
Norwegian Environment Agency
National Oceanic and Atmospheric Administration