Toxic chemicals make for poisonous plastic

Toxic chemicals are yet another weapon in plastics arsenal, in its apparent bid to destroy marine life (don't get me wrong, this is people's fault, not the plastics). Plastic debris has the potential to act as both a sink and a source for toxic chemicals, transferring them to marine organisms. If that doesn't concern you, the toxic chemicals accumulate up the food chain until they reach our plates. So just remember: what goes in the ocean goes in you.

Toxic chemicals can make their way up the food chain from plastic-ingesting marine species into human diets. Source.

Plastic as a sink for toxic chemicals

Plastic can act as a sink when it adsorbs toxic chemicals in the ambient seawater. These chemicals include persistent, bioaccumulative and toxic substances (PBTs), dichlorodiphenyltrichloroethane (DDT), polychlorinated biphenyls (PCBs), and persistent organic pollutants (POPs). PBTs, DDT, PCBs and POPs present a health concern to humans and the environment. The chemicals can be released into the oceans through pesticides, industrial processes, leaching, or being released from objects into the environment. The interactions between plastic and chemicals in the ocean is complex, but it is more likely than not that PBTs will preferentially sorb to plastic debris, as they have a low water solubility. 

Resin pellets (or nurdles) are the raw material used to make plastics in industry. Mato et al. (2001) studied 4 sites on the Japanese coast, examining PCBs, and DDE (a DDT derivative) in polypropylene resin pellets. The concentrations found in the resin pellets were equivalent to that of the seawater and sediments where they were found. A control experiment measured the absorption of virgin resin pellets, finding a regular and significant increase in PCBs and DDE concentration with exposure to the seawater. Mato et al. (2001) showed that the source of PCBs and DDE in the resin pellets was the ambient seawater, as the pellets absorb the chemicals through a process of enrichment.

Plastic resin pellets act as a sink for toxic chemicals in the ocean. Source.

   

Plastic as a source for toxic chemicals

Plastic act as a source for toxic chemicals due to the compounds added during manufacture to give the plastic certain desirable properties, such as pliability. When the plastic is ingested, the chemicals can leach from the plastic directly into the organism. Here are some examples of the chemicals:

• Phthalates. Added to PVC for softness and pliability. 
• Bisphenol A (BPA). A monomer used to make polycarbonate plastics. Can have toxic and biological effects on humans. 
• Brominated flame retardants. Added to reduce flammability. 

With all the added chemicals, plastic debris has the potential to be a source of toxic chemicals for months or decades.  

In a study of 12 short-tailed shearwaters collected from a research trawler in the North Pacific Ocean, Tanaka et al. (2013) demonstrated that chemicals are not only transferred to the birds from prey but also from ingested plastics. The abdominal fat tissue of the birds was analysed for polybrominated diphenyl ethers (PBDEs), a POP which is added as a flame retardant. 6 lanternfish and 1 squid were also collected and analysed, being common prey for the birds. In 9 of the 12 birds lower-brominated congeners were the dominant form of PBDEs found. The lower-brominated congeners were also dominant in the prey, indicating accumulation through the food chain. However, in the other 3 birds higher-brominated congeners were dominant - this doesn't match the profile of the prey. Plastics can provide the answer here. Higher-brominate congeners are present in plastics, including those found in the stomachs of the 3 birds. The results indicate that ingestion of plastic is the likely source of the higher-brominated congeners.   

Plastic debris acts as both a sink and a source for toxic chemicals. Source.

Interactions between plastic, toxic chemicals and the food chain

There are 3 key terms to know about the interaction between toxic chemicals and the food web:

1. Bioconcentration. Species living in chemical polluted waters concentrate the chemicals in their tissues
2. Bioaccumulation. Species face exposure to toxic chemicals from bioconcentration and ingestion. If the exposure is occurring faster than the chemicals can be eliminated, this is bioaccumulation. 
3. Biomagnification. Chemicals are found in progressively higher concentrations in progressively higher trophic levels in the food chain.

These 3 processes mean that even a small addition of chemicals can lead to large concentrations in species higher up the food chain. That includes humans too. Toxic chemicals and their interactions with plastic debris are a real concern for all people who eat seafood. Plastic is a sink and a source for toxic chemicals, but it is really a vector for transferring toxic chemicals from water, into marine organisms, and into us. Have a think about that next time you eat seafood. 

Biomagnification of toxic chemicals up the food chain. Source. 

A second life: ocean plastics re-imagined

Perhaps we can re-imagine the plastic in our oceans. It is still a pollutant, of course, but it could have greater potential. Adidas and Parley for the Oceans have been working on a project which would use 3D printing to turn ocean plastic into trainers. That's right. The plastic in our oceans could even be a resource! Projects like this could transform the lifecycle of plastic, reforming it into something useful again.

Trainers made from ocean plastic. Source: Adidas group. 

Little plastics, big problem: microplastics

Great for washing your face, terrible for marine environments: it's microplastics. They may be little, but they are a massive problem.

Microplastics: admittedly quite pretty, but definitely deadly. Source.

Microplastics (<5mm) have become an increasingly common ingredient in toiletries such as facial cleansers and toothpaste. They are often called 'micro-beads' by the toiletries industry and praised for "exfoliating skin and clearing out pores". The average consumer ends up using microplastics on a daily basis. There's a few reasons why these tiny pieces of plastic are seriously bad news:

• Microplastics are too small to be caught by wastewater screens, so they go directly into our oceans ...
• ... but they're plenty small enough to be easily consumed by small marine animals, such as filter feeders, which can starve from satiation, reduced food consumption or intestinal blockages 
• They then have the potential to pass up the food chain and accumulate
• Smaller pieces means a bigger surface area, which means more potential for binding and up-taking of toxic contaminants 

They're in our oceans and they're in our rivers too. In a study from earlier this week it was found that the Rhine has the highest levels of microplastic pollution in any recorded river, transporting an unbelievable 191 million plastic particles every single day! Microplastics are bad for you as well our waterways. Microplastics in toothpastes can end up embedding plastic in your gums.... Definitely count me out!

Plastic in my toothpaste? No thanks! Source.

However, not all hope is lost! As consumers, we can all make an effort to avoid products containing microplastics. What's more, major toiletry brands such as L'Oreal and Johnson have made commitments to phase out microplastics and replace them with natural alternatives. Even more encouragingly, laws are beginning to be drawn up which will ban microplastics. Addressing the issue of microplastics rapidly and effectively is a win for ocean conservation!

Hitching a ride on plastic: alien invasions and the plastisphere

As if directly injuring and killing animals wasn't enough, plastic poses a whole other host of threats to biodiversity and marine ecological systems.

Marine plastic debris is resilient, non-biodegradable and abundant, making it the perfect substrate for species wanting to hitch a ride across the oceans. There are a number of natural substrates which usually fulfil this role, such as wood, feathers and macroalgae. But plastic is outnumbering all the natural substrates and vastly increasing the potential for species transport and dispersal. This enables species to travel further and in greater numbers. Marine plastic is another example of anthropogenic activities causing the spread of invasive species, which have serious consequences for biodiversity.

A wide variety of animals use marine debris as a mode of transportation and dispersal, especially bryozoans, barnacles, polychaete worms and molluscs. In 2002 Barnes studied 30 remote islands across all the worlds oceans, on which over 200 items were found washed ashore. Of these items, anywhere between 20 and 80% were anthropogenic.

Map of study sites, with inset debris on beach. Source: Barnes, 2002.

It was found that the proportion of anthropogenic debris increases with latitude (fig. 2 a). The increase in anthropogenic debris represents an increase in potential for transporting organisms. Figure 2 b shows the effects of remoteness of the island on colonisation of debris. Distance from the mainland is given as  hundreds of kilometres (circles), tens of kilometres (triangles) or less than 10 kilometres (squares). The distance from the mainland doesn't have a significant effect on the proportion of colonised debris. It can also be seen that there were no samples recorded as colonised beyond 60 degrees, most likely due to the persistent low temperatures found at such high latitudes. Global warming will exacerbate the issue, enabling plastic colonisers to travel further poleward. Figure 2 c shows the ratio of colonisation on non-anthropogenic debris to colonisation on anthropogenic debris, with the amount on anthropogenic debris rapidly increasing up to 60 degrees. From this Barnes has concluded that anthropogenic debris in the oceans has approximately doubled the spread of fauna in the sub-tropics, and increased it more than 3 times at latitudes over 50 degrees!

Figure 2: comparison of anthropogenic and colonised debris with latitude. Note: white symbols are in the Northern hemisphere; black symbols are in the Southern hemisphere. Source: Barnes, 2002.

Plastic is doing more than just transporting species: delving down to a much smaller scale we find the 'plastisphere', a new ecological realm. Zettler et al. (2013) carried out an comprehensive study classifying the microbial communities found on polypropylene and polyethylene fragments (used in packaging and single-use plastic items) in the North Atlantic sub-tropical gyre. Using SEM micrographs they found over 50 distinct morphotypes and over 1000 species equivalents of operational taxonomic units.

Figure 2: examples of different morphotypes of bacteria. Source: Zettler et al., 2013.

Their key finding is that the 'plastisphere' microbial communities are distinct from those in surrounding seawater. What this means is that anthropogenic plastic pollution into the oceans has created a novel ecological habitat! I won't go into detailing every single microbe they found, but figure 4 is a bar chart of all different operational taxonomic units, and it is obvious how different the seawater communities are from those found on plastic fragments.

Figure 4: bar chart of different microbial operational taxonomic units illustrating the difference between plastisphere communities and surrounding seawater communities. Source: Zettler et al., 2013.  

Figure 5: Venn diagram illustrating the significant differences between plastisphere communities and surrounding seawater communities. Source: Zettler et al., 2013. 

The Venn diagram shows the disparity between plastisphere communities and seawater communities - there is only a limited overlap. Also, different types of plastic appear to have largely different communities too. What this all means is that we have created a new plastic ecosystem, further evidence for the significant effects of anthropogenic activities on Earth!

Deadly plastic oceans: entanglement and ingestion

One of the most shocking and saddening impacts of marine plastic debris is entanglement of and ingestion by marine species. Entanglement and ingestion is thought to have affected at least 267 different species. The range of species affected is huge including turtles, seabirds, whales and dolphins, penguins, seals and sea lions, sea otters, manatees, fish, and crustaceans. There is a vast amount of literature recording entanglement and ingestion. I have selected a few examples to illustrate how pervasive the problem is.

Entanglement

• Over a 23 year period Waluda and Staniland (2013) observed 1033 Antarctic fur seals entangled in marine debris at Bird Island, South Georgia. Plastic packaging bands were the most common cause of entanglement (43%), followed by synthetic fishing line (25%) and fishing net (17%). 44% of seals entanglement were juvenile males - who will have a lot of growing left to do. 
• Page et al. (2004) estimate that 1478 seals die from entanglement each year in Australia alone. Studying Australian sea lions and New Zealand fur seals at Kangaroo Island, Australia, Page et al. found that the rate of entanglement had not decreased in recent years, despite government and fishing industry efforts aimed to reduce the impact of fishing activities on non-target species. When lost or abandoned fishing gear 'catches' seals, fish or other species, this is known as 'ghost fishing' (Gregory, 2009). Australian sea lions and New Zealand fur seals have the 3rd and 4th highest entanglement rates for any seal species. 
• A northern gannet colony in Wales was studied for two weeks in October from 1996-1997 and 2005-2010. Votier et al. (2011) looked at 6 nests representative of the overall colony (in terms of individual nest size), and calculated that the average nest contained 469.91g of plastic, predominantly synthetic rope. The estimated colony total was 18.46 tonnes of plastic. On average this led to 63 birds entangled each year, or 525 individuals over the 8 study years. Votier et al. believe that this level of entanglement is unlikely to have population-level effects.
• Sightings of pods of endangered humpback whales showed at least 7 whales towing tangled rope or other debris (Gregory, 2009). 
• Sharks are often entangled in 'debris collars' (Gregory, 2009). 

Cartoon entanglement, from Happy Feet. Source.

Ingestion

• Several sea turtle species are seriously endangered from ingestion of plastic debris. Turtles often mistake floating plastic bags for jellyfish (Gregory, 2009). Analysing the stomach and oesophagus content of sea turtles in Southern Brazil, Bugoni et al. (2001) found that plastic bags were the most common form of debris ingestion, predominantly clear or white pieces, i.e. those most resembling jellyfish. 13.2% of green turtle deaths were accountable to the ingestion of anthropogenic debris. 
• There are over 100 species of bird known to ingest plastic (Gregory, 2009). These birds include albatrosses, of which Jimenez et al. (2015) studied 128 specimens of 7 different species. The amount of plastic fragments ingested varied significantly between species, indicating a difference in feeding and foraging patterns. Only 2% of mollymawk albatross (Thalassarche spp.) had ingested plastic, whereas 25.6% of great albatross (Diomedea spp.) had ingested plastic, with the highest being Diomedea sanford at 38.9%! 
• In the first study to examine plastic ingestion in common planktivorous fish, Boerger et al. (2010) found that approximately 35% of fish collected from surface waters of the North Pacific Central Gyre had ingested plastic, averaging 2.1 pieces of plastic per fish.

From these few examples it clear just how deadly the oceans have become for marine species, due to the anthropogenic input of plastic debris.