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Microplastics in seafood

microplastic in seafood

Ivan Bartolo and William Lart explore the uptake of microplastics by fish, crustaceans and shell fish and consider the implications of this for the environment and human health.

Introduction

Plastic waste is very much under the spotlight. We are all becoming more aware of the vast amounts of plastic litter in the marine environment and more conscious of the need to be more responsible in the way we use and dispose of plastics. We are learning that plastic pollution is not just the plastic litter that we see but also the less visible plastic microparticles. Microplastics have been found in marine and aquatic habitats, in the air, in sludge fertiliser applied to agricultural land and in house dust. As evidence mounts that microplastics are ubiquitous, there is growing concern about how they are taken up by wildlife, how they become incorporated into food chains and what health effects they may be having on us.

What are microplastics?

Microplastics are particles made from synthetic polymers with an upper size limit of 5 mm. Their shapes are variable, with microplastic fragments and fibres being the most commonly reported forms. Other characteristics, such as density, colour and chemical composition, are also variable and reflect the range of plastics that have been produced over time. Table 1 gives a list of some of the types of plastics that are commonly encountered in the marine environment. Nanoplastics form a subset of the microplastics. The European Food Safety Authority (EFSA) defines particles as nanoparticles when 50% or more of the particles have one or more external dimensions in the 1–100nm size range.

The majority of studies on microplastics have so far focused on the marine environment and seafood; scientific techniques for assessing microplastics in these substrates are available. Comparing the findings of different studies can be difficult, however, because sampling, extraction, purification and analytical methods for enumerating and characterising microplastics are not yet fully standardised. Compounded with this, contamination from airborne fibres during sample handling is a serious problem[1] and not all studies incorporate adequate measures to protect samples from airborne microfibre contamination. Standardisation of the approaches is underway, for example as part of the Joint Programming Initiative (JPI) Oceans project.

Once extracted, microplastics can be examined by a variety of techniques (summarised in Table 2). Optical microscopy can aid sorting in order to separate microplastics from non-plastic particulate matter of similar size. Spectroscopic techniques using the infrared (IR) part of the spectrum have proved reliable for identifying plastic fragments. Fourier transform (FT)-IR analysis has been successfully used for identifying microplastics in environmental and biological samples. Another useful method is pyrolysis followed by gas chromatography– mass spectrometry (Pyr-GC-MS). No single technique is suitable for all plastic types and for all particle sizes, with the choice of techniques becoming more restricted at the nanolevel. Using a suite of analytical methods rather than a single method may be necessary[2].

Sources of microplastics

Plastic production has risen steadily since the emergence of the plastic industry in the mid-1950s, with plastics and synthetic fibres reaching an annual production of 322m and 61m tonnes respectively in 2015 (Figure 1). The majority of plastic is used to make packaging and for construction. ‘Primary’ microplastics are plastics originally manufactured to be that size and include microbeads in cosmetics, powders used for blasting surfaces clean and resins used for the manufacture of plastic products. ‘Secondary’ microplastics are those that result from the wear and tear of larger plastic items and from the fragmentation of plastic litter. The wear and tear of paints and tyres, for example, gives rise to microplastics on land. Fragmentation occurs in the marine environment through prolonged photo-oxidation catalysed by ultraviolet light and through physical abrasion.

Most of the microplastics in the marine environment are secondary microplastics from degraded plastic litter. Plastic litter from badly managed landfills and general plastic litter inland finds its way into the sea via rivers, as runoff or directly from the coastline. There is also marine plastic waste, consisting of waste from shipping and fishing activities. It is estimated that the seas now contain 150 million tonnes of plastic, with 5–13 million tonnes being added every year[4].

Microplastics, such as microbeads from cosmetics and microfibres from clothes washing, enter wastewater drains; they are not removed by sewage treatment plants and are consequently transported via rivers to the sea[5]. Fallout of microplastics from the atmosphere also occurs.

Figure 1 Worldwide production of plastics in millions of tonnes including plastic materials (thermoplastics, thermosets, polyurethanes, adhesives, coatings and sealants) and synthetic fibres (nylon, polyethylene, polypropylene, polyurethane,
polyethylene terephthalate, acrylic and polyester fibres).
Reproduced from Lusher et al. (2017)[11].

Environmental impact

Microplastics are known to be ingested by marine species at all trophic levels, from plankton to macro fauna. They have been found in the stomachs of fish and seabirds. Laboratory-based investigations on marine organisms from lower trophic levels have identified sub-lethal microplastic effects on health, feeding, growth and survival. The effects were seen only in the laboratory and only when the levels of microplastic exposure were far higher than would be encountered normally. There is very little evidence of any effects, harmful or otherwise, of microplastics in nature. Further study is necessary in order to understand the real-life impacts on marine wildlife[6].

While there is no ecotoxi-cological threat identified so far from the physical and chemical composition of microplastics, there are also indirect threats to be considered. The hydrophobic nature of microplastics allows them to sorb harmful organic contaminants, such as poly­chlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs) and organochlorine pesticides, for example dichloro-diphenyltrichloroethylene (DDT). The microplastic particles also contain endogenous chemical additives, incorporated during the manufacture of the plastic. These include additives incorporated to inhibit photo-degradation, to impart flexibility or rigidity, or to retard microbial degradation. They include heavy metals and potential endocrine disruptors.

In terms of ecological health, the sorbed chemicals would be able to, in principle, exert an effect following ingestion by an organism or if they are leached into the water and then incorporated into the food chain through bioaccumulation. There is still uncertainty about whether these processes occur in nature and significant research is being carried out to address this[6,7]. Also in the ‘uncertain’ category are the effects of nanoplastics. Nanoplastics interact with biological systems in different ways to microplastics. They are more likely to internalise into tissues and cells and participate in biochemical processes; these effects have been demonstrated in the laboratory.

In all cases the microplastics were detected in the stomach and gut of the animals, which tend to be removed and discarded, and not eaten by humans.

Human exposure to micro-plastics through seafood

Microplastics have been detected in fish, crustaceans and bivalve molluscs. Fish found to contain microplastics include pelagic and demersal fish from the English Channel, North Sea, Baltic Sea, Mediterranean Sea, Portugal, Indonesia, California (USA) and the North Pacific Central Gyre. Microplastics (predominantly fibres) were detected in brown shrimp (Crangon crangon) caught in various locations in the English Channel and Norway lobster (Nephrops norvegicus) caught in the Clyde Sea. In all cases the microplastics were detected in the stomach and gut of the animals, which tend to be removed and discarded, and not eaten by humans[5].

The extent to which microplastics can migrate in fish from the digestive tract to the flesh is still being elucidated, and some investigations have shown that microplastics can translocate to fish liver[8].

Although fish guts tend to be removed before fish is consumed, there are occasions where the seafood is ingested without prior gut removal. Small fish (sardines, anchovies, young herring) can be consumed whole, fish for canning can be improperly gutted or not gutted at all and of course bivalve molluscs, such as mussels and oysters, are consumed whole. Mussels have been of considerable research interest. Studies of mussels from the Atlantic coast report 0.2–0.4 microparticles per gram of mussel wet weight.

Higher levels were found in Chinese bivalve molluscs. Perhaps not surprisingly, given the Belgian preference for mussels, the highest reported consumption of microplastics is thought to be by older Belgians[9]. Individuals in this group of high consumers are estimated to consume 72g of mussels per person per day, which would correspond to 11,000 microplastic particles per year.

How this level of exposure to microplastics through seafood compares with exposure from all sources is unknown. A recent study suggests that the amounts of microplastics ingested by eating mussels might be dwarfed by the amounts landing fortuitously on our food as household dust[10].

Effect on human health

Ingested microplastics could affect our health in a number of ways:

1 They could interfere with biological processes due to their physical characteristics.

2 They contain additives that might interfere with biological processes.

3 Through their hydrophobic nature, microplastics can adsorb potentially harmful organic molecules and act as vectors for them.

4 They can host microorganisms.

In vivo human data for the absorption of microplastics is limited, but there is mammalian data that can be used. It is very likely that microplastics with a size of 150 µm or more cannot cross the epithelium of the gut wall and are not absorbed. Those in the range 1.5–150 µm are absorbed at a rate of 0.3%. Microplastics in this range have not been shown to exert any adverse effect in humans, but abnormal inflammatory reactions and immunological effects have been observed in rodents. Though not reported in humans, these effects remain a possibility and may be more relevant in individuals with gastrointestinal afflictions. Nanoplastics smaller than 1.5 µm may penetrate organs, but information of their effects on the body is unavailable.

When plastic polymers are made, their characteristics (colour, rigidity, inflammability, resistance to photodegradation) can be modified by the incorporation of chemical additives. These can include pigments, heat stabilisers, UV stabilisers, plasticisers, flame retardants, biocides, smoke suppressors and slip agents. Heat stabilisers and slip agents can contain cadmium, lead and zinc. Biocides may contain arsenic and organic tin compounds. Plasticisers are usually phthalates, such as diisoheptyl phthalate (DIHP), bis(2- ethylhexyl) phthalate (DEHP) and dibutyl phthalate (DBP). Stabilisers and antioxidants include bisphenol A (BPA), nonylphenol compounds and cadmium and lead compounds. While the use of the more harmful of these additives is nowadays restricted, older plastics contain ‘legacy additives’ that were permitted at the time of manufacture.

Although many of these compounds have the potential to cause harm, they are not present in microplastics at sufficient levels to have any meaningful effect through seafood ingestion. EFSA considered a meal consisting of 225g of Chinese mussels containing a worst-case contaminant scenario of 4 particles/g.

The mass of plastic ingested with the meal was calculated to be 7µg. If the microplastics contained 4% of plasticiser (e.g. bisphenol A) and this was completely released in the body, it would contribute only a small fraction (2% in the case of bisphenol A) towards the total amount of plasticiser to which an adult would normally be exposed through food[5]. Whereas EFSA considered a single seafood meal, the authors of an FAO Technical Paper[11] considered the intake of a range of additives directly from microplastics in seafood through people’s ordinary dietary intake using a worst case additive scenario and reported additive intakes via microplastics in seafood versus additive intakes through the diet (Table 3).

Through their hydrophobic properties, microplastics can sorb organic contaminants, such as PCBs, PAHs, dioxins and other halogenated organic compounds. These substances are persistent, bioaccumulative and have toxic effects, and most national jurisdictions set limits for them in food. There is little doubt that given the right partitioning environment, a microplastic in a fish or bivalve mollusc would release these sorbed compounds. As with the discussion on additives in plastics, the additional contaminant load experienced by someone eating the fish or shellfish is unlikely to be significant when compared to the overall exposure experienced by the consumer (Table 3). Uncertainty remains on the fate and effect of compounds sorbed onto nanoparticles. Because nanoparticles can translocate into tissues and cells, there is a possibility of localised effects of the compounds. There is an information gap in this area: no data is available on nanoplastics in foods[5] and analytical methods for nanoplastics are in their infancy.

Microorganisms are known to colonise microplastics and in the marine environment the microbiome on the microplastic is known to be distinct from microbial communities in surrounding water. However the relevance to seafood and the consequences to human health are unknown[11].

In conclusion, microplastics are ubiquitous. We have been unwittingly exposed to them for several decades through the air we breathe and the food we eat. With the current state of our knowledge, we can assume that microplastics in seafood are unlikely to cause us any harm either directly or by their ability to act as vectors of contaminants. However, there is much that we do not know about some of the other possible effects of microplastics on us as consumers. The effects of microplastics smaller than 150 µm, including nanoplastics, have not been properly characterised and much work remains to be carried out in this area.

Table 3 Comparison of the calculated intake of contaminants and additives (worst case scenario) directly from microplastics in seafood and the total dietary intake of these compounds[11]. PCBs, polychlorinated biphenyls; PAHs, polycyclic aromatic hydrocarbons; DDT, dichlorodiphenyltrichloroethane; PBDEs, polybrominated diphenyl
ethers. NA, not available from EFSA or JECFA. For references and further information on intakes see Lusher et al. (2017)[11].

Ivan Bartolo

Regulatory Affairs Advisor with the Sea Fish Industry Authority, UK

Bartolo holds degrees in Pharmaceutical Technology, Food Technology and Applied Toxicology, and has held various food science roles in industry and government. Ivan is President of the Seafood Importers and Processors Alliance (SIPA). He held the post of Chair of the IFST Scientific Committee from 2011 to 2013, where he continues to serve as a member, and is a Fellow of the IFST and Registered Scientist (RSci).

Email ivan.bartolo@seafish.co.uk

William Lart

Sustainability and Data Advisor with the Sea Fish Industry Authority, UK

He holds degrees in Marine Biology, Fisheries Science and Applied Statistics, and has specialised in research into applied marine science. He has co-ordinated a number of European research projects concerned with fisheries and the environment.

Email william.lart@seafish.co.uk

Conflict of Interest The authors are employees of the Sea Fish Industry Authority, a UK non-departmental public body that has the promotion of seafood consumption as one of its goals.

References

1. Torre M, Digka N, Anastasopoulou A, Tsangaris C, Mytilineou C (2016). Anthropogenic microfibres pollution in marine biota. A new and simple methodology to minimize airborne contamination. Marine Pollution Bulletin 113(1–2),55–61.

2. Vollertsen J (2017). Overview of methods and challenges for microplastic analysis. Paper presented to IWA Sweden Microplastics Conference, Malmö, Sweden, 8 November 2017. Available from URL: http://www.svensktvatten.se/globalassets/utbildning/konferenser-och-seminarier/microplastics/03_vollertsen_microplastic-intro.pdf (accessed 20 June 2018).

3. Rocha-Santos T, Duarte AC (2015). A critical overview of the analytical approaches to the occurrence, the fate and the behavior of microplastics in the environment. Trends in Analytical Chemistry 65, 47–53.

4. Jambeck JR, Geyer R, Wilcox C, Siegler TR, Perryman M, Andrady A, Narayan R, Law KL (2015). Plastic waste inputs from land into the ocean. Science 347(6223), 768–771.

5. EFSA (2016). Presence of microplastics and nanoplastics in food, with particular focus on seafood. EFSA Panel on Contaminants in the Food Chain (CONTAM). EFSA Journal 14(6), 4501.

6. GESAMP (2016). Sources, fate and effects of microplastics in the marine environment: part two of a global assessment (Kershaw PJ, Rochman CM, eds) (IMO/FAO/UNESCO-IOC/UNIDO/WMO/IAEA/UN/UNEP/UNDP Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection). GESAMP Reports & Studies No 93.

7. Lart W (2018). Sources, fate, effects and consequences for the seafood industry of micro and nanoplastics in the marine environment. Seafish Information Sheet No FS 92.04.19. Grimsby, Seafish.

8. Collard F, Gilbert B, Compère P, Eppe G, Das K, Jauniaux T, Parmentier E (2017). Microplastics in livers of European anchovies (Engraulis encrasicolus, L.) Environmental Pollution, 229, 1000–1005.

9. Van Cauwenberghe L, Janssen CR (2014). Microplastics in bivalves cultured for human consumption. Environmental Pollution 193, 65–70.

10. Catarino AI, Macchia V, Sanderson WG, Thompson RC, Henry TB (2018). Low levels of microplastics (MP) in wild mussels indicate that MP ingestion by humans is minimal compared to exposure via household fibres fallout during a meal. Environmental Pollution 237, 675–684.

11. Lusher A, Hollman P, Medonza-Hill (2017). Microplastics in fisheries and aquaculture. Status of knowledge on their occurrence and implications for aquatic organisms and food safety. FAO Fisheries and Aquaculture Technical Paper No 615. Rome, FAO.



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