Plastics in the ocean

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Since the 1950s, huge amounts of plastic have been produced worldwide. Plastic is a collective name for certain synthetic polymers, mainly polyurethane (PUR), polyethylene (PE), polyamide (PA), polyethylene terephthalate (PET), polystyrene (PS), polyvinyl chloride (PVC) and polypropylene (PP). Part of the plastic waste ends up in the oceans. This waste consists of macroscopic plastic debris (> 0.5 mm) and microscopic pieces (< 0.5 mm) as a result of physical, chemical and biological weathering and fragmentation of macroscopic waste. The smallest plastic fragments (< 100 nm) are called nanoplastics. Many studies have been conducted to determine the amount of plastic in the oceans and the effects on the marine ecosystem. This article summarizes some important results of these studies.


Budget of plastics in the ocean

The current (2024) global production of plastics is estimated at 350-400 million tons. Products made of plastic or containing plastics eventually become waste. Most waste products are destroyed (incinerated), some are recycled and some are released in the natural environment (‘mismanaged’). A small fraction ends up in the ocean, mostly via rivers[1], but also via in situ sources (e.g. discarded fishing gear) and the atmosphere. Estimates of the amount of plastic waste entering the sea vary widely, from 0.5 Mton/y to more than 10 Mton/y [2][3][4][1]. The lower values likely underestimate the supply rates of plastics to the ocean. They do not include atmospheric inputs [5] and plastic fragments smaller than <300 μm [6]. Sea-based sources account for about 20% of the total input of plastics[4].

Beach covered with plastic garbage. Photo credit: Mel D Cole/World Bank

There is also much uncertainty about the amount of plastic accumulated in the oceans since the mass production began in the 1950s. Early estimates for the total amount of plastic in the surface ocean water range from 0.1–0.8 Mton for microplastics and around 0.8 Mton for plastic macrodebris [7][8][4]. This is a very small amount compared to the total input of plastics to the oceans over the past 60 years. This total input has been estimated at around 25 Mton (4-5% of the mismanaged plastic waste generated in the period 1961-2017[4][9]), but could be much larger[6]. A large part of the floating plastic macrodebris is trapped along the shorelines (stranded and buried)[10][8][11]. Another part (probably smaller, estimated at about 3 Mton[12]) consists of plastic debris accumulated on the ocean floor through the incorporation of plastics into organic particles (e.g., marine snow and fecal material)[9].

More recent studies attribute the major contribution to ocean plastic pollution to smaller size classes of microplastic and nanoplastic. Using nets with a mesh size of 100 μm resulted in 2.5-fold and tenfold higher microplastic concentrations than with 333 μm and 500 μm meshes, respectively[13]. Samples from the 200 m surface layer of the Atlantic ocean were analyzed for the three polymer groups polyethylene, polypropylene and polystyrene. The majority of the microplastics were <100 μm, with the peak size distribution observed in the range 50–75 μm for all the polymer groups. Only a small fraction of all three polymer groups were >300 μm. The three polymer groups represented a weight estimated at 11.6–21.1 Mton for the whole Atlantic ocean[6].

Micro- and nanoplastics are likely to originate mainly from land-based sources: effluents from wastewater treatment plants (e.g. components of cosmetic and care products, fibres from washed synthetic clothing), grindings from synthetic materials (e.g. tyre wear, carpet wear) transported by run-off or airborne, weathering of plastic waste along beaches that is washed to sea, etc. [14]. Breakdown of microplastic debris in the ocean into nano-fibers is extremely slow and therefore considered less important[15]. Several studies report that the smaller the plastic particles, the more toxic they are to marine biota[16]

Degradation of plastics in the marine environment

Degradation mechanisms for macroplastics occurring in the marine environment are: (i) colonization of plastics by microorganisms that form a biofilm on the polymer surface and induce biodegradation via surface erosion; (ii) depolymerization by abiotic hydrolysis of functional groups (esters, carbonates and amides), which causes molecular weight reduction; and (iii) photodegradation by exposure to UV light, resulting in molecular weight reduction and cracking of the material[17]. Microbes that degrade plastics attack plastics extracellularly to (partially) depolymerize the complex and large molecule into compounds that can be used as carbon and energy sources[18]. Fungi, like bacteria, can also use plastics as carbon source. Laccase enzymes play an important role in fungi attachment[17].

In the marine environment, complex hydrocarbon degrading bacteria are often found in association with plastics. These bacteria have genes encoding for monooxygenases, peroxidases and dehydrogenases, that can facilitate the initial degradation of plastics[18]. These enzymes act mainly on polyethylene terephthalate (PET) and ester-based polyurethane (PUR). The currently known PUR- and PET-active enzymes and microorganisms have a very low conversion rate (on the order of decades to centuries[19]). No enzymes are known that act on the polymers polystyrene, polyamide, polyvinyl chloride, polypropylene, ether-based polyurethane and polyethylene. Together, these polymers account for more than 80% of the annual plastic production[20].

Some marine algae (e.g. Chlorella sp. and Cyanobacteria sp.) are capable to degrade polyethylene terephthalate (PET). As for bacteria, the process starts with the colonization of the polymer surface (fouling), which affects the strength and physical performance of immersed polymers. Degradation by algae follows two pathways: (i) polymer molecular weight reduction, which is the breakdown of large molecules initiated by enzymatic action; (ii) oxidation of low-molecular-weight molecules [17].

Weathering and photo-oxidation are generally considered the most important processes for the degradation of plastics. They result in a change in the chemical, physical and mechanical properties of the plastics. Abiotic factors, such as shear stress caused by wave action, ultraviolet radiation from the sun and heat make plastics brittle and break them up. Fragmentation of macroplastics results in micro- and nanoparticles. Photo-oxidation decomposes the polymers into nanoplastics, low molecular weight polymer fragments and hydrocarbon gases. These degradation products of UV radiation can be a useful carbon source for microbes, but can also interfere with enzyme degradation processes[20]. Polystyrene can be completely mineralized by photooxidation[21]. Photo-oxidative fragmentation of plastic debris in the upper ocean layer is inhibited by several factors: short residence times before sinking, attenuation of UV radiation below the upper 1 meter, fouling by biota, low water temperature and low dissolved oxygen[15]. Origins of the abundant micro- and nanoplastics found in the oceans must therefore be attributed primarily to photo-oxidation of plastic debris on land prior to reaching water.

Ecosystem impacts

Sea turtle eating a plastic bag. Image from https://tontoton.com/how-does-ocean-plastic-affect-marine-wildlife/#

Some important observed impacts of plastics on marine ecosystems and fauna are:

  • Macroplastics cause smothering of organisms when getting trapped in coastal ecosystems such as corals and mangroves. For example, corals can be detached from the hard substrate, with broken or dead branches. Exposure to light, oxygen exchange, prey capture and growth can be impaired and pathogens can get a foothold for invasion. On the other hand, plastic debris can provide a substrate for colonization by corals[22].
  • Macroplastics can cause entanglement of marine animals. This happens especially for abandoned fishing gear (74% of the cases). Entanglements causing the amputation of limbs have been reported in all species of sea turtles. Entanglement with and ingestion of plastic debris have been reported in 35 species including dolphins, whales, seals, sea lions, sea otters, polar bears and manatees[22].
  • The ingestion of macroplastics or microplastics can cause injury to the intestines of species and accumulate in the animal's organs, causing blockages of the gut tract that result in pseudo-satiety sensation and physiological stress, alteration of the feeding and retardation of growth, reduction in fertility, fecundity and survival rate of progeny[23]. A compilation of a large number of studies shows that on average 39% of all examined seabirds had ingested macroplastics, and on average 33% microplastics, while ingested microplastics where found in all sea turtles sampled in the Atlantic, Mediterranean and Pacific[22]. There is clear evidence of the ingestion of microplastics by marine zooplankton, as well as the transfer of microplastic particles from mesozooplankton to macrozooplankton. Therefore a real risk exists of microplastics invading marine food webs[24].
  • Chemical additives or pollutants adsorbed to plastic debris from seawater can become bioavailable when ingested by organisms, disrupting essential metabolic processes. Microplastics can be taken up by a much wider range of species than large plastics, so their potential to enter marine food webs and move up trophic levels through predation is higher. Especially filter-feeding animals, from small mussels to large whale sharks, can ingest considerable amounts of microplastics[22].

To give plastics certain properties, various substances are widely used as additives. Such substances include bisphenol A (BPA), brominated flame retardants, PCBs and a group of substances called phthalates. They belong to the so-called endocrine disruptors, which disrupt the hormonal functions of animals. They affect the movement, behaviour, sexual development, growth (including abnormalities), survival and reproduction of aquatic animals and can bioaccumulate in organisms such as fish and tadpoles. Early life stages, amphibians, fish and invertebrates are particularly sensitive[22].

Micro- and nanoplastics have a high surface area to volume ratio and can therefore also adsorb high amounts of hydrophobic contaminants from the environment. Plastics of these size classes are easily taken up by many organisms, within which the contaminants can be released again[18]. Adsorbed contaminants such as pesticides, herbicides and heavy metals (e.g. Al, Cd, Co, Cr, Cu, Fe, Hg, Mn, Pb and Zn) can lead to changes in metabolic and reproductive activity, impaired immune response, oxidative stress, cellular or subcellular toxicity, inflammation and cancer[23].

On the other hand, chemicals accumulated in the body can adsorb to microplastics and be excreted, reducing the burden on the organism. Similarly, plastics in water can accumulate chemicals, leading to a lower concentration of chemicals in the water[22].


Related articles

Coastal pollution and impacts
Endocrine disrupting compounds in the coastal environment


References

  1. ↑ 1.0 1.1 Strokal, M., Vriend, P., Bak, M.P., Kroeze, C., van Wijnen, J. and van Emmerik, T. 2023. River export of macro- and microplastics to seas by sources worldwide. Nature Communications 14, 4842
  2. ↑ Jambeck, J.R., Geyer, R., Wilcox, C., Siegler, T.R., Perryman, M., Andrady, A., Narayan, R. and Law, K.L. 2015. Plastic waste inputs from land into the ocean. Science 347: 768–771
  3. ↑ Schmidt, C., Krauth, T. and Wagner, S. 2017. Export of plastic debris by rivers into the sea. Environ. Sci. Technol. 51: 12246–12253
  4. ↑ 4.0 4.1 4.2 4.3 Isobe, A. and Iwasaki, S. 2022. The fate of missing ocean plastics: Are they just a marine environmental problem? Science of the Total Environment 825, 153935
  5. ↑ Allen, D., Allen, S., Abbasi, S., Baker, A., Bergmann, M., Brahney, J., Butler, T., Duce, R. A., Eckhardt, S., Evangeliou, N., Jickells, T., Kanakidou, M., Kershaw, P., Laj, P., Levermore, J., Li, D., Liss, P., Liu, K., Mahowald, N., Masque, P., Materic, D., Mayes, A.G., McGinnity, P., Osvath, I., Prather, K.A., Prospero, J.M., Revell, L.E., Sander, S.G., Shim, W.J., Slade, J., Stein, A., Tarasova, O. and Wright, S. 2022. Microplastics and Nanoplastics in the Marine-Atmosphere Environment. Nature Reviews Earth & Environment 3: 393–405
  6. ↑ 6.0 6.1 6.2 Pabortsava, K. and Lampitt, R.S. 2020. High concentrations of plastic hidden beneath the surface of the Atlantic Ocean. Nat. Commun. 11, 4073
  7. ↑ van Sebille, E., Wilcox, C., Lebreton, L., Maximenko, N., Hardesty, B.D., van Franeker, J.A. and Eriksen, M., Siegel, D., Galgani, F. and Law, K.L. 2015. A global inventory of small floating plastic debris. Environ Res Lett 10, 124006
  8. ↑ 8.0 8.1 Lebreton, L., Egger, M. and Slat, B. 2017. A global mass budget for positively buoyant macroplastic debris in the ocean. Scientific Reports 9, 12922
  9. ↑ 9.0 9.1 Zhu, X., Rochman, C.M., Hardesty, B.D. and Wilcox, C. 2024. Plastics in the deep sea – A global estimate of the ocean floor reservoir. Deep Sea Research Part I: Oceanographic Research Papers 206, 104266
  10. ↑ Chenillat, F., Huck, T., Maes, C., Grima, N. and Blanke, B. 2021. Fate of floating plastic debris released along the coasts in a global ocean model. Marine Pollution Bulletin 165, 112116
  11. ↑ Ritchie, H. 2023. How much plastic waste ends up in the ocean? https://ourworldindata.org/how-much-plastic-waste-ends-up-in-the-ocean
  12. ↑ Harris, P.T., Maes, T. and Raubenheimer, K. 2023. A marine plastic cloud - Global mass balance assessment of oceanic plastic pollution. Continental Shelf Research 255, 104947
  13. ↑ Lindeque, P.K., Cole, M., Coppock, R.L., Lewis, C.N., Miller, R.Z., Watts, A.J.R., Wilson-McNeal, A., Wright, S.L. and Galloway, T.S. 2020. Are we underestimating microplastic abundance in the marine environment? A comparison of microplastic capture with nets of different mesh-size. Environ. Pollut. 265, 114721
  14. ↑ Mishra, S., Rath, C.C. and Das, A.P. 2019. Marine microfiber pollution: A review on present status and future challenges. Marine Pollution Bulletin 140: 188–197
  15. ↑ 15.0 15.1 Andrady, A.L. 2022. Weathering and fragmentation of plastic debris in the ocean environment. Marine Pollution Bulletin 180, 113761
  16. ↑ Leistenschneider, C., Wu, F., Primpke, S., Gerdts, G. and Burkhardt-Holm, P. 2024. Unveiling high concentrations of small microplastics (11–500 μm) in surface water samples from the southern Weddell Sea off Antarctica. Science of the Total Environment 927, 172124
  17. ↑ 17.0 17.1 17.2 Viel, T., Manfra, L., Zupo, V., Libralato, G., Cocca, M. and Costantini, M. 2023. Biodegradation of Plastics Induced by Marine Organisms: Future Perspectives for Bioremediation Approaches. Polymers 2023, 2673
  18. ↑ 18.0 18.1 18.2 Vaksmaa, A., Hernando-Morales, V., Zeghal, E. and Niemann, H. 2021. Microbial Degradation of Marine Plastics: Current State and Future Prospects 5. Biotechnology for Sustainable Environment, Eds. S. J. Joshi et al. Springer Nature Singapore
  19. ↑ Krueger, M.C., Harms, H. and Schlosser, D. 2015. Prospects for microbiological solutions to environmental pollution with plastics. Appl. Microbiol. Biotechnol. 99: 8857–8874
  20. ↑ 20.0 20.1 Danso, D., Chow, J. and Streit, W.R. 2019. Plastics: Environmental and Biotechnological Perspectives on Microbial Degradation. Applied and Environmental Microbiology 85: 1095-1119
  21. ↑ Ward, C.P., Armstrong, C.J., Walsh, A.N., Jackson, J.H. and Reddy, C.M. 2019. Sunlight converts polystyrene to carbon dioxide and dissolved organic carbon. Environ. Sci. Technol. Lett. 6: 669–674
  22. ↑ 22.0 22.1 22.2 22.3 22.4 22.5 Tekman, M. B., Walther, B. A., Peter, C., Gutow, L. and Bergmann, M. 2022. Impacts of plastic pollution in the oceans on marine species, biodiversity and ecosystems. WWF Germany, Berlin, pp. 221
  23. ↑ 23.0 23.1 Guzzetti, E., Sureda, A., Tejada, S. and Faggio, C. 2018. Microplastic in marine organism: Environmental and toxicological effects. Environmental Toxicology and Pharmacology 64: 164–171
  24. ↑ Ugwu, K., Herrera, A. and Gomez, M. 2021. Microplastics in marine biota: A review. Marine Pollution Bulletin 169, 112540


The main author of this article is Job Dronkers
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