Difference between revisions of "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, including Nylon, Kevlar, Nomex), 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 ('macroplastics' and 'mesoplastics', > 1 mm) and microscopic pieces ('microplastics', < 1 mm), which are a result of physical, chemical and biological weathering and fragmentation of macroscopic waste. The smallest plastic fragments (< 1 μm) are called nanoplastics<ref>Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESAMP) 2015. Sources Fate and Effects of Microplastics in the Marine Environment: A Global Assessment</ref>. 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.
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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 ([https://en.wikipedia.org/wiki/Polyurethane PUR]), polyethylene ([https://nl.wikipedia.org/wiki/Polyetheen PE]), polyamide ([https://nl.wikipedia.org/wiki/Polyamide PA]), polyethylene terephthalate ([https://en.wikipedia.org/wiki/Polyethylene_terephthalate PET]), polystyrene ([https://en.wikipedia.org/wiki/Polystyrene PS]), polyvinyl chloride ([https://en.wikipedia.org/wiki/Polyvinyl_chloride PVC]) and polypropylene ([https://en.wikipedia.org/wiki/Polypropylene PP]). Part of the plastic waste ends up in the oceans. This waste consists of plastic debris visible with the naked eye ('macroplastics' > 2.5 mm and 'mesoplastics' > 1 mm, according to the GESAMP expert group<ref>Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESAMP) 2015. Sources Fate and Effects of Microplastics in the Marine Environment: A Global Assessment</ref>) and microscopic pieces. At an international research workshop hosted by NOAA on the effects and fate of microplastic marine debris, participants suggested an upper size limit of 5 mm for microplastics<ref>Arthur, C. D., Opfer, S. E. and Bamford, H. A. 2009. Standardization of field methods for collection of microplastic marine debris: pelagic, shoreline and sediment protocols. New Orleans, USA, 19-23 November 2009</ref>. Microplastics (MP) are particles that are either originally manufactured at micro size (primary MP) or are the result of physical, chemical and biological weathering and fragmentation of macroscopic debris (secondary MP). The smallest plastic fragments (< 1 μm) are called nanoplastics.
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Microplastics occur in various shapes:<ref name=K24>Kushwaha, M., Shankar, S., Goel, D., Singh, S., Rahul, J., Rachna, K. and Singh, J. 2024. Microplastics pollution in the marine environment: A review of sources, impacts and mitigation. Marine Pollution Bulletin 209 (2024) 117109</ref> fragments (irregularly shaped pieces that result from the break down of larger plastic items),  fibers (thin, elongated strands often originating from synthetic textiles or fishing nets), beads (small, spherical particles commonly found in personal care products like exfoliating scrubs and toothpaste), foams (lightweight, porous particles originating from products like polystyrene foam), films (thin, flat pieces that result from the degradation of plastic bags, wrappers, and similar items), pellets (pre-production plastic beads used in manufacturing, often spilled and dispersed into the environment).  
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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==
 
==Budget of plastics in the ocean==
 
The current (2024) global production of plastics is estimated at about 400 million tons. About 8-9 %
 
The current (2024) global production of plastics is estimated at about 400 million tons. About 8-9 %
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of fossil fuels consumed annually are used in the creation of plastics (4–5% as feedstock and 3–4% as energy<ref>Nielsen, T.D., Hasselbalch, J., Holmberg, K., Stripple, J., 2020. Politics and the plastic crisis: a review throughout the plastic life cycle. Wiley Interdiscip Rev Energy Environ. 9, e360</ref>). Polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC) are the most dominant plastics globally (80 % by weight). Products made of plastic or containing plastics eventually become waste. Most waste products are destroyed (incinerated) or stored in landfills, some are recycled and some are released in the natural environment (‘mismanaged’). A small fraction ends up in the ocean, mostly via rivers<ref name=S23>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</ref>, 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 <ref>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</ref><ref>Schmidt, C., Krauth, T. and Wagner, S. 2017. Export of plastic debris by rivers into the sea. Environ. Sci. Technol. 51: 12246–12253</ref><ref name=II>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</ref><ref name=S23/>. The lower values likely underestimate the supply rates of plastics to the ocean. They do not include atmospheric inputs <ref>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</ref> and plastic fragments smaller than <300 μm <ref name=PL>Pabortsava, K. and Lampitt, R.S. 2020. High concentrations of plastic hidden beneath the surface of the Atlantic Ocean. Nat. Commun. 11, 4073</ref>. Sea-based sources account for about 20% of the total input of plastics<ref name=II/>.  
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of fossil fuels consumed annually are used in the manufacturing of plastics (4–5% as feedstock and 3–4% as energy<ref>Nielsen, T.D., Hasselbalch, J., Holmberg, K., Stripple, J., 2020. Politics and the plastic crisis: a review throughout the plastic life cycle. Wiley Interdiscip Rev Energy Environ. 9, e360</ref>). Polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC) are the most dominant plastics globally (80 % by weight). Products made of plastic or containing plastics eventually become waste. Most waste products are destroyed (incinerated) or stored in landfills, some are recycled and some are released in the natural environment (‘mismanaged’). A small fraction ends up in the ocean, mostly via rivers<ref name=S23>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</ref>, but also via sea-based sources (mainly discarded fishing gear) and the atmosphere. About half of the plastic debris larger than 0.5 mm in the Great Pacific Garbage Patch originates from fishing gear<ref>Lebreton, L., Slat, B., Ferrari, F., Sainte-Rose, B., Aitken, J., Marthouse, R., Hajbane, S., Cunsolo, S., Schwarz, A., Levivier, A., Noble, K., Debeljak, P., Maral, H., Schoeneich-Argent, R., Brambini, R. and Reisser, J. 2018. Evidence that the Great Pacific Garbage Patch is rapidly accumulating plastic. Sci. Rep. 8, 4666</ref>. Estimates of the amount of plastic waste entering the sea vary widely, from 0.5 Mton/yr to more than 10 Mton/yr <ref>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</ref><ref>Schmidt, C., Krauth, T. and Wagner, S. 2017. Export of plastic debris by rivers into the sea. Environ. Sci. Technol. 51: 12246–12253</ref><ref name=II>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</ref><ref name=S23/>. The lower values likely underestimate the supply rates of plastics to the ocean. They do not include atmospheric inputs <ref>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</ref> and plastic fragments smaller than <300 μm <ref name=PL>Pabortsava, K. and Lampitt, R.S. 2020. High concentrations of plastic hidden beneath the surface of the Atlantic Ocean. Nat. Commun. 11, 4073</ref>. Sea-based sources account for 10-20% of the total input of plastics<ref name=II/>.  
  
 
[[File:PlasticGarbageBeach.jpg|thumb|450px|right| Beach covered with plastic garbage.  Photo credit: Mel D Cole/World Bank]]
 
[[File:PlasticGarbageBeach.jpg|thumb|450px|right| Beach covered with plastic garbage.  Photo credit: Mel D Cole/World Bank]]
  
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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 <ref>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</ref><ref name=L17>Lebreton, L., Egger, M. and Slat, B. 2017. A global mass budget for positively buoyant macroplastic debris in the ocean. Scientific Reports 9, 12922</ref><ref name=II/>. 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<ref name=II/><ref name=Z24>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</ref>), but could be much larger<ref name=PL>Pabortsava, K. and Lampitt, R.S. 2020. High concentrations of plastic hidden beneath the surface of the Atlantic Ocean. Nat. Commun. 11, 4073</ref>. A large part of the floating plastic macrodebris is trapped along the shorelines (stranded and buried)<ref>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</ref><ref name=L17/><ref>Ritchie, H. 2023. How much plastic waste ends up in the ocean? https://ourworldindata.org/how-much-plastic-waste-ends-up-in-the-ocean</ref>. Another part (probably smaller, estimated at about 3 Mton<ref name=H23>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</ref>) consists of plastic debris accumulated on the ocean floor through the incorporation of plastics into organic particles (e.g., marine snow and fecal material)<ref name=Z24/>.  
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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<ref>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</ref><ref name=L17>Lebreton, L., Egger, M. and Slat, B. 2017. A global mass budget for positively buoyant macroplastic debris in the ocean. Scientific Reports 9, 12922</ref><ref name=II/>. 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<ref name=II/>), but could be an order of magnitude larger<ref name=PL>Pabortsava, K. and Lampitt, R.S. 2020. High concentrations of plastic hidden beneath the surface of the Atlantic Ocean. Nat. Commun. 11, 4073</ref><ref name=Z24>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</ref>.  
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There is compelling evidence from observations and model simulations that the majority of the plastic waste entering the sea does not (directly) reach the global ocean, but is retained in the coastal zone<ref>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</ref><ref name=L17/><ref>Harris, P.T. 2020. The fate of microplastic in marine sedimentary environments: A review and Synthesis. Marine Pollution Bulletin 158, 111398</ref>. Large debris are broken down by mechanical fracturing in high-energy coastal and shelf environments during bedload transport and buried in sediments. Buoyant plastic debris generally become stranded on beaches<ref>Onink, V., Jongedijk, C.E., Hoffman, M.J., Van Sebille, E. and Laufkotter, C. 2021. Global simulations of marine plastic transport show plastic trapping in coastal zones. Environ. Res. Lett. 16, 064053</ref>. Another part of the plastic waste (estimated at about 3-11 Mton<ref name=H23>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</ref>) consists of microdebris accumulated on the ocean floor through the incorporation of plastics into sinking organic particles (e.g., marine snow and fecal material)<ref name=Z24/>.  
  
 
==Micro- and nanoplastics==
 
==Micro- and nanoplastics==
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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<ref>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</ref>. 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<ref name=PL>Pabortsava, K. and Lampitt, R.S. 2020. High concentrations of plastic hidden beneath the surface of the Atlantic Ocean. Nat. Commun. 11, 4073</ref>.  
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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<ref>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</ref>. 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<ref name=PL>Pabortsava, K. and Lampitt, R.S. 2020. High concentrations of plastic hidden beneath the surface of the Atlantic Ocean. Nat. Commun. 11, 4073</ref>. According to these figures, concentrations of micro and nanoplastics in the oceans fall in the range 0.01-10 mg/m<sup>3</sup>.
  
 
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. <ref>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</ref>. Breakdown of microplastic debris in the ocean into nano-fibers is extremely slow and therefore considered less important<ref name=A22>Andrady, A.L. 2022. Weathering and fragmentation of plastic debris in the ocean environment. Marine Pollution Bulletin 180, 113761</ref>. Several studies report that the smaller the plastic particles, the more toxic they are to marine biota<ref>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</ref>.
 
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. <ref>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</ref>. Breakdown of microplastic debris in the ocean into nano-fibers is extremely slow and therefore considered less important<ref name=A22>Andrady, A.L. 2022. Weathering and fragmentation of plastic debris in the ocean environment. Marine Pollution Bulletin 180, 113761</ref>. Several studies report that the smaller the plastic particles, the more toxic they are to marine biota<ref>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</ref>.
  
 
==Degradation of plastics in the marine environment==
 
==Degradation of plastics in the marine environment==
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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<ref name=V23>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</ref>. 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<ref name=V21>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</ref>. Fungi, like bacteria, can also  use plastics as carbon source. Laccase enzymes play an important role in fungi attachment<ref name=V23/>.  
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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<ref name=V23>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</ref>. 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<ref name=V21>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</ref>. Fungi, like bacteria, can also  use plastics as carbon source. Laccase enzymes play an important role in fungi attachment<ref name=V23/>. However, the energy yield of enzymatic degradation of microplastics is low, compared to the energy that can be retrieved from other carbon sources. The selective advantage for microbes to degrade microplastics is limited. This may explain why no significant adaptation of the ocean microbiome has occurred in spite of ubiquitous micro- and nanoplastics<ref>Gambarini, V., Drost, C.J., Kingsbury, J.M., Weaver, L., Pantos, O., Handley, K.M. and Lear, G. 2024. Uncoupled: investigating the lack of correlation between the transcription of putative plastic-degrading genes in the global ocean microbiome and marine plastic pollution. Environmental Microbiome 19, 34</ref>. 
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There is no firm evidence that the worldwide most produced synthetic polymers with a pure carbon backbone (including polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC) and polystyrene (PS)) can be degraded by microbes. No enzymes are known that act on these polymers<ref name=D19>Danso, D., Chow, J. and Streit, W.R. 2019. Plastics: Environmental and Biotechnological Perspectives on Microbial Degradation. Applied and Environmental Microbiology 85: 1095-1119</ref><ref name=L22>Lear, G., Maday, S.D,M., Gambarini, V., Northcott, G., Abbel, R., Kingsbury, J.M., Weaver, L., Wallbank, J.A. and Pantos, O. 2022. Microbial abilities to degrade global environmental plastic polymer waste are overstated. Environ. Res. Lett. 17, 043002</ref>.
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Microbial degradation requires (artificially) aged polymer since physicochemical ageing processes increase the abundance of monomers and oligomers such that they may be degraded by microbial activity.
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Weathering and photo-oxidation are generally considered to be the most important processes for the degradation of plastics. They result in a change in the chemical, physical and mechanical properties of the plastics and included chemical additives<ref>Bridson, J.H., Masterton, H., Theobald, B., Risani, R., Doake, F., Wallbank, J.A., Maday, S.D.M., Lear, G.,  Abbel, R., Smith, D.A., Kingsbury, J.M., Pantos, O., Northcott, G.L. and Gaw, S. 2024. Leaching and transformation of chemical additives from weathered plastic deployed in the marine environment. Marine Pollution Bulletin 198, 115810</ref>. 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. UV photodegradation breaks the polymer backbone of exposed plastics down, releasing low molecular-weight dissolved organic matter like carboxylic acids, monomers and more complex hydrocarbons or halogenated compounds<ref name=M24>Menger, F., Romerscheid, M., Lips, S., Klein, O., Nabi, D., Gandrass, J., Joerss, H., Wendt-Potthoff, K., Bedulina, D., Zimmermann, T., Schmitt-Jansen, M., Huber, C., Bohme, A., Ulrich, N., Beck, A.J., Profrock, D., Achterberg, E.P., Jahnke, A. and Hildebrandt, L. 2024. Screening the release of chemicals and microplastic particles from diverse plastic consumer products into water under accelerated UV weathering conditions. Journal of Hazardous Materials 477, 135256</ref>. These degradation products of UV radiation can be further decomposed by microbes. Chemical additives can also be a target for microbial degradation but may interfere with enzyme degradation processes<ref name=D19>Danso, D., Chow, J. and Streit, W.R. 2019. Plastics: Environmental and Biotechnological Perspectives on Microbial Degradation. Applied and Environmental Microbiology 85: 1095-1119</ref>. Polystyrene can be completely mineralized by photooxidation<ref>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</ref>.  
  
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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<ref name=V21/>. 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<ref>Krueger, M.C., Harms, H. and Schlosser, D. 2015. Prospects for microbiological solutions to environmental pollution with plastics. Appl. Microbiol. Biotechnol. 99: 8857–8874</ref>). 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<ref name=D19>Danso, D., Chow, J. and Streit, W.R. 2019. Plastics: Environmental and Biotechnological Perspectives on Microbial Degradation. Applied and Environmental Microbiology 85: 1095-1119</ref>.
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Photodegradation is one of the major driving forces inducing the disintegration of plastics in the environment, including the reduction of chain length, surface oxidation, formation of microplastics and loss of mechanical properties<ref name=M24/>. It can be a key mechanism for fragmentation of plastic debris floating in the oceans. However, 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. This suggests that most of the abundant micro- and nanoplastics found in the oceans originate from primary sources (e.g. cosmetic products, washing/grinding of synthetic materials) or from photodegradation of macroplastic debris on land (especially stranded along shores) prior to reaching water<ref name=L17>Lebreton, L., Egger, M. and Slat, B. 2017. A global mass budget for positively buoyant macroplastic debris in the ocean. Scientific Reports 9, 12922</ref><ref>Andrady, A.L. 2022. Weathering and fragmentation of plastic debris in the ocean environment. Marine Pollution Bulletin 180, 113761</ref>.
  
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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 <ref name=V23/>.
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Effective microbial degradation has been demonstrated for heterochain plastic polymers such as polylactic acid (PLA), polycaprolactone (PLC), polyurethane (PU) and polyethylene terephthalate (PET). These polymers contain oxygen, nitrogen or other heteroatoms in their backbone structure, which increases the susceptibility to degradation by biotic and abiotic hydrolysis reactions<ref name=L22/>. In the marine environment, bacteria found in association with polyethylene terephthalate (PET) and ester-based polyurethane (PUR) have genes encoding for monooxygenases, peroxidases and dehydrogenases, that can facilitate the initial degradation of these plastics<ref name=V21/>. However, currently known PUR- and PET-active enzymes and microorganisms have a very low conversion rate (on the order of decades to centuries<ref>Krueger, M.C., Harms, H. and Schlosser, D. 2015. Prospects for microbiological solutions to environmental pollution with plastics. Appl. Microbiol. Biotechnol. 99: 8857–8874</ref>). Even for plastics marketed as biodegradable, such as PLA, prior treatment (such as heating) is necessary to accelerate microbial degradation<ref name=L22/>.
  
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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 and included chemical additives<ref>Bridson, J.H., Masterton, H., Theobald, B., Risani, R., Doake, F., Wallbank, J.A., Maday, S.D.M., Lear, G.,  Abbel, R., Smith, D.A., Kingsbury, J.M., Pantos, O., Northcott, G.L. and Gaw, S. 2024. Leaching and transformation of chemical additives from weathered plastic deployed in the marine environment. Marine Pollution Bulletin 198, 115810</ref>. 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<ref name=D19>Danso, D., Chow, J. and Streit, W.R. 2019. Plastics: Environmental and Biotechnological Perspectives on Microbial Degradation. Applied and Environmental Microbiology 85: 1095-1119</ref>. Polystyrene can be completely mineralized by photooxidation<ref>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</ref>. 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<ref name=A22>Andrady, A.L. 2022. Weathering and fragmentation of plastic debris in the ocean environment. Marine Pollution Bulletin 180, 113761</ref>. Origins of the abundant micro- and nanoplastics found in the oceans must therefore be attributed primarily to photo-oxidation of plastic debris on land (especially along the shorelines) prior to reaching water.   
+
Yoshida et al. (2016<ref>Yoshida, S., Hiraga, K., Takehana, T., Taniguchi, I., Yamaji, H., Maeda, Y., Toyohara, K., Miyamoto, K., Kimura, Y. and Oda, K. 2016. A bacterium that degrades and assimilates poly(ethylene terephthalate). Science 351: 1196–1199</ref>) discovered that the bacterium ''Ideonella sakaiensis'' produces enzymes able to degrade polyethylene terephthalate (PET), using it as a carbon and energy source. However, only low-crystallinity PET can be degraded this way, whereas most PET water bottles have a high crystalline content which is not amenable to microbial degradation without prior treatment<ref>Wallace, N.E., Adams, M.C., Chafin, A.C., Jones, D.D., Tsui, C.L. and Gruber, T.D. 2020. The highly crystalline PET found in plastic water bottles does not support the growth of the PETase-producing bacterium ''Ideonella sakaiensis''. Environ. Microbiol. Rep. 12: 578–582</ref>. Another bacterium, ''Pseudalkalibacillus'' sp. MQ-1, has shown skill to increase the hydrophilicity of polyethylene and to decrease the crystallinity<ref>Meng , Q., Yi, X., Zhou, H., Song, H., Liu, Y., Zhan, J. and Pan, H. 2024. Isolation of marine polyethylene (PE)-degrading bacteria and its potential degradation mechanisms. Marine Pollution Bulletin 207, 116875</ref>.   
  
 
==Ecosystem impacts==
 
==Ecosystem impacts==
Line 32: Line 43:
 
* 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<ref name=T22>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</ref>.  
 
* 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<ref name=T22>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</ref>.  
 
*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<ref name=T22>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</ref>.
 
*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<ref name=T22>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</ref>.
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*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<ref name=G18>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</ref>. 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<ref name=T22/>. There is 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<ref>Ugwu, K., Herrera, A. and Gomez, M. 2021. Microplastics in marine biota: A review. Marine Pollution Bulletin 169, 112540</ref>. Microplastics impair the capability of fish larvae to evade predators<ref>Pannetier, P., Morin, B., Le Bihanic, F., Dubreil, L., Clérandeau, C., Chouvellon, F., van Arkel, K., Danion, M. and Cachot, J. 2020.  Environmental samples of microplastics induce significant toxic effects in fish larvae. Environment International 134, 105047</ref>. As these organisms are more likely to be eaten, the concentration of microplastics in their predators will be biomagnified<ref>Hajji, A.L. and Lucas, K.N. 2024. Anthropogenic stressors and the marine environment: From sources and impacts to solutions and mitigation. Marine Pollution Bulletin 205, 116557</ref>.
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*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<ref name=G18>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</ref>. 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<ref name=T22/>. (Substantially lower percentages were reported in another literature survey<ref>Kibria, G. 2024. Contamination of coastal and marine bird species with plastics: Global analysis and synthesis. Marine Pollution Bulletin 206, 116687</ref>) There is 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<ref>Ugwu, K., Herrera, A. and Gomez, M. 2021. Microplastics in marine biota: A review. Marine Pollution Bulletin 169, 112540</ref>. Microplastics impair the capability of fish larvae to evade predators<ref>Pannetier, P., Morin, B., Le Bihanic, F., Dubreil, L., Clérandeau, C., Chouvellon, F., van Arkel, K., Danion, M. and Cachot, J. 2020.  Environmental samples of microplastics induce significant toxic effects in fish larvae. Environment International 134, 105047</ref>. As these organisms are more likely to be eaten, the concentration of microplastics in their predators will be biomagnified<ref>Hajji, A.L. and Lucas, K.N. 2024. Anthropogenic stressors and the marine environment: From sources and impacts to solutions and mitigation. Marine Pollution Bulletin 205, 116557</ref>. As an example of biomagnification, a krill-feeding blue whale consumes 2.51 to 43.6 kg of microplastics per day<ref>Kahane-Rapport, S.R., Czapanskiy, M.F., Fahlbusch, J.A., Friedlaender, A.S., Calambokidis, J., Hazen, E.L., Goldbogen, J.A. and Savoca, M.S. 2022. Field measurements reveal exposure risk to microplastic ingestion by filter-feeding megafauna. Nat. Commun. 13, 6327</ref>.
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* Adverse effects have been observed in many organisms exposed to synthetic fibres, particularly in species at the lower end of the food chain. Laboratory studies report, among other things, reduced fitness of zooplankton populations, reduced filtration rates of bivalves, increased mortality in crabs and gill damage in fish, together with increased oxidative stress and altered metabolism<ref name=C23>Concato, M., Panti, C., Baini, M., Galli, M., Giani, D. and Fossi, M.C. 2023. Detection of anthropogenic fibres in marine organisms: Knowledge gaps and methodological issues. Marine Pollution Bulletin 191, 114949</ref>.  
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* Micro and nanoplastics can cross cell membranes<ref name=D19/>. Adverse effects have been observed in many organisms exposed to synthetic fibres, particularly in species at the lower end of the food chain. Laboratory studies on the exposure of marine biota to microplastics report, among other things, reduced fitness of zooplankton populations, reduced filtration rates of bivalves, increased mortality in crabs and gill damage in fish, together with increased oxidative stress and altered metabolism<ref name=C23>Concato, M., Panti, C., Baini, M., Galli, M., Giani, D. and Fossi, M.C. 2023. Detection of anthropogenic fibres in marine organisms: Knowledge gaps and methodological issues. Marine Pollution Bulletin 191, 114949</ref>.  
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*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<ref name=T22/>.
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*Chemical additives or pollutants adsorbed to plastic debris from seawater can become bioavailable when ingested by organisms. 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<ref name=G18>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</ref>. Micro- and nanoplastics have a high surface area to volume ratio and can therefore adsorb greater amounts of hydrophobic contaminants from the environment than macroplastics. They can also be taken up by a much wider range of species, 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 micro- nanoplastics<ref name=T22/>. 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<ref name=T22/>.
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During manufacture, plastics are commonly provided with additives to impart desirable and beneficial properties. Such added substances can include [https://en.wikipedia.org/wiki/Bisphenol_A bisphenol-A (BPA)], [https://en.wikipedia.org/wiki/Per-_and_polyfluoroalkyl_substances per- and polyfluoroalkyl (PFAS)], brominated flame retardants, [[PCB]]s and a group of substances called phthalates. For example, PFASs are added for water, heat, and grease resistance, while bisphenols are added for flexibility properties. They belong to the so-called endocrine disruptors, which disrupt the hormonal functions of animals (including humans). In zooplankton, exposure to PFAS yields developmental retardation, reduced fecundity, oxidative stress, irregular apoptosis, cellular proliferation, and generates reactive oxidative species which further damages DNA<ref name=C23/>. Bisphenols have significant negative impacts on reproductive function in marine animals, where males experience more prominent and lasting effects. Other added endocrine disruptors have similar detrimental effects when they enter the gastrointestinal tract of organisms after leaching from ingested microplastics. They affect movement, behavior, sexual development, growth (including growth abnormalities), survival and reproduction. They can bioaccumulate in organisms such as fish and tadpoles. Early life stages, amphibians, fish and invertebrates are particularly sensitive<ref name=T22/>.
 
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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<ref name=V21/>. 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<ref name=G18>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</ref>.
 
  
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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<ref name=T22/>.
+
==Toxic leachates==
 +
During manufacture, plastics are commonly provided with additives to impart desirable and beneficial properties. In some cases, additives represent up to 30% of the plastic weight<ref name=L22/>. Release of these additives is boosted by UV weathering<ref name=M24/>. Frequently added chemicals include [https://en.wikipedia.org/wiki/Bisphenol_A bisphenol-A (BPA)], [https://en.wikipedia.org/wiki/Per-_and_polyfluoroalkyl_substances per- and polyfluoroalkyl (PFAS)], brominated flame retardants, [[PCB]]s and a group of substances called phthalates<ref>Gallo, F., Fossi, C., Weber, R., Santillo, D., Sousa, J., Ingram, I., Nadal, A. and Romano, D. 2018. Marine litter plastics and microplastics and their toxic chemicals components: the need for urgent preventive measures. Environ Sci. Eur. 30, 13</ref>. For example, PFASs are added for water, heat, and grease resistance, while bisphenols are added for flexibility properties. These chemicals belong to the so-called endocrine disruptors, which disrupt the hormonal functions of animals (including humans). In zooplankton, exposure to PFAS yields developmental retardation, reduced fecundity, oxidative stress, irregular apoptosis, cellular proliferation, and generates reactive oxidative species (ROS) which further damages DNA<ref name=C23/>. Bisphenols have significant negative impacts on reproductive function in marine animals, where males experience more prominent and lasting effects. Other added endocrine disruptors have similar detrimental effects when they enter the gastrointestinal tract of organisms after leaching from ingested microplastics. They affect movement, behavior, sexual development, growth (including growth abnormalities), survival and reproduction. They can bioaccumulate in organisms such as fish and tadpoles. Early life stages, amphibians, fish and invertebrates are particularly sensitive<ref name=T22/>. Additives also include toxicologically relevant trace metals and metalloids such as As, Cd, Sb, Sn or Pb <ref name=M24/>
  
 
It must be noted that not all fibres in the environment are of synthetic origin. Fibres of natural origin (e.g. cotton, viscose) are also widespread. These natural fibres are often chemically treated with dyes, additives, and flame retardants and therefore pose a similar threat<ref name=C23>Concato, M., Panti, C., Baini, M., Galli, M., Giani, D. and Fossi, M.C. 2023. Detection of anthropogenic fibres in marine organisms: Knowledge gaps and methodological issues. Marine Pollution Bulletin 191, 114949</ref>.
 
It must be noted that not all fibres in the environment are of synthetic origin. Fibres of natural origin (e.g. cotton, viscose) are also widespread. These natural fibres are often chemically treated with dyes, additives, and flame retardants and therefore pose a similar threat<ref name=C23>Concato, M., Panti, C., Baini, M., Galli, M., Giani, D. and Fossi, M.C. 2023. Detection of anthropogenic fibres in marine organisms: Knowledge gaps and methodological issues. Marine Pollution Bulletin 191, 114949</ref>.
  
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To date (2020), the problem of nanoplastic effects in the marine environment has been assessed almost exclusively on the basis of laboratory exposure studies. Linking laboratory results to natural exposure levels is essential to avoid under- or overestimation of the problem. Improvement of existing techniques is needed to obtain reliable and reproducible information on exposure levels in natural environments<ref>Piccardo, M., Renzi, M. and Terlizzi, A. 2020. Nanoplastics in the oceans: Theory, experimental evidence and real world. Marine Pollution Bulletin 157, 111317</ref>.
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To date (2022), the problem of micro- and nanoplastic effects in the marine environment has been assessed almost exclusively on the basis of laboratory exposure studies. These studies typically look at concentrations of micro- and nanoplastics that are much higher (up to g/l) than concentrations in the natural marine environment (< 10 µg/l) <ref>Ahmed A.S.S., Billah, M.M., Ali, M.M., Bhuiyan, M.K.A., Guo,L., Mohinuzzaman,M., Hossain, M.B., Rahman, M.S., Islam, M.S., Yan, M.and Cai, W. 2023. Microplastics in aquatic environments: A comprehensive review of toxicity, removal, and remediation strategies. Science of the Total Environment 876, 162414</ref><ref name=K24/>. It was shown, for example, that at such high concentrations, plastic leachates impair photosynthesis in common picophytoplankton species, such as ''Prochlorococcus'' <ref>Tetu, S.G., Sarker, I., Schrameyer, V., Pickford, R., Elbourne, L.D.H., Moore, L.R. and Paulsen, I.T. 2019. Plastic leachates impair growth and oxygen production in Prochlorococcus, the ocean’s most abundant photosynthetic bacteria. Commun Biol. 2019;2, 184</ref>, whereas no effect of microplastics on photosynthesis was found in studies with naturally occurring concentrations<ref>Sjollema, S.B., Redondo-Hasselerharm, P., Leslie, H.A., Kraak, M.H.S. and Vethaak, A.D. 2016. Do plastic particles affect microalgal photosynthesis and growth? Aquat. Toxicol. 170: 259–261</ref>. Linking laboratory results to natural exposure levels is essential to avoid overestimation of the problem. Improvement of existing techniques is needed to obtain reliable and reproducible information on exposure levels in natural environments<ref>Piccardo, M., Renzi, M. and Terlizzi, A. 2020. Nanoplastics in the oceans: Theory, experimental evidence and real world. Marine Pollution Bulletin 157, 111317</ref>.
  
  

Latest revision as of 11:28, 20 December 2024

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 plastic debris visible with the naked eye ('macroplastics' > 2.5 mm and 'mesoplastics' > 1 mm, according to the GESAMP expert group[1]) and microscopic pieces. At an international research workshop hosted by NOAA on the effects and fate of microplastic marine debris, participants suggested an upper size limit of 5 mm for microplastics[2]. Microplastics (MP) are particles that are either originally manufactured at micro size (primary MP) or are the result of physical, chemical and biological weathering and fragmentation of macroscopic debris (secondary MP). The smallest plastic fragments (< 1 μm) are called nanoplastics.

Microplastics occur in various shapes:[3] fragments (irregularly shaped pieces that result from the break down of larger plastic items), fibers (thin, elongated strands often originating from synthetic textiles or fishing nets), beads (small, spherical particles commonly found in personal care products like exfoliating scrubs and toothpaste), foams (lightweight, porous particles originating from products like polystyrene foam), films (thin, flat pieces that result from the degradation of plastic bags, wrappers, and similar items), pellets (pre-production plastic beads used in manufacturing, often spilled and dispersed into the environment).

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 about 400 million tons. About 8-9 % of fossil fuels consumed annually are used in the manufacturing of plastics (4–5% as feedstock and 3–4% as energy[4]). Polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC) are the most dominant plastics globally (80 % by weight). Products made of plastic or containing plastics eventually become waste. Most waste products are destroyed (incinerated) or stored in landfills, some are recycled and some are released in the natural environment (‘mismanaged’). A small fraction ends up in the ocean, mostly via rivers[5], but also via sea-based sources (mainly discarded fishing gear) and the atmosphere. About half of the plastic debris larger than 0.5 mm in the Great Pacific Garbage Patch originates from fishing gear[6]. Estimates of the amount of plastic waste entering the sea vary widely, from 0.5 Mton/yr to more than 10 Mton/yr [7][8][9][5]. The lower values likely underestimate the supply rates of plastics to the ocean. They do not include atmospheric inputs [10] and plastic fragments smaller than <300 μm [11]. Sea-based sources account for 10-20% of the total input of plastics[9].

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[12][13][9]. 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[9]), but could be an order of magnitude larger[11][14].

There is compelling evidence from observations and model simulations that the majority of the plastic waste entering the sea does not (directly) reach the global ocean, but is retained in the coastal zone[15][13][16]. Large debris are broken down by mechanical fracturing in high-energy coastal and shelf environments during bedload transport and buried in sediments. Buoyant plastic debris generally become stranded on beaches[17]. Another part of the plastic waste (estimated at about 3-11 Mton[18]) consists of microdebris accumulated on the ocean floor through the incorporation of plastics into sinking organic particles (e.g., marine snow and fecal material)[14].

Micro- and nanoplastics

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[19]. 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[11]. According to these figures, concentrations of micro and nanoplastics in the oceans fall in the range 0.01-10 mg/m3.

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. [20]. Breakdown of microplastic debris in the ocean into nano-fibers is extremely slow and therefore considered less important[21]. Several studies report that the smaller the plastic particles, the more toxic they are to marine biota[22].

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[23]. 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[24]. Fungi, like bacteria, can also use plastics as carbon source. Laccase enzymes play an important role in fungi attachment[23]. However, the energy yield of enzymatic degradation of microplastics is low, compared to the energy that can be retrieved from other carbon sources. The selective advantage for microbes to degrade microplastics is limited. This may explain why no significant adaptation of the ocean microbiome has occurred in spite of ubiquitous micro- and nanoplastics[25].

There is no firm evidence that the worldwide most produced synthetic polymers with a pure carbon backbone (including polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC) and polystyrene (PS)) can be degraded by microbes. No enzymes are known that act on these polymers[26][27]. Microbial degradation requires (artificially) aged polymer since physicochemical ageing processes increase the abundance of monomers and oligomers such that they may be degraded by microbial activity.

Weathering and photo-oxidation are generally considered to be the most important processes for the degradation of plastics. They result in a change in the chemical, physical and mechanical properties of the plastics and included chemical additives[28]. 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. UV photodegradation breaks the polymer backbone of exposed plastics down, releasing low molecular-weight dissolved organic matter like carboxylic acids, monomers and more complex hydrocarbons or halogenated compounds[29]. These degradation products of UV radiation can be further decomposed by microbes. Chemical additives can also be a target for microbial degradation but may interfere with enzyme degradation processes[26]. Polystyrene can be completely mineralized by photooxidation[30].

Photodegradation is one of the major driving forces inducing the disintegration of plastics in the environment, including the reduction of chain length, surface oxidation, formation of microplastics and loss of mechanical properties[29]. It can be a key mechanism for fragmentation of plastic debris floating in the oceans. However, 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. This suggests that most of the abundant micro- and nanoplastics found in the oceans originate from primary sources (e.g. cosmetic products, washing/grinding of synthetic materials) or from photodegradation of macroplastic debris on land (especially stranded along shores) prior to reaching water[13][31].

Effective microbial degradation has been demonstrated for heterochain plastic polymers such as polylactic acid (PLA), polycaprolactone (PLC), polyurethane (PU) and polyethylene terephthalate (PET). These polymers contain oxygen, nitrogen or other heteroatoms in their backbone structure, which increases the susceptibility to degradation by biotic and abiotic hydrolysis reactions[27]. In the marine environment, bacteria found in association with polyethylene terephthalate (PET) and ester-based polyurethane (PUR) have genes encoding for monooxygenases, peroxidases and dehydrogenases, that can facilitate the initial degradation of these plastics[24]. However, currently known PUR- and PET-active enzymes and microorganisms have a very low conversion rate (on the order of decades to centuries[32]). Even for plastics marketed as biodegradable, such as PLA, prior treatment (such as heating) is necessary to accelerate microbial degradation[27].

Yoshida et al. (2016[33]) discovered that the bacterium Ideonella sakaiensis produces enzymes able to degrade polyethylene terephthalate (PET), using it as a carbon and energy source. However, only low-crystallinity PET can be degraded this way, whereas most PET water bottles have a high crystalline content which is not amenable to microbial degradation without prior treatment[34]. Another bacterium, Pseudalkalibacillus sp. MQ-1, has shown skill to increase the hydrophilicity of polyethylene and to decrease the crystallinity[35].

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[36].
  • 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[36].
  • 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[37]. 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[36]. (Substantially lower percentages were reported in another literature survey[38]) There is 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[39]. Microplastics impair the capability of fish larvae to evade predators[40]. As these organisms are more likely to be eaten, the concentration of microplastics in their predators will be biomagnified[41]. As an example of biomagnification, a krill-feeding blue whale consumes 2.51 to 43.6 kg of microplastics per day[42].
  • Micro and nanoplastics can cross cell membranes[26]. Adverse effects have been observed in many organisms exposed to synthetic fibres, particularly in species at the lower end of the food chain. Laboratory studies on the exposure of marine biota to microplastics report, among other things, reduced fitness of zooplankton populations, reduced filtration rates of bivalves, increased mortality in crabs and gill damage in fish, together with increased oxidative stress and altered metabolism[43].
  • Chemical additives or pollutants adsorbed to plastic debris from seawater can become bioavailable when ingested by organisms. 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[37]. Micro- and nanoplastics have a high surface area to volume ratio and can therefore adsorb greater amounts of hydrophobic contaminants from the environment than macroplastics. They can also be taken up by a much wider range of species, 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 micro- nanoplastics[36]. 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[36].

Toxic leachates

During manufacture, plastics are commonly provided with additives to impart desirable and beneficial properties. In some cases, additives represent up to 30% of the plastic weight[27]. Release of these additives is boosted by UV weathering[29]. Frequently added chemicals include bisphenol-A (BPA), per- and polyfluoroalkyl (PFAS), brominated flame retardants, PCBs and a group of substances called phthalates[44]. For example, PFASs are added for water, heat, and grease resistance, while bisphenols are added for flexibility properties. These chemicals belong to the so-called endocrine disruptors, which disrupt the hormonal functions of animals (including humans). In zooplankton, exposure to PFAS yields developmental retardation, reduced fecundity, oxidative stress, irregular apoptosis, cellular proliferation, and generates reactive oxidative species (ROS) which further damages DNA[43]. Bisphenols have significant negative impacts on reproductive function in marine animals, where males experience more prominent and lasting effects. Other added endocrine disruptors have similar detrimental effects when they enter the gastrointestinal tract of organisms after leaching from ingested microplastics. They affect movement, behavior, sexual development, growth (including growth abnormalities), survival and reproduction. They can bioaccumulate in organisms such as fish and tadpoles. Early life stages, amphibians, fish and invertebrates are particularly sensitive[36]. Additives also include toxicologically relevant trace metals and metalloids such as As, Cd, Sb, Sn or Pb [29]

It must be noted that not all fibres in the environment are of synthetic origin. Fibres of natural origin (e.g. cotton, viscose) are also widespread. These natural fibres are often chemically treated with dyes, additives, and flame retardants and therefore pose a similar threat[43].

To date (2022), the problem of micro- and nanoplastic effects in the marine environment has been assessed almost exclusively on the basis of laboratory exposure studies. These studies typically look at concentrations of micro- and nanoplastics that are much higher (up to g/l) than concentrations in the natural marine environment (< 10 µg/l) [45][3]. It was shown, for example, that at such high concentrations, plastic leachates impair photosynthesis in common picophytoplankton species, such as Prochlorococcus [46], whereas no effect of microplastics on photosynthesis was found in studies with naturally occurring concentrations[47]. Linking laboratory results to natural exposure levels is essential to avoid overestimation of the problem. Improvement of existing techniques is needed to obtain reliable and reproducible information on exposure levels in natural environments[48].


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References

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