Microplastics are small plastic particles in the environment and have become a paramount issue especially in the marine environment. Not unequivocally defined, some marine researchers define microplastics as all plastic particles smaller than 1 mm pertaining to their microscopic size range while others in turn define them as smaller than 5 mm recognizing the common use of 333 μm mesh neuston nets for field sampling. However, scientifically, their integral impact on wildlife is not well established.
There are several suspected sources of microplastics:
- Microplastics which are produced either for direct use, such as for industrial abrasives, exfoliants, cosmetics or rotomilling or for indirect use as precursors (so called resin pellets or nurdles) for the production of manifold consumer products ("primary microplastics").
- Microplastics formed in the environment as a consequence of the breakdown of larger plastic material, especially marine debris, into smaller and smaller fragments (so called "secondary microplastics"). The breakdown is caused by mechanical forces (e.g. waves) and/or photochemical processes triggered by sunlight (especially UVB)
- The shedding of synthetic fibers from textiles by domestic clothes washing. A comparison of microplastic particles in sewage effluent from clothes washing with the makeup of microplastics in the environment has suggested that much of the microplastic pollution sized less than 1 mm may consist of shed synthetic textile fibers.
The abundance and global distribution of microplastics in the oceans has steadily increased over the last few decades with rising plastic consumption worldwide.
Potential impacts on the marine environment 
The scientists that participated in the first International Research Workshop on the Occurrence, Effects and Fate of Microplastic Marine Debris, held September 9–11, 2008 on the University of Washington Tacoma campus in Tacoma, Washington, USA, agreed that microplastics may pose problems in the marine environment based on the following:
- the documented occurrence of microplastics in the marine environment,
- the long residence times of these particles (and, therefore, their likely buildup in the future), and
- their demonstrated ingestion by marine organisms.
So far, research has mainly focused on larger plastic items. Widely recognized problems are associated with entanglement, ingestion, suffocation and general debilitation often leading to death and/or strandings. This raises serious public concern.
In contrast, microplastics are not as conspicuous, being less than 5 mm. Particles of this size are available to a much broader range of species and have been shown to be ingested by deposit-feeding lugworms (Arenicola marina) and filter-feeding mussels (Mytilus edulis), to name just two examples. Ingestion of microplastics by species at the base of the food web causes concern as little is known about its effects. It remains unknown if microplastics may be transferred across trophic levels.
Possible effects of microplastics on marine organisms after ingestion are threefold:
- physical blockage or damage of feeding appendages or digestive tract,
- leaching of plastic component chemicals into organisms after digestion, and
- ingestion and accumulation of sorbed chemicals by the organism.
Small animals are at risk of reduced food intake due to false satiation and resulting starvation or other physical harm. However, long term impacts on marine organisms are currently unknown.
Plastic debris has also been shown to serve as carrier for the dispersal of biota, thus greatly increasing dispersal opportunities in the oceans, endangering marine biodiversity worldwide. The dispersal of aggressive alien and invasive species is as much a topic as the dispersal of cosmopolitan species.
Approximately half of the plastic material introduced to the marine environment is buoyant, but fouling by organisms can induce the sinking of additional plastic debris to the sea floor, where it may interfere with sediment-dwelling species and sedimental gas exchange processes. However, this is of more importance for larger plastic debris.
Microplastics and persistent organic pollutants (POPs) 
Furthermore, plastic particles may highly concentrate and transport synthetic organic compounds (e.g. persistent organic pollutants, POPs) commonly present in the environment and ambient sea water on their surface through adsorption. It still remains unknown if microplastics can act as agents for the transfer of POPs from the environment to organisms in this way, but evidence suggest this to be a potential portal for entering food webs. Of further concern, additives added to plastics during manufacture may leach out upon ingestion, potentially causing serious harm to the organism. Endocrine disruption by plastic additives may affect the reproductive health of humans and wildlife alike.
At current levels, microplastics are unlikely to be an important global geochemical reservoir for POPs such as PCBs, dioxins, and DDT in open oceans. It is not clear, however, if microplastics play a larger role as chemical reservoirs on smaller scales. A reservoir function is conceivable in densely populated and polluted areas, such as bights of mega-cities, areas of intensive agriculture and effluents flumes.
Oil based polymers ('plastics') are virtually non-biodegradable. However, renewable natural polymers are now in development which can be used for the production of biodegradable materials similar to that of oil-based polymers. Their properties in the environment, however, require detailed scrutiny before their wide use is propagated.
See also 
- Plastic particle water pollution (Nurdles)
- Great Pacific Garbage Patch
- Endocrine disruption
- Biodegradable plastic
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- NOAA Marine Debris Program
- Algalita Marine Research Foundation
- The Great Pacific Garbage Patch on YouTube
- Capt. Charles Moore on the seas of plastic – video at TED.com
- Microplastics (Swiss-German)
- Science Daily, Nov. 2, 2007. "'Microplastics' May Pose Previously Unrecognized Pollution Threat" (http://www.sciencedaily.com/releases/2007/10/071029092034.htm)