[<< wikibooks] Perspectives of Aquatic Toxicology/Chapter 3: Micro-plastics: An Emerging Pollutant in an Aquatic Ecosystem
= Chapter 3: Microplastics: An Emerging Pollutant in an Aquatic Ecosystem =

== Introduction ==

=== Introduction to Microplastics ===
If you were to causally glance at your surroundings, chances are you would see a multitude  of products that are made of, or utilize plastics. Plastics are long chains of polymers synthesized from both organic and inorganic materials such as carbon, hydrogen, silicon, oxygen, and chloride which are usually acquired from natural resources such as natural gas, oil, and coal (Ivar do Sul et al., 2013). Plastic is considered to be a fairly recent invention.  In 1907, the first plastic Bakelite was synthesized. However, it wasn’t until the 1940s that plastic production began in earnest (Cole et al., 2011). Copious amounts of plastics have been synthesized and released into the environment. One of the major draws of plastic is its durability, which may ultimately lead to its downfall. Tons of plastic pieces find their way each year into various watersheds, and subsequently the oceans. Many studies have demonstrated the hazards of plastics to aquatic wildlife. (Figure 2 & 3)

These studies focus on macroplastics: plastics that are easy to see with the naked eye. However, plastics come in a variety of sizes: macroplastics, microplastics, and nanoplastics. There is no standard definition for microplastics, and thus the term “microplastic” is controversial. The term does not refer to the standard micro unit as seen in the International System of Units (1-999 μm). A microplastic can be a particle that has a diameter of < 10 mm, or < 5 mm, or 2-6 mm, or <1 mm (Cole et. al., 2011) (Figure 4). Nor is there a set definition for nanoplastics (plastic particles that are smaller than microplastics) and that absence further complicates matters. Again, the term “nano” in nanoplastic does not refer to the nano size typically seen in a laboratory setting. For this chapter, the term microplastic means any plastic particle that is < 5 mm in diameter but greater than 100 nm.  The term nanoplastic refers to plastics that are between 1-100 nm (Jovanović, 2017). 

=== Types of Microplastics in the Environment ===
There are two major categories of microplastics: primary and secondary. Primary microplastics are plastics manufactured in the microplastic size range and often used in cosmetics, facial cleansers, facial exfoliants, air blasting media, and even medicine. Occasionally, these microplastics are called micro-beads or micro-exfoliants. Primary microplastics can also include virgin plastic production pellets. Secondary microplastics are microplastic particles that have fragmented from macroplastics (Cole et al., 2011). Common synthetic plastics used today include polypropylene, polyethylene, polystyrene, polyvinyl chloride, polyethylene terephthalate, and as well as others (Ivar do Sul et al., 2013) (Figure 5).

=== Quantification and Location of Microplastics Worldwide ===
As previously mentioned, the first major plastic, Bakelite, was produced in 1907; however, the mass production of plastic containing compounds began much later in the 1940s (Cole et al., 2011).  Since then, the amount of plastic produced worldwide has continued to increase at a rapid rate. In 2017, it was estimated that over 350 million metric tons of plastic were produced, and since then plastic production has increased even further (PlasticsEurope, 2018). Of the plastic manufactured each year, roughly up to 10% of it will subsequently end up in aquatic environments where it will reside for extended periods of time. Thus, the quantify of plastic accumulating in aquatic environments is continuously increasing. By 2050 it is estimated that the amount of plastic present in the oceans will either be equal to or greater than the total weight of fish in that same environment (Jovanović, 2017). In 2014, van Sebille, et al. estimated the amount of microplastics on the surface of the ocean to be between 15-51 trillion particles, together weighing between 93-236 thousand metric tons. 
Microplastic particles have been located worldwide, even in areas that should be devoid of plastics. Additionally, the type of microplastic present in such environments does not substantially differ from region to region: it is possible to find polystyrene microplastics everywhere (Koelmans, et al., 2016). Microplastics are present at various depths in the water column. Typically, microplastics such as polyethylene and polypropylene are very buoyant and may remain on the surface of water. However, it is also possible for other microplastics such as polyvinyl chloride to be less buoyant or for the buoyancy of the particle to decrease or increase due to surface growth of microbial films. These microplastics particles may then be suspended in the water column or be located on, or in the sediment (Ivar do Sul, et al., 2013). 
Models used to estimate the prevalence of microplastics in the ocean often focus on microplastics located on the surface of the water. As such, they capture a brief snapshot of microplastic presence in one specific area of the water column. However, vertical mixing of particles in the water column may occur as a result of wind (Ivar do Sul, et al., 2013.) Additionally, more microplastic particles enter into an aquatic ecosystem after storms, flooding, or other types of extreme weather (Cole, et al., 2011). This makes it difficult to obtain an accurate image of how many pieces of microplastic are present on the ocean surface. Nevertheless, there are areas more or less abundant in the number of microplastic particles compared to other areas. For instance, there are fewer microplastics present in the tropics above 45ᵒN and below 45ᵒS, and the remote coastline off Western Australia. The highest concentrations are found in centers of subtropical gyres located in North Atlantic and North Pacific regions. The highest concentration in subtropical gyres is estimated to be 108 particles km-2. The highest counts of microplastics are located in Mediterranean and North Pacific basins while microplastics present in the North Pacific basin contain the largest total mass (Figure 6). This is likely due to the vast area of the North Pacific basin and the large quantity of plastic waste entering the ocean from the coastlines of the United States and Asia. Ultimately, the majority of microplastics lie in regions of low concentration. Further data is needed to verify these estimates, especially in less analyzed regions such as South America (van Sebille, et al., 2015). 

== Methods Used to Study Microplastics ==
Various methods are used to quantify the number of microplastics present in an aquatic environment. These include beach combing, sediment testing, biological sampling, marine observational surveys, and marine trawls. Beach combing involves collecting, identifying, and quantifying all litter items on the shore of a specific coastline region, and If done periodically it can illustrate the debris accumulation during a specific time frame. This process is best suited for quantifying macroplastics and occasionally larger microplastics. For example, it is possible to quantify plastic resin pellets called Mermaid’s Tears while beach combing.  Sediment testing involves taking sediment samples from the sediment of an aquatic environment and quantifying the amount of microplastics in benthic regions. Biological sampling involves quantifying the amount of microplastics consumed by aquatic organisms (See Section C for examples). Marine observational sampling is conducted by divers or onboard observers who record the size, location and types of plastic observed in the water. However, the small nature of microplastics allows them to be unobservable and ergo, undetected. Marine trawls constitute one of the more popular methods used to capture, identify and quantify microplastics where fine meshes or nets are dragged behind boats. It is common to use a plankton net in these studies; the size of the net can vary between 0.1 mm to 0.5 mm (Cole et al., 2011; van Sebille et al., 2015).
Microplastics are typically quantified either by sediment testing or marine trawls. It is important to filter out impurities from the samples and to promote the migration of microplastics to various solution surfaces after samples have been collected (Figure 7). Water samples are run through a coarse filter to remove microplastic particles and other larger contaminants. Meanwhile, sediment samples are slurried with saline water to promote the migration of microplastics to the sample’s surface. Additionally, mineral salts may be added to either the water sample or the slurried sample to increase water density, as more microplastics may float to the surface of a sample when water density is greater. Evaporation is then commonly used to concentrate the microplastic particles at the surface of the sample. Subsequently, regions of the samples with microplastics will have been isolated and can be removed for quantification. Following removal, microplastics in surface water samples can be stained with a lipophilic dye such as Nile Red in order to increase ease of visualization. Microbiota that are present in the solution or on the microplastic pieces are not stained with the dye. Sometimes hot dilute mineral acid is used to remove any additional biomass. Finally, the microplastic pieces identified and quantified using various types of microscopy such as Raman spectroscopy, FTIR spectroscopy, electron microscopy, and optical microscopy (Andrady, 2011).

== Ingestion of Microplastics ==
Plastics are ubiquitous: it is not uncommon to find bits and microplastic pieces in and around aquatic wildlife. Haunting images of turtles trapped inside plastic have invaded the media for years. Other images have shown tons of plastics almost spilling out of the guts of aquatic birds or whales. (Figure 8) In fact, at least 44% of all marine bird species have ingested plastics (Andrady, 2011).
Both macroplastics and microplastics have been ingested by a variety of aquatic wildlife at different trophic levels. Microplastics do not discriminate against species (although some species may discriminate against microplastics--e.g. selective filter feeders) and have been found in and around both invertebrates and vertebrates. (Table 1)

Table 1: Examples of species that have ingested microplastics

=== Ingestion of Microplastics by Invertebrates ===
Colonies of microscopic organisms can grow microbial biofilms on various microplastics, and these biofilms can accumulate on microplastics in less than a week. As a result, species of invertebrates and algae have been able to grow on the surfaces of these particles (Cole, et.al, 2011). Additionally, insects such as Halobates micans and H. sericeus use plastic pellets as oviposition sites (Majer et al., 2012; Goldstein et al., 2012). For example,  the numbers of eggs of H. sericeus juveniles and adults in the North Pacific Ocean were significantly and positively correlated with the number of microplastics present. Furthermore, 34% of all plastic pellets found in the western Atlantic had eggs attached to them (Barnes, et al., 2009). 
Laboratory experiments have been used often to highlight how invertebrates ingest microplastic particles. In 2004, the first experiment to test microplastic ingestion by barnacles (Semibalanus balanoides), amphipods (Orchestia gammarellus), and lugworms (Arenicola marina) was performed, and demonstrated that these three species could ingest microplastics (Thompson et al., 2004). Other studies have tested various mollusks, crustaceans, annelids, and echinoderms. For example, one of the more commonly studied mussels in regard to microplastic ingestion is Mytilus edulis. This species ingests and accumulates microplastics within 12 hours of exposure. Interestingly, not only were microplastic pieces found in the intestines, they were also present in the gills and hemolymph of the mussels (Browne et al., 2008; von Moos et al., 2012). Additionally, microplastics found in these mussels were later transferred to a higher trophic level: Carcinus maenus. As a result, microplastic particles have been found in the hemolymph and various other tissues of these crabs (Farrell and Nelson, 2012). Thus far, studies of microplastic ingestion by benthic crustaceans have been limited. However, 83% of sampled lobsters in the Clyde Sea had microplastic particles in their stomachs. Additionally, lobsters were shown to ingest microplastics within 24 hours of being exposed (Murray and Cowie, 2011). Microplastic ingestion has even been identified in humboldt squids (Dosidicus gigas), and sea cucumbers (Holothuria) have been known to preferentially feed on nylon and PVC fragments over sediment grains (Braid et al., 2012; Graham and Thompson, 2009). Finally, many types of zooplankton have also been studied for the possibility of microplastic ingestion, and It was confirmed that many taxa do indeed ingest the particles (Cole et al., 2013). The possibility of ingestion of microplastics by various types of invertebrates seems endless.

=== Ingestion of Microplastics by Vertebrates ===
Microplastic particles have been discovered in the guts of various vertebrates since 1972. Some of the first particles detected in larvae and juvenile fish were found in Psuedopleuronects flounder, while some of the first particles detected in adult fish were found in Morone america and Protonus evolans (Carpenter et al., 1972; Hoss and Settle, 1990). The potential for possible ingestion of microplastics became concerning when a study (Boerger et al., 2010) found synthetic fragments in 35% of planktivorous fish located in the North Pacific Central Gyre. A study (Lusher et al., 2013) that evaluated demersal and pelagic fish in the coastal waters around the United Kingdom found further evidence of microplastic ingestion among various species, with 36% of the species tested having ingested fibers and microplastic fragments. Additionally, mesopelagic fish located in the North Pacific were found to have ingested microplastic particles and fibers (Davison and Asch, 2011). To further illustrate the enormity of the problem, 40% of lantern fish tested in the Marianna Islands--a region that contains less microplastic particles--were found to have ingested microplastics, further illustrating how ubiquitous microplastics are in terms of ingestation (Van Noord, 2013). 
Microplastic ingestion by vertebrates is not limited to the marine environment. Studies have found these particles in fish living in estuaries as well. For example, a study conducted in the western South Atlantic Ocean reported that mojarras (Gerreidae), estuarine drums (Sciaenidae), and catfishes (Ariidae) all ingested microplastics. All three species feed on or just below the surface of the sediment, and the most popular ingested microplastic was blue nylon thread. They most likely ingested the microplastic particles accidentally through normal suction feeding, eating contaminated prey, and/or by actively feeding on plastics with biofilm (Ramos et al., 2012; Dantas et al., 2012; Possatto et al., 2011). Microplastics have been found ingested by fish in all areas of the world and at all levels of the water column. 
Although not commonly thought of as aquatic vertebrates, shore birds have also been known to ingest microplastics. In fact, the ingestion of plastics by birds has been studied for the past four decades, although many of these studies did not distinguish between microplastics and macroplastics. Microplastic pellets have been found in the guts of migratory petrels--shearwaters and prions--in the Atlantic and south-western Indian Ocean among others since the 1980s. Plastic pellets and other plastic fragments were found in 80% of two Fulmarus glacialis colonies in the Canadian Arctic (Provencher et al., 2009). Additionally, the Fulmarus glacialis species has been monitored in both the North Sea and the Netherlands for the past 30 years. During this time, it was discovered that while the number of plastic pellets decreased by 50% in 20 years, the number of plastic fragments tripled. Interestingly, juvenile Fulmarus glacialis ingested more plastic than the adults. It was also noted that there was an increased presence of ingested microplastics in areas near highly industrialized areas specializing in either shipping or fishing (Van Franeker et al., 2011; Kühn and van Franeker, 2012). Furthermore, around 90% of all fulmars sampled in the North Pacific Ocean had ingested microplastics (Avery-Gomm et al., 2012). Microplastics were found ingested by birds located in Iceland as well; but overall, fewer had ingested the particles. Thus, it has been hypothesized that birds further north are less likely to be contaminated by microplastics because there are fewer microplastics available on the surface of the water in these regions (Van Franeker et al., 2011; Kühn and van Franeker, 2012). This hypothesis was also proposed by a study done on the ingestion of microplastics by Uria lomvia in Nunavut, Canada in which 11% of the birds investigated had ingested microplastics; however, the authors hypothesized that the low number may be due to these birds feeding below the water surface and are therefore less likely to ingest floating plastics (Provencher et al., 2010). It is interesting to note that birds never exposed to marine environments may ingest microplastics as well. For example, 83.5% of fledgling cory shearwaters (Calonectris diomedea) were found to have ingested microplastics. These chicks were exposed to pieces of microplastic by eating the regurgitated food from their parents (Rodríguez et al., 2012). Other bird species known to ingest microplastics include Larus glaucescens, P. nigripes, Phoebastria immutabilis, among others (Lindborg et al., 2012; Gray et al., 2012; Avery-Gomm et al., 2013). 
Research related to ingestion of microplastics by marine mammals is limited; however, it has been shown by analyzing their scat that marine mammals do indeed ingest microplastics. For example, fur seal scat (A. gazelle and Arctocephalus tropicalis) collected on Macquaire Island contained plastic fragments and pellets that were 2-5 mm in size. These plastics were believed to have come from the animal’s prey that ingested microplastics present in or on the water (Eriksson and Burton, 2003). Future studies will likely highlight the presence of microplastics found in aquatic mammal intestines. 

== Degradation of Microplastics ==
Macroplastics eventually become microplastics, and microplastic pieces eventually degrade into nanoplastics. It is estimated that it will take approximately 320 years for a 1 mm sized microplastic to degrade into a nanoplastic within a marine environment (Koelmans et.al, 2015). Degradation is the chemical process by which the average molecular weight of a polymer is reduced. Plastic degradation to any size will eventually weaken the material and the particles may become so weak they fall apart into a powdery substance subsequently degraded further. Plastic is said to have mineralized when all carbon present in the polymer is converted into CO2: mineralization is the goal of degradation. The process of degradation can fall into one of five major categories: thermal degradation, photodegradation, thermooxidative degradation, biodegradation, and hydrolysis. 

=== Different types of Degradation ===
Thermal degradation is the degradation that occurs via high temperatures. This is not considered a mechanism by which microplastics degrade in the environment. Photodegradation is the process by which light, usually sunlight, degrades a material. UV-B is the primary component in photodegradation. It is also considered to be one of the fastest methods of degradation for microplastics, working best for particles that are exposed to air or lying on a beach; however, the degradation rate decreases when the microplastic particles enter the water.  Additionally, photodegradation is often the precursor to other types of degradation, and it is often followed by thermooxidative degradation. Thermooxidative degradation is the process by which particles are slowly broken down by oxidation in moderate temperatures--a process which can continue to degrade the microplastic particle as long as oxygen surrounds the microplastic. The quantity of oxygen present decreases in an aquatic environment, and a lack of oxygen and colder temperature will often decrease the degradation rate of a microplastic. Additionally, the growth of organisms on a microplastic can decrease the rate of degradation. This process is different from biodegradation. Biodegradation is the process by which living organisms degrade microplastics and it is typically carried out by microscopic organisms; however, these organisms are rare in nature. Finally, hydrolysis is the process by which water is used to degrade a compound; it is one of the slowest in the marine environment and rarely occurs. Once again, photodegradation and thermooxidative degradation are the most common ways in which microplastic particles are degraded into smaller pieces and eventually into nanoplastics (Andrady, 2011).

=== Equilibrium Partitioning Equations ===
It is helpful to discuss the concept of equilibrium partitioning before discussing the relative toxicity of each microplastic. Chemicals often partition in either organic matter or water, as discussed in previous chapters. Additionally, toxicants can have sorption to the plastic particles themselves. However, microplastics do not follow typical equilibrium partitioning concepts: they do not have a log Kow. Instead, specialized equilibrium factors for microplastics must be used to determine how a toxicant may partition. There are many different factors that affect the overall kinetics of the equilibrium partitioning process such as the equilibrium between water-biota, water-sediment, and water-plastic. The equilibrium partitioning equation used to determine if a toxicant will sorb to a piece of microplastic (KPL) is defined as:
KPL = CPL/Cwwhere CPL (μg/kg) is the concentration of the toxicant in plastic and CW is the concentration of the toxicant in water. If KPL > CPL/CW, the toxicant can desorb from plastic to water, but if KPL  5 mm in diameter.
9.    Marine observational sampling: Involves divers or onboard observers who record the size, location, and types of plastics in the water.
10.  Marine trawls: Use of fine meshes or nets to collect microplastic particles or marine organisms.
11.  Microplastic: Plastic particle that is < 5 mm in size but greater than 100 nm.
12.  Nanoplastic: Plastics that are between 1-100 nm.
13.  Persistent organic pollutants (POPs): Lipophilic chemicals that prefer to concentrate on hydrophobic regions of microplastics.
14.  Photodegradation: Process by which light, usually sunlight, degrades a material.
15.  Physical Adverse Response: Within the context of this chapter, a negative physiological response to a microplastic particle.
16.  Plastics: Long chains of polymers that are synthesized from both organic and inorganic materials such as carbon, hydrogen, silicon, oxygen, and chloride; usually acquired from natural resources such as natural gas, oil and coal.
17.  Primary microplastics: Plastics manufactured in the microplastic size range.
18.  Secondary microplastics: Microplastic particles that have fragmented from macroplastics.
19.  Thermal degradation: Degradation that occurs via high temperatures.
20.  Thermooxidative degradation: Process by which particles slowly break down through oxidation in moderate temperatures.
21.  Toxicant: Human-made chemicals that cause a deleterious or harmful response to an organism.

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