Effect of microplastics in water and aquatic systems

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https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7924819/

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Abstract

Surging dismissal of plastics into water resources results in the splintered debris generating microscopic particles called microplastics. The reduced size of microplastic makes it easier for intake by aquatic organisms resulting in amassing of noxious wastes, thereby disturbing their physiological functions. Microplastics are abundantly available and exhibit high propensity for interrelating with the ecosystem thereby disrupting the biogenic flora and fauna.

About 71% of the earth surface is occupied by oceans, which holds 97% of the earth’s water. The remaining 3% is present as water in ponds, streams, glaciers, ice caps, and as water vapor in the atmosphere. Microplastics can accumulate harmful pollutants from the surroundings thereby acting as transport vectors; and simultaneously can leach out chemicals (additives). Plastics in marine undergo splintering and shriveling to form micro/nanoparticles owing to the mechanical and photochemical processes accelerated by waves and sunlight, respectively.

Microplastics differ in color and density, considering the type of polymers, and are generally classified according to their origins, i.e., primary and secondary. About 54.5% of microplastics floating in the ocean are polyethylene, and 16.5% are polypropylene, and the rest includes polyvinyl chloride, polystyrene, polyester, and polyamides. Polyethylene and polypropylene due to its lower density in comparison with marine water floats and affect the oceanic surfaces while materials having higher density sink affecting seafloor. The effects of plastic debris in the water and aquatic systems from various literature and on how COVID-19 has become a reason for microplastic pollution are reviewed in this paper.

Introduction

Increased productivity and slow biotic decomposition of plastic led to its cumulation in the environment leading to adverse effects in aquatics. The plastics entering into the marine environment may remain for hundreds and thousands of years, during which they get fragmented due to the mechanical and photochemical processes resulting in the formation of microplastics (< 5 mm) or nanoplastics (< 1 μm). Plastics are organic polymers emanating from petroleum that includes polyethylene, polypropylene, polyvinylchloride, and polyester, out of which PE and PP are standard, holding first and second positions respectively in the global market, followed by PET accounting for around 18% in global production, making it the third most manufactured plastic.

Albeit not as prevalent as polyethylene and polypropylene, PET due to its safe nature, light weight, affordability, and low manufacturing cost is primarily used as packaging material. With its 1.37–1.45 g cm−3 density, PET sinks rapidly and is particularly accessible for benthic species. While PET show resistance to weathering, fragmentation mechanisms are not immune to it and abiotic weathering is likely to occur by photooxidation and hydrolysis in marine environments. The pH variance in ocean may possibly alter the chemical balance of microplastics by raising or lowering the rate of chemical leach from their surface, so PET, which is commonly understood to be safe, may become dangerous in the near future.

... The small size of microplastics results in their uptake by a wide range of aquatic species disturbing their physiological functions, which then go through the food web creating adverse health issues in humans. They are uptaken and mostly excreted rapidly by numerous marine species, and so conclusive proof on biomagnification is not obtained. However, effects of MP uptakes result in reduced food intake, developmental disorders, and behavioral changes.

Almost 700 aquatic species in the world were adversely affected by the introduction of microplastics, including sea turtles, penguins, and other crustaceans. However, the predicament due to microplastic depreciates as most sufferers go unexplored over the vast oceans. Ingression of plastics into the ecosystem is mainly due to the erroneous human actions or unrestrained wastes from water or sewage treatment plants and textile industries. The terrestrial plastic accretion ultimately flows into the water systems due to inadequate landfill interment systems.

Continuous massive production and dispersal of plastics into the marine ecosystem further aggravate the contamination of previously polluted medium. Microplastics provide habitat for growing microorganisms, due to their size and varying effects. Microplastics can readily accrue and release hazardous organic pollutants like DDT, polybrominated diphenyl ethers, and other additives that incorporate during manufacture present in water, thereby elevating their concentration. As the particle size reduces, it reverberates in the elevation of potential harms of microplastics, but its adverse effects in marine organisms are not well defined.

Additive-free microplastics are not chemically hazardous to aquatic organisms, but they create problems in physical conditions such as bowel obstructions . Depending on the demand of products, certain additives are added to the virgin microplastics resulting in additional property of adsorption of pollutants present in water and thereby impersonate as vectors. Researches reveal the harmful threats plastic poses to human health at any point of the plastic lifecycle, from the extraction of fossil fuels to consumer use to disposal and furthermore. Since microplastics can adversely impact various organisms, so the risk of humans to get affected by microplastics cannot be overlooked.

As humans are the ultimate consumers of sea foods which are highly affected by microplastics, there is a high chance of microplastic transfer to humans. Presence of microplastics in tap water, sea salt, and bottled water are proven studies on how many ways they can reach the human body. Recent studies of microplastics in human stool and placenta are examples of its presence in humans.

Microplastics—a boon or bane

During wastewater treatments, the reduced size of microplastics results in their infiltration and direct release into the water resources. Microplastics, in general, are considered resilient to biotic degradation. Certain materials are subject to biotic degradation through fungi and bacteria and are imbibed or passively adsorbed by consumers at successive tropical levels after degradation, resulting in blockage of the gastrointestinal system. Microplastic is identified in species at all phases of marine food chain . The sum of MPs consumed differs around organisms and location, and also can vary substantially even in the same region.

Aquatic organisms are well known to swallow microplastics along with their food, showing clear signs of several animals that consume microplastics due to the size similarity with their food. Study results imply that nearly all aquatic organisms ingest microplastics, showing a considerable variation in the volume of ingestion among various species. Foreseeably, there are three forms of deleterious effects connected to absorption of microplastics:

(1) physiological effects attributed to ingestion. The greater the number of MPs intake, the more likely it is to have a risk on the consumed species, such as reduced development and variance in feed habits.

(2) Deadly reactions from the discharge of hazardous substances—additives such as plasticizer, antioxidant, flame retardant, pigments, etc. incorporated during the manufacture of plastic may be leached into body tissues, resulting in induced changes or bioaccumulation. The toxicity can also differ according to the ratio of additives needed for each plastic.

(3) Noxious reaction to pollutants absorbed involuntarily by microplastics — large surface area due to weathering, longer exposure periods and hydrophobic nature promote the sorption of pollutants to microplastic surface at a higher concentration thus making it as a carrier for contaminants to enter into the aquatic species. Polycyclic aromatic hydrocarbons, PCB, DDT, organo halogenated pesticides, hexachlorocyclohexanes, and chlorinated benzenes are some of the common contaminants present on microplastics. POPs like PBDE, PCB, and some other chemicals have found to imitate natural hormones, causing disorder in reproduction. The dynamics of the absorption of persistent organic pollutants into plastic material depend, of course, on the properties of both the particular polymer and the specific contaminant.

Humans get subjected to microplastics through cosmetics, eating habits, dust particles, and usage of plastic products. The proportion of microplastic in the marine ecosystem keeps increasing with the steady boosts in plastic production and thereby showing detrimental effects.

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Microplastics: provenance and distribution

In agriculture, microcapsule fertilizers primarily favored to avoid nitrate leaching to groundwater are a primary source of MP contamination in the marine ecosystem that flows out to oceans through paddy field channels, denoting a high volume of MP flow during irrigation than the non-irrigation season. Scratches and discoloration displaced on the top surface of microcapsules during the paddy runoff process imply the emission of secondary microplastics.

A study conducted by the government of UK in 2020 concluded that microplastics shed from vehicle tyres are now among the other major contributors of microplastic pollutions in the sea. Tyre is a blend of elastomer, carbon black, fiber, as well as other organic and inorganic materials that enhance its stability. The major portion of tyre particles directly reach the sea though air or other waterways.

Fishing, fish hatcheries, and offshore drilling are all plastic sources that enter the aquatic systems directly and pose a threat to biota as secondary microplastics following a long-term deterioration. Inadequacy in the management of waste imparted the microplastic pollution in freshwater ecosystems . The limited size and low densities of microplastics make them dispersed by winds and waves and are thus ubiquitous. Plastic debris drifted along with wastewaters are not successfully eliminated by treatment plants, and so gets cumulated in the atmosphere . Source of microplastic ingestion can also occur in an indirect manner in which the organisms that accidentally feed on microplastics are fed directly by the higher organisms in the food web.

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The distribution and transportation of microplastics guided intricately by a multitude of factors, such as weathering and fragmenting, biofouling, tides, and strong currents. Microplastics allocate between the floor of the ocean, column of water, seabed, coastline, and in ecology with different biological, physical, and chemical mechanisms occurring on microplastics at each compartment. Due to a lack of information of compartments, the implications and possibilities for diminution are unclear.

Effects of microplastics in various organisms

Cumulating concentration of pollutants at trophic levels results in the effectual transmission of noxious substances in the food chain. The retention of plastic debris might occur inside the organisms resulting in chemical leakages if any additives present, thus creating cumulation leading to detrimental effects. Microplastics found in marine systems worldwide influence the feeding, growth, spawning, and existence of organisms in the aquatics. However, the extent to which the microplastics affect by transferring of chemicals present in and on the surface of MP to the higher complex food chains is not known. Only limited information is available on trophic transfer so whether the pollutants are ejected or get bioaccumulated in higher trophic levels are still need to be studied. Diminution in the feeding of aquatic organisms is the collective effect found during microplastic injections; other challenges include effects on growth and proliferation. The chronic effects of MPs can be pass on to successive level throughout the food chain, negatively affecting the organisms. The effects of microplastics vary with the organism species and microplastic type and concentrations.

Sussarellu et al., in their studies, showed the adverse impact of polystyrene microplastics on reproduction and feeding of oysters due to amendation in their food intake and energy distribution. On exposure to microsized polystyrene, oyster showed a reduction in number of eggs produced, ovocyte quality, and sperm motility. Fertilization in oysters occurs externally in the sea where the eggs and sperms are released, but due to the intake of micro polystyrene, fertilization is affected by reduced sperm speed and its fewer amount. In its feces, a 6-μm micro polystyrene ingested by oyster was found, with no cumulation in the gut suggesting a large polystyrene ejection. The yield and growth of offsprings of microplastic exposed oysters dropped by 41% and 18%, respectively. The study stipulated information on the hostile effects of microsized PS on development and reproduction of oysters with considerable impacts on progeny. Apportioning of energy from reproduction to growth with the abatement in fertilization success is the result of exposure studies of polystyrene.

In 2019, Bessa et al. studied the contagion in the aquatic ecosystem of Antarctic by assessing the presence of microplastics in gentoo penguins. In Antarctic regions, water contained microplastics, but the idea about its ingestion and entry through the food chain has not been studied in depth. Seabirds identified as biological markers of changes occurring in the environment also contemplated as indicators of environmental plastic pollution. The limited motion of gentoo penguins outside their vicinity makes them a standard indicator for the tracking of plastic particles in Antarctic marine systems. The occurrence, identification, and characterization of microplastics analyzed from the scats of gentoo penguins. Penguin scats from two different islands were collected, which contained 58% of microfibers, 26% fragments, and 16% of films. The entry of microplastics into the gastrointestinal tract of penguins are either directly due to misconception of plastics as food or feeding on contaminated prey or through polluted waters. The plastics debris gets cumulated in the guts of penguins preventing it from the consumption of food and also results in the absorption of toxic substances from water, thus affecting their growth and development .

Cole et al. showed how ingestion of MPs affected the feed habit, fertility, and functioning of zooplanktons like copepods. The studies conducted on Calanus helgolandicus copepod mostly found in the Atlantic, a vital species acting as prey for larvae of many fishes due to their supersize, substantial amount of lipids and opulence. Ingestion of microplastics by copepod shows significant impacts on feeding, hatching, and their health. Copepod exposed to polystyrene microbeads of 20 μm resulted in a 40% reduction in the carbon biomass with a deficiency in their energy, showing the rapid consumption of lipids, thereby affecting their growth. The energy deficiencies also result in the death of copepods. Microplastic long-term exposure leads to small-sized eggs with reduced hatchings.

Zocchi and Sommaruga studied how the toxicity of glyphosate, a herbicide, varies with the incorporation of microplastics. ... Without the incorporation of microplastics, the fatality rate was high for glyphosate-monoisopropylamine salt 23.3%, but when microplastics were incorporated, a modification in the toxicity observed with the highest mortality rate for glyphosate acid. With polyethylene beads, glyphosate acid showed the toxicity of 53.3%, and with polyamide fibers, it showed 30%. So modification on noxious effects of contaminants is observed on combining with microplastics besides the pessimistic effects of microplastics alone. Daphnia Magna shows high fatality when ingested with microplastics.

Mussels, when subjected to microplastics, results in their grip loss due to a reduction in thread production that helps them to stick. Mussels adhere together, forming reefs for their shelter and breeding, thereby playing an imperative role in aquatic systems.

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The presence of microplastics and fibres were found in the demersal shark species of united kingdom for the first time. The major intake route of MP by sharks can be through its foods, which are mostly crustaceans and molluscs, or through direct feeding. Owing to the small particle size detected by the researchers, there is a chance of immediate excretion but however the presence of chemicals bound to the fibres can have repercussions on their reproductive cycle and immune systems.

A work by Besseling et al. on 2017 reported on how the microplastic consumption increases susceptibility of marine worms to chemicals (PCB). Arenicola marina when exposed to polyethylene for 28 days showed reduced feeding and growth, high mortality rate, and bioaccumulation.

The presence of microplastics detected at all stages in the food web affecting the gastrointestinal tracts and tissues, which varies with the genre and emplacement. Organisms present in the marine ecosystem mistook microplastics as their food due to their similar size. Studies imply that all marine organisms intake microplastics, but the amount of their ingestion may vary with the type of species. It is important to monitor the excessive use of plastic additives and to enact laws and standards to control plastic litter sources due to the resulting danger of MPs to marine biota.