The Harmful Effects of Microplastic Pollution on Aquatic Organisms

In recent years, microplastics have been recognized as the most popular pollutants in marine and fresh waters. Plastic is one of the most used materials in all areas of our lives due to its cheap, light, and durable properties. Because they are used as main or auxiliary materials in almost all industries and branches of industry, the disappearance of plastics, which are in our lives, requires very long processes. Since petroleum-derived plastic wastes, which bacteria cannot consume directly, are decomposed by solar heat and radon, only the resulting compounds can be consumed by bacteria. For this reason, awareness-raising efforts to reduce plastic consumption in daily use all over the world have been accelerated. In parallel with this situation, research on microplastic pollution in both seas and inland waters is carried out intensively. Considering the current consumption habits of us humans, who are at the top of the food chain, the rates of microplastics we are exposed to are at a substantial level. The determination of the amount of microplastics contained in seafood consumed as the main protein source is of great importance in terms of public health and shows the necessity of further scientific research on this subject. The purpose of our study is to reveal the effects of microplastic pollution on aquatic organisms both in sea and inland waters by compiling studies on this subject and to draw attention to microplastic pollution in waters.


Introduction
Water is the main source of life on earth and is the most important substance that all living beings need to maintain their vital functions.Nowadays, our water resources are polluted day by day due to climatic 1305 (2024) 012006 IOP Publishing doi:10.1088/1755-1315/1305/1/012006 2 changes, increasing population, uncontrolled urbanization, unconscious water consumption, and wastes from various industries.In recent years, microplastics have been recognized as the most popular pollutants in marine and fresh waters.Plastics are organic polymers emanating from petroleum that include polyethylene, polypropylene, polyvinyl chloride, and polyester [1].Plastic materials are among the most used materials in industrial and domestic applications due to their features such as being cheap, light, and durable [2].The presence of microplastics, which require long periods of time to disappear, has been extensively investigated in different environments.It is widely accepted that microplastic bioaccumulation in aquatic organisms is a major consequence of global plastic pollution.
The purpose of our study is to reveal the effects of microplastic pollution on aquatic organisms both in sea and inland waters by compiling studies on this subject and to draw attention to microplastic pollution in waters.

Methodology of the study
To access the literature information required to carry out our research, the articles especially from the last 5 years (2019-2023) were scanned in Scopus (https://www.scopus.com/sources.uri)database."Microplastics" and "aquatic organisms" were used as keywords.Excel program from Microsoft Office was used for analysing the trend in the article number during the investigated years.The program VOSviewer [3] was used for visualising the occurrence of the selected keywords from Scopus.Selected articles were then processed with VOSviewer and a map was created based on the author keywords from these articles.

Definition of Microplastics
The term "microplastic", which was first used in 1968, was defined as particles that occur as a result of deformation in plastic materials exposed to high stresses (https://thesciencetech.com/environment/microplastic/).Microplastic particles were reported in the aquatic environment for the first time in 1975 in the Sargasso Sea surface water, and in 2004 these small particles were named "microplastics".The microscopic plastics are fragments of any type of plastic that are less than 5 mm in length, according to the U. S. National Oceanic and Atmospheric Administration (NOAA) and the European Chemicals Agency.The United Nations Environment Programme (UNEP) defines microplastics as any solid plastic particle of 5 mm or less which are insoluble in water.The International Organization for Standardization (ISO) ISO/TC 61 (Plastic)/SC 14 (Environmental Aspect) defines microplastics as any solid plastic particle insoluble in water with dimension between 1 μm and 1000 μm in the ISO/TR 21960:2020 standard terms and definitions.
According to their origin, they can be classified as primary and secondary microplastics.Very small plastic particles (<5 mm) discharged into the environment, such as microbeads or synthetic fibers used in toothpaste, cosmetics, laundry, and textiles, are known as primary microplastics.Those formed as a result of the breakdown of large plastic items after entering the environment are called secondary microplastics [1,2,4].

The sources of microplastics
While excessive urbanization is cited as one of the main causes of microplastic pollution, pollution from industry and land runoff is also included [6].Some predictions have revealed that the weight of microplastics in the ocean can exceed the total weight of fish in the ocean without appropriate plastic waste management [7].

Land-based microplastics via discharges
Microplastics may primarily originate from land-based sources, including inputs from residential areas (e.g., garbage from food packaging, fibers in clothing, and household waste).Additionally, personal hygiene products (e.g., shower gels, face cleansers, and toothpastes) are known to contain microplastics.
Alternatively, microfibers can also originate fromhousehold laundry reported that a single wash cycle can yield up to 1900 fibers [8].Cellulose-based fibers such as rayon have been reported in municipal wastewater [9].

Sea-based sources
River discharges.Fishery and aquaculture activities are considered potential sources of microplastics.Aquaculture materials such as plastic ropes, nets and fishing gear, which are common in fishing, can cause microplastic pollution in waters due to weathering or breakdown [10].Additionally, various meteorological and hydrodynamic events have the capacity to strongly affect microplastic levels in freshwater and marine systems [7].Plastics in the sea break down and shrink to form micro/nanoparticles due to mechanical and photochemical processes accelerated by waves and sunlight, respectively.Atmosferic fallout can be another source of microfiber [11].Meteorological and hydrodynamic events may strongly microplastics levels in European river systems [6].

Studies on Microplastic in Waters
In recent years, microplastics have become one of the most important environmental problems affecting air, soil, and aquatic environments worldwide.It is confirmed by the fact that during the last five years the total number of articles concerning microplastics and aquatic organisms in Scopus database has significantly increased with more than 76%, from 78 in 2019 to 333 in 2023 (Figure 1).Research has been mostly done in the seas, but the number of studies in freshwater is very limited [12].

Harmful Effects on Aquatic Organisms
In Scopus database, the term "microplastics" has the highest co-occurrence frequency with the term "microplastic" because they have the same meaning, but the VOSviewer program determines them as different words.Based on "microplastics" and "aquatic organisms" as searching keywords, 988 articles were selected in which the other most frequent terms that occurred were "nanoplastics" and "oxidative stress" (Figure 2).Here we can see the strong relation of microplastics/microplastic with nanoplastics as a result of the microplstics destruction and the most observed and studied effect on the aquatic organisms which is the oxidative stress.
The increased surface area of microplastics, which promotes the adsorption of various organic (PCBs, PAHs, pesticides, etc.) and inorganic (e.g.metal ions) pollutants in the aquatic environment, makes them of even greater concern.It is reported that improper disposal of plastic waste basically threatens the homeostasis of flora and fauna [2].One of the most polluted seas globally are the Black Sea [13], the bay of Maowei Sea, China, [14] and the the Adriatic Sea around Italy [15], which besides the effluent microplastics could also be related to the topographic features and properties of these seas [7].Lake systems in Asia show the highest microplastic pollution, followed by those in Europe, America, and Africa, as the main factors that determine the level and type of microplastics pollution are the primary economic source, local development levels and the economic structure of the country [16].
Microplastics in the environment can mainly be transferred to organisms via ingestion and transported up the food chain [17].This direct uptake has been reported in several marine animals, including zooplankton shellfish, fish, crustaceans, and megafauna, including cetaceans [18,19].In a study investigating microplastic pollution in water, sand and fish samples along the east coast of India, especially in the states of West Bengal, Odisha and Andhra Pradesh, a significant amount of microplastic was detected in water samples compared to other studies conducted in the world; Approximately 29.6% of fish samples were determined to contain various types of plastic particles ranging in size from ~200 μm to ~2 mm.The amount of plastic found in fish was found to be higher than most previously reported studies worldwide [2].Studies conducted both in seas and freshwaters have shown that microplastics, in particular, cause great damage to aquatic organisms [20].
It was reported that nearly 700 aquatic species around the world have been negatively impacted by the input of microplastics, including sea turtles, penguins, and other crustaceans.According to studies conducted in Brazil's Goiana Estuary, the concentration of microplastics was found to be half that of fish larvae and almost the same as the density of fish eggs [7].The average microplastic concentration in oyster and mussel samples found off the Korean coast was determined as 0.33 ± 0.23 n/g and 1.21 ± 0.68 n/individual, respectively.Also, the annual microplastic intake from bivalves was calculated as 587 n/person⋅year [49].

Harmful effects on Human Health
A pilot study carried out in the Hooghly River Estuary found a direct positive relationship between surface microplastic concentration and the human population density in the surrounding area and the volume of sewage discharged into the Hooghly River Estuary.These data clearly show that the main causes of microplastic pollution are human activities, industrial wastes, and untreated sewage discharges [50].In the research conducted on the comparative evaluation of microplastics in water and sediment in the Elbe River, the largest river in Europe, high microplastics concentrations in the sediment, especially in some Elbe sampling areas, clearly emphasize that microplastics pollution is not limited to certain countries or continents but should be seen as a global problem that we must approach [51].
Microplastics have been detected in fish species like Tilapia sparrmanii (Smith, 1840) of the river systems of Okavango Delta, Botswana [52], Eleutheronema tetradactylum, Rastrelliger sp., Nemipterus japonicus, and Trichiurus lepturus from Lengkong Fish Auction Point, Central Java, Indonesia [53], Oreochromis niloticus from lake Amatitlán [54].The data obtained have reported that microplastic bioaccumulation by fish [55], bivalves [56,57], squids [58] and other edible aquatic organisms, poses a high risk for human health when they are consumed.Since the toxic effects of micro and nanoplastics increase as a result of their reactions with other pollutants, it is of great importance to comprehensively investigate the combined ecotoxicological effects on the food web in aquatic systems [2].
Although it is known that treated water contains fewer particles than raw water, many people unfortunately drink untreated water [59].Microplastics have been detected in the blood, placenta, lungs and even feces of human beings at the top of the food pyramid.It has been reported that an adult human swallows from 39000 to 52000 microplastics particles per year [60].Therefore, research on this subject, which is directly related to human health, should be increased and continued, and extra importance should be given to the detection and monitoring of microplastics along with other pollutants in water resources.

Conclusion
The data clearly show that not only our waters but also all the world are facing a global threat from plastics.The fact that they lead to the formation of nanoplastics due to their corrosion and their potential serious effects on environmental homeostasis necessitates detailed research on microplastics.
First of all, the first step should be to seriously reduce individual plastic use and not to throw plastic waste into the environment uncontrollably.It is of great importance that the relevant institutions of the state show the necessary sensitivity to the issue, and work on recycling waste plastics with new relevant legislation and developing environmentally friendly products that are alternatives to the use of plastic.An effective struggle against microplastics seems possible only with low-cost, high-quality, and environmentally sustainable plastic waste management.In addition, from an ecological perspective, necessary support should be provided for the determination of microplastic pollution in our inland waters and seas and for continuous monitoring research.

Figure 1 .
Figure 1.Annual trends in the number of articles in Scopus database for the period 2019-2023.

Figure 2 .
Figure 2. A map of the co-occurrence of author keywords from Scopus database, using the bibliometric tool VOSviewer based on the "microplastics" and "aquatic organisms" keywords.