Occurrence and Distribution of Antibiotics in the Water, Sediment, and Biota of Freshwater and Marine Environments: A Review

Antibiotics, as pollutants of emerging concern, can enter marine environments, rivers, and lakes and endanger ecology and human health. The purpose of this study was to review the studies conducted on the presence of antibiotics in water, sediments, and organisms in aquatic environments (i.e., seas, rivers, and lakes). Most of the reviewed studies were conducted in 2018 (15%) and 2014 (11%). Antibiotics were reported in aqueous media at a concentration of <1 ng/L–100 μg/L. The results showed that the highest number of works were conducted in the Asian continent (seas: 74%, rivers: 78%, lakes: 87%, living organisms: 100%). The highest concentration of antibiotics in water and sea sediments, with a frequency of 49%, was related to fluoroquinolones. According to the results, the highest amounts of antibiotics in water and sediment were reported as 460 ng/L and 406 ng/g, respectively. In rivers, sulfonamides had the highest abundance (30%). Fluoroquinolones (with an abundance of 34%) had the highest concentration in lakes. Moreover, the highest concentration of fluoroquinolones in living organisms was reported at 68,000 ng/g, with a frequency of 39%. According to the obtained results, it can be concluded that sulfonamides and fluoroquinolones are among the most dangerous antibiotics due to their high concentrations in the environment. This review provides timely information regarding the presence of antibiotics in different aquatic environments, which can be helpful for estimating ecological risks, contamination levels, and their management.


Introduction
Today, drugs are an integral part of life and are used to treat diseases of humans and other organisms [1]. Antibiotics are substances produced or derived from an organism that kills other microorganisms and can also inhibit their growth [2]. Since the discovery of the first antibiotic by Fleming (1929), various groups of antibiotics have been identified and developed around the world and applied to treat human, animal, and plant diseases groundwater [21], lakes [22], surface water [23], soils [24], and sewage [25]-and due to the threat posed by antibiotics to human health and other organisms [26], reviewing the latest published works on antibiotics is informative and important. Accordingly, this study was designed to review the occurrence of antibiotics in different parts of marine environments (i.e., water, sediments, and biota) worldwide. In addition, the present review aimed to present the concentrations of antibiotics in sediments, water, and biota in water bodies worldwide and investigate the spatial distribution of antibiotics in the mentioned environments.

Methodology
The Web of Science database was used to find published articles related to the present study. Additionally, for a specialized search, keywords such as 'Antibiotics + Marine environment', 'Antibiotics + Sediment', 'Live creatures + Antibiotics', 'Antibiotic + river + lake + marine + Sea + sediment + organism + water body', and 'Review Antibiotics + microorganism + biota' were used. There were 99 articles on antibiotics in sediments, water, and biota in the environments (aquatic environments), as well as several review studies. To draw the figures, first, the studies were categorized by year, parts of aquatic environments, country, and continent, and then the graphs related to each item were drawn using Excel software. Figure 1a-c show the percentages of studies conducted on antibiotics in marine environments, rivers, and lakes on different continents. The figure shows that the majority of studies for all water resources were conducted in Asia. Figure  1d represents the percentages of studies conducted in different years. According to this figure, more studies in the different environments were conducted in 2018 than in any other year. After the reviews, the information of some articles (e.g., [9,[17][18][19]) was found to be more complete than others, and we considered these articles for summarization. To determine the distribution of antibiotics in different aquatic environments, the geographical coordinates (i.e., longitude and latitude) mentioned in the articles and the concentrations of available antibiotics (sediment: ng/g, water: ng/L) in each study were collected; then, using the Arc Geographic Information System (ArcGIS) 10.3, the map of distribution was prepared.

Classification of Antibiotics
According to their chemical structures, antibiotics are classified into several groups, including macrolides, beta-lactams, tetracyclines, quinolones, fluoroquinolones, sulfonamides, phenicols, and penicillin [14]. The tendency of antibiotics to be present in different environments-such as water, soil, and the atmosphere-depends on their physicochemical properties, octanol/water dividing coefficient (K ow ), distribution coefficient (K d ), separation constants (pK a ), vapor pressure, and Henry's law constant (K H ) [27]. Antibiotics such as penicillin are easily decomposed in the environment. At the same time, fluoroquinolones and tetracyclines are more stable, so they can exist for longer in the environment, spread more, and eventually accumulate at higher concentrations [6]. The presence of beta-lactam rings in the structure of amoxicillin causes its degradation in the environment. Ciprofloxacin and erythromycin are also resistant to degradation because they lack beta-lactam in their structure [28]. Fluoroquinolones and sulfonamides are the most dangerous antibiotics in the environment; however, these antibiotics may be degraded by sunlight [29]. Accumulation of nitrite and nitrogen oxide (a strong greenhouse gas) in aquatic environments due to possible processes of nitrification and denitrification is among the effects of these two antibiotics (fluoroquinolones and sulfonamides) on the environment [30]. Antibiotics can be classified into different categories based on their chemical structure and function, such as beta-lactams (BLs), fluoroquinolones (FQs), macrolides (MLs), sulfonamides (SFs), tetracyclines (TCs), etc. Table 1 summarizes the physicochemical properties of the most common antibiotics. An overview of the physicochemical properties of common antibiotics is briefly presented below.
Macrolide (ML) antibiotics form a group of 12-16 organ lactone rings that are replaced with one or more sugars (amino sugars). Other characteristics of this group include being lipophilic, having low solubility in water, and being weakly acidic. Macrolides are generally bacteriostatic; however, some of these drugs may be bactericidal at very high concentrations [33,37]. This group is often used to treat infections in the respiratory tract, skin, and soft tissue [38].
Beta-lactam antibiotics include a wide range of molecules that contain at least one beta-lactam ring in their molecular structures. These drugs are active against many Grampositive, Gram-negative, and anaerobic organisms, interfering with the cell wall synthesis of reproducing bacteria [39]. The presence of a typical four-membered (β)-lactam ring in the structure of this group gives them unstable thermal properties [40]. Beta-lactams, including penicillin-which accounts for 50-70% of antibiotics-are the most widely used antibiotics [41].
Sulfonamides (SAs) are derived from a p-amino-benzene-sulfonamide functional group. This group has acidic and basic properties [42]. These antibiotics are among the most widely used antibiotics in the world. The decomposition half-life of these antibiotics under light and heat is more than one year. They also have a relatively low adsorption capacity to solid matrices compared to other antibiotics. In addition, other applications of this type of antibiotics are as corrosion inhibitors and in the production of polymers. The most frequent use of these antibiotics is in veterinary medicine [33].
Tetracyclines are one of the main groups of antibiotics used for veterinary and human medicine, agriculture, and as food additives to enhance the growth of animals. These are amphoteric and degradable antibiotics that are unstable in bases but stable in acids. These antibiotics cause severe environmental problems and serious damage to human health. Since conventional WWTPs are not able to fully eliminate these micropollutants, the removal efficiency of tetracyclines in treatment plants has been reported between 12 and 80% [33,43]. Studies have shown that approximately 25-75% or 70-90% of tetracyclines used for treating animals enter the environment through urine and feces [44].  [8,36] The quinolones' properties are fat-solubility and resistance to acidic hydrolysis, alkalinity, high temperatures, and ultraviolet radiation damage. This group has many applications, including the treatment of infectious diseases and the promotion of livestock and aquaculture. They enter aquatic environments through untreated human and animal wastewater or direct discharge from aquaculture products [45].

Occurrence of Antibiotic Pollution in Seawater
Over the past decades, the presence of antibiotics in the environment has created concerns around the world. Antibiotics are widely used in human and veterinary medicine and enter the environment through different pathways. Figure 2 shows the routes of entry of antibiotics into aqueous media. A significant portion of antibiotics enter marine environments through effluents from wastewater treatment plants and river inlets [46]. Sulfamethoxazole, azithromycin, and ciprofloxacin are some antibiotics used by humans, and their residues have been traced in the environment. Sulfamethoxazole, trimethoprim, azithromycin, and enrofloxacin are converted into ciprofloxacin in living organisms and are the most commonly used antibiotics for biota [34].
The presence of antibiotics in the sludge and effluent of municipal treatment plants, hospitals, industrial centers, and livestock farms has resulted in the occurrence of pollution in marine environments, surface water, soil, and groundwater [47]. Consumed drugs and their metabolites are introduced into natural ecosystems through excretion (i.e., urine and feces) after a short time in the bodies of humans/animals [48,49]. The stability and accumulation of antibiotics in marine environments can endanger human health [28]. Chen et al. declared that about 92,700 tons of antibiotics was used in China, of which 53,800 tons was released into the environment. According to these results, it can be concluded that antibiotics are present in surface waters, groundwater, and coastal waters [15]. The presence of these compounds in water resources has become one of the most important public health concerns. Many studies have been conducted to determine the fates and effects of antibiotics in aquatic environments [50]. In marine environments, the concentrations of antibiotics in cold seasons are higher than in warm seasons [51]. The continued entry of antibiotics into the environment may pose potential risks to ecosystems and humans through food chains. Studies in the European Union have shown that ciprofloxacin (CIX), ofloxacin (OFX), erythromycin (ETM), and sulfadiazine (SDZ) can pose high environmental risks to aquatic life [10]. More than 100 types of antibiotics are used by humans and other living organisms. So far, 70 antibiotics have been observed in surface waters and sediments [47]. Figure 3a,b show the distribution of antibiotics in seawater and sediments in marine areas worldwide. According to Figure 3a, the waters most contaminated with antibiotics were found in Iran and China.
applications, including the treatment of infectious diseases and the promotion of livesto and aquaculture. They enter aquatic environments through untreated human and anim wastewater or direct discharge from aquaculture products [45] .

Occurrence of Antibiotic Pollution in Seawater
Over the past decades, the presence of antibiotics in the environment has creat concerns around the world. Antibiotics are widely used in human and veterina medicine and enter the environment through different pathways. Figure 2 shows t routes of entry of antibiotics into aqueous media. A significant portion of antibiotics ent marine environments through effluents from wastewater treatment plants and river inle [46]. Sulfamethoxazole, azithromycin, and ciprofloxacin are some antibiotics used humans, and their residues have been traced in the environment. Sulfamethoxazo trimethoprim, azithromycin, and enrofloxacin are converted into ciprofloxacin in livi organisms and are the most commonly used antibiotics for biota [34]. The presence of antibiotics in the sludge and effluent of municipal treatment plan hospitals, industrial centers, and livestock farms has resulted in the occurrence pollution in marine environments, surface water, soil, and groundwater [47]. Consum drugs and their metabolites are introduced into natural ecosystems through excretion (i. urine and feces) after a short time in the bodies of humans/animals [48,49]. The stabil and accumulation of antibiotics in marine environments can endanger human health [2 Chen et al. declared that about 92,700 tons of antibiotics was used in China, of whi 53,800 tons was released into the environment. According to these results, it can concluded that antibiotics are present in surface waters, groundwater, and coastal wate [15]. The presence of these compounds in water resources has become one of the mo important public health concerns. Many studies have been conducted to determine t fates and effects of antibiotics in aquatic environments [50]. In marine environments, t  [54]. In the Yellow Sea of China, it was concluded that the antibiotics ENR and CIP had the highest frequency in seawater samples, at concentrations of 0.56-125.96 ng/L and 14.94-48.26 ng/L, respectively [51]. Table 2 shows the abundance of antibiotics in seawater and sediments in the evaluated seas around the world. used by humans and other living organisms. So far, 70 antibiotics have been obse surface waters and sediments [47]. Figure 3a,b show the distribution of antibi seawater and sediments in marine areas worldwide. According to Figure 3a, the most contaminated with antibiotics were found in Iran and China.

Occurrence of Antibiotics in Sediments
Sediments are considered to be reservoirs of pollution and receive large amounts of organic pollutants [15]. Antibiotics enter aquatic environments through effluents of agricultural and aquaculture activities and eventually reach sediments. The concentrations of antibiotics in sediments that are exposed to the entry of the aforementioned effluents are higher than in other areas [16]. Sediments act as reservoirs of antibiotics due to their high absorption capacity. Contaminants such as heavy metals and antibiotics can be absorbed by the sediment and cause antibiotic resistance [50]. The concentrations of antibiotics in sediments depend on seasonal changes, water flow, sediment characteristics, and the amounts of antibiotics used in the area [67]. Microbial activity in sediments can reduce the concentrations of antibiotics over time. However, some antibiotics or their metabolites can remain in the environment for a long time, depending on the conditions. Many antibiotics can bind to sediment particles, and only a tiny amount are bioactive [68]. Many studies have also shown that the absorption of antibiotics into sediments is influenced by changes in the flow and volume of water and its physicochemical properties (e.g., water Antibiotics 2022, 11, 1461 9 of 23 temperature and the pH of water and sediments) [69]. Based on previous studies, the mobility of antibiotics increases in the presence of dissolved organic carbon and colloidal matter [70,71]. Kafaei 2016) have stated that the uptake of antibiotics by sediments is influenced by the pH and the contents of clay, silt, organic matter, and ionic matter of sediments [50,56,72]. Sediments are an ideal environment for the accumulation and propagation of antibiotic-resistance genes [73]. To determine quantitative relationships between antibiotics in the sediment and water phases, a quasi-partitioning coefficient (k d , s) is used. However, because there is no dynamic adsorption between sediment and water, this value cannot be considered an accurate partition coefficient [56]. Negligible concentrations of antibiotics that are soluble in water have been reported in sediments [52]. Factors such as the absorption or excretion of antibiotics over time and increasing ambient temperature reduce the concentrations of antibiotics in aquatic environments and, ultimately, sediments [74]. High concentrations of quinolones in sediments can be attributed to their chelation with cations and binding with particulate matter, delaying their degradation [69]. Other factors related to high concentrations of quinolones in sediments include a high dividing coefficient, low solubility, and low biodegradation [15]. The concentrations of eight antibiotics (four sulfonamides and four tetracyclines) in the sediment samples of two lakes in China were reported to be 117.97 ng/g (TCs) and 77.73 ng/g (SFs) in the sediment of East Dongting Lake and 1519.40 ng/g (SFs) and 126.27 ng/g (TCs) in the Lake Honghu sediment [73]. Zhang et al. concluded that fluoroquinolones and tetracyclines at concentrations of 12 ng/g and 11.8 ng/g, respectively, were the most abundant antibiotics in the evaluated sediment samples [75]. Arikan et al. reported that the concentration of the antibiotic clarithromycin in the sediment samples of the studied area was 1-180 ng/g, and it was the most abundant antibiotic [76]. Antibiotics studied in sediments in other parts of the world are summarized in Table 2. According to the sediment columns in Tables 2-4, it can be stated that the average concentrations of fluoroquinolones in sediments of seas, rivers, and lakes are 431.06 ng/g, 1020.58 ng/g, and 167.74 ng/g, respectively. Since sediments act as reservoirs of various pollutants, the concentrations of antibiotics in sediments are higher than those in water (except for lakes).

Occurrence of Antibiotics in Rivers
Investigating the occurrence of antibiotics in aquatic environments helps us to assess their potential threat to ecosystem balance [104]. Antibiotic contamination in small streamlike rivers is mainly due to wastewater discharge [28]. Many rivers face serious problems due to antibiotic contamination. The presence of antibiotics in aquatic environments causes the accumulation of these contaminants in biota [69]. In recent years, antibiotics have come to the fore because of their potential threat to aquatic ecosystems and public health in river systems [15]. Various antibiotics have been frequently identified in surface water, groundwater, and drinking water. High concentrations of antibiotics in rivers also affect human populations, living organisms, and water flows [67]. The concentrations of pollutants (including antibiotics) in rivers passing through urban and rural areas have been increasing, which is caused by the discharge of sewage into these water sources. Moreover, population density downstream of the river compared to upstream, dehydration, and insufficient continuous flow of seasonal runoff to dilution are the main factors of high antibiotic concentrations in the lower parts of the rivers [125]. Studies on the cognition and mechanisms of action of antibiotics in humans show that their function is different in fish, algae, birds, and other species that live in rivers [126]. Concentrations of antibiotics in rivers that are exposed to effluent from sewage are higher than in those that are not exposed to effluent [52]. Estuarine sediments act as reservoirs for antibiotics and potential sources of secondary contamination that are affected by changes in environmental conditions [83]. Reducing the concentrations of antibiotics in rivers can be linked to dilution effects. Variable concentrations in wastewater and effluents are due to the chemical transformation of pollutants-which become metabolites-along with purification systems that have little impact in removing antibiotics [102,127]. Many studies have been performed to evaluate antibiotics in rivers around the world. Table 3 shows the concentrations of antibiotics in the sediments and water of rivers around the world. According to a study on a river in France, the concentrations of sulfamethizole, norfloxacin, and trimethoprim were 544 ng/L, 163 ng/L, and 45 ng/L, respectively [104]. Wang et al., while examining river waters in China, concluded that the concentrations of doxycycline, oxytetracycline, and tetracycline were 56.09 ng/L, 18.98 ng/L, and 11.16 ng/L, respectively [93]. In 2019, the mean amoxicillin concentration in the effluent of WWTPs in Italy was reported to be 1258 ± 7.6 ng/L [112]. The authors have stated that if these pollutants enter the rivers, they can create a high ecological risk. Figure 4a,b show the distribution of antibiotic concentrations in the water and sediments of rivers worldwide. Based on data analysis of published articles, it can be stated that the average concentration of sulfonamides in rivers is 191.11 ng/L. The high concentrations of sulfonamides can be attributed to the widespread use of these antibiotics and their relatively high stability (one year).

Occurrence of Antibiotics in Lakes
As the use of antibiotics is increasing in industrialized and developing countries, it has led to the identification of these contaminants in surface waters, sediments, and biota around the world. The entry of antibiotics into aqueous media takes place through several sources. Most wastewater treatment plants are not able to effectively remove antibiotics, so their output can contaminate surface waters with antibiotics [16]. The antibiotic-laden effluents from agriculture enter environments such as rivers and lakes. Lakes, unlike rivers with their high water exchange, have low water circulation and, therefore, are more exposed to antibiotic contamination [73]. Although lakes have high potential for long-term storage of antibiotics, information on antibiotic contamination in lakes is much scarcer than in rivers. The entry of effluents from human and aquaculture activities increases the concentrations of these pollutants in lakes [128]. The accumulation of metals and antibiotics in urban and rural lake sediments is a serious threat to public health due to their potential for antibiotic resistance [7]. Among the various antibiotics, quinolones can bind to particulate matter in lakes because they are susceptible to optical degradation, while the biodegradation of macrolides has caused them to exist at low levels in surface water [69]. Table 4 shows the studies on antibiotic contamination in the sediments and water of lakes around the world. In a survey of a lake in Turkey, amoxicillin had a concentration of 1.1-1.15 ng/L [28]. In another study by Su et al., erythromycin, with a concentration of 7.26-99.22 ng/g, had the highest concentration in the sediments of the studied lake [57]. In the sediments of Lake Michigan, the antibiotics with the highest concentrations were azithromycin (147.28 ng/g), clarithromycin (67.66 ng/g), and ciprofloxacin (33 ng/g). Azithromycin (12.5 ng/L) and sulfamethoxazole (10.22 ng/L) also had the highest concentrations in Lake Michigan's water [116]. Ciprofloxacin, norfloxacin, and ofloxacin, at concentrations of 75.5 ng/g, 55.2 ng/g, and 108.9 ng/g, respectively, were the most abundant antibiotics in the sediments of Dianchi Lake (China); moreover, sulfamethoxazole and ofloxacin, at concentrations of 17.6 ng/L and 713.6 ng/L, respectively, had the highest levels among antibiotics in the water samples of this lake [120]. Based on Table 4, it can be concluded that the average concentration of fluoroquinolones in the water of the studied lakes was 369.74 ng/L. According to the results of previous studies, it can be concluded that the concentrations of antibiotics in lake water are higher than in rivers and seas. This can be attributed to the stagnant flow of water in lakes and the continuous influx of agricultural, aquacultural, and human wastewaters containing antibiotic pollution. Figure 5a,b show the studies on the levels of antibiotics in the water and sediments of lakes. Based on this figure, the water and sediments of lakes in China and Peru have been more contaminated with antibiotics than in other parts of the world. and 45 ng/L, respectively [104]. Wang et al., while examining river waters in China, concluded that the concentrations of doxycycline, oxytetracycline, and tetracycline were 56.09 ng/L, 18.98 ng/L, and 11.16 ng/L, respectively [93]. In 2019, the mean amoxicillin concentration in the effluent of WWTPs in Italy was reported to be 1258 ± 7.6 ng/L [112]. The authors have stated that if these pollutants enter the rivers, they can create a high ecological risk. Figure 4a,b show the distribution of antibiotic concentrations in the water and sediments of rivers worldwide. Based on data analysis of published articles, it can be stated that the average concentration of sulfonamides in rivers is 191.11 ng/L. The high concentrations of sulfonamides can be attributed to the widespread use of these antibiotics and their relatively high stability (one year).

Occurrence of Antibiotics in Biota
Antibiotics are widely used to treat bacterial infections in humans and animals. Some are also used as growth stimulants in pigs [129] and plants [130]. The chemical structure of antibiotics is such that it causes a positive performance for the growth of some macrobiota [3,131]. The entry of these pollutants into aquatic environments from various sources causes biological accumulation and magnification in marine organisms. According to recent studies, contamination of water sources with antibiotics can double the toxic effects of cocktails in water (released from homes and commercial sectors) by creating more toxic compounds [132,133]. There is limited information on the potential for the bioaccumulation of antibiotics in organisms and food chains [69]. The bioaccumulation of antibiotics in marine organisms such as oysters is affected by several

Occurrence of Antibiotics in Biota
Antibiotics are widely used to treat bacterial infections in humans and animals. Some are also used as growth stimulants in pigs [129] and plants [130]. The chemical structure of antibiotics is such that it causes a positive performance for the growth of some macrobiota [3,131]. The entry of these pollutants into aquatic environments from various sources causes biological accumulation and magnification in marine organisms. According to recent studies, contamination of water sources with antibiotics can double the toxic effects of cocktails in water (released from homes and commercial sectors) by cre-ating more toxic compounds [132,133]. There is limited information on the potential for the bioaccumulation of antibiotics in organisms and food chains [69]. The bioaccumulation of antibiotics in marine organisms such as oysters is affected by several factors, including the degree of ionization and the K ow partition coefficient. If the degree of ionization is not high and the K ow partition coefficient is not between 2 and 6, the concentrations of antibiotics in oysters decrease [46]. Extensive and improper use of antibiotics causes large amounts of these contaminants in the tissues of aquatic organisms. The maximum virtual values in fish tissue for florfenicol (1000 µg/kg), total enrofloxacin and ciprofloxacin (100 µg/kg), oxytetracycline (100 µg/kg), tetracycline (100 µg/kg), chlortetracycline (100 µg/kg), and sulfonamide (100 µg/kg) were set by the European Commission [134]. Figure 6 presents the study of antibiotics in biota in aquatic environments worldwide. Most of the reported studies in this field have been implemented in China. Chen et al., during a study on fish (Lutjanus russelli, Lutjanus erythopterus, and Trachinotus ovatus), mollusks (Atrina pectinata Linnaeus, Meretrix lusoria, Trisidos kiyoni, and Crassostrea rivularis Gould), crabs (Calappa philargius), and shrimps (Fenneropenaeus penicillatus) in southern China, concluded that enrofloxacin was a high-concentration antibiotic in the muscles of the studied samples [46]. Sulfamethoxazole and norfloxacin were detected by Zhang et al. in a study on shrimps and tilapia [54]. In Bangladesh, sulfamethoxazole and trimethoprim had the highest concentrations in the studied organisms (i.e., finfish and shellfish) [88]. In another study conducted in Iran, enrofloxacin and fluoroquinolones were the predominant antibiotics in the farmed rainbow trout (Oncorhynchus mykiss) [135]. Table 5 lists the studies that reported the concentrations of antibiotics in biota in aquatic environments around the world. According to the information in the table, it can be concluded that among the different continents, the number of studies conducted in Asia was the highest. Among different antibiotics, the concentrations of fluoroquinolones were reported to be higher than those of other antibiotics. Furthermore, among different Asian countries, the average concentrations of fluoroquinolone were reported as 273.67 ng/g and 17.88 ng/g in China and Iran, respectively. of ionization is not high and the Kow partition coefficient is not between 2 and 6, the concentrations of antibiotics in oysters decrease [46]. Extensive and improper use of antibiotics causes large amounts of these contaminants in the tissues of aquatic organisms. The maximum virtual values in fish tissue for florfenicol (1000 µ g/kg), total enrofloxacin and ciprofloxacin (100 µ g/kg), oxytetracycline (100 µ g/kg), tetracycline (100 µ g/kg), chlortetracycline (100 µ g/kg), and sulfonamide (100 µ g/kg) were set by the European Commission [134]. Figure 6 presents the study of antibiotics in biota in aquatic environments worldwide. Most of the reported studies in this field have been implemented in China. Chen [54]. In Bangladesh, sulfamethoxazole and trimethoprim had the highest concentrations in the studied organisms (i.e., finfish and shellfish) [ 88 ] . In another study conducted in Iran, enrofloxacin and fluoroquinolones were the predominant antibiotics in the farmed rainbow trout (Oncorhynchus mykiss) [ 135 ] . Table 5 lists the studies that reported the concentrations of antibiotics in biota in aquatic environments around the world. According to the information in the table, it can be concluded that among the different continents, the number of studies conducted in Asia was the highest. Among different antibiotics, the concentrations of fluoroquinolones were reported to be higher than those of other antibiotics. Furthermore, among different Asian countries, the average concentrations of fluoroquinolone were reported as 273.67 ng/g and 17.88 ng/g in China and Iran, respectively.

Conclusions and Remarks
The present study reviewed the reports of antibiotic concentrations in different parts of aquatic environments (i.e., seas, rivers, lakes, and marine organisms living in these environments). It can be concluded that among the different antibiotics, fluoroquinolones and sulfonamides had the highest concentrations in most of the studied environments. The average concentration of fluoroquinolones in sea, lake, and river sediments was 539.79 ng/g, while in sea and lake water it was 204.85 ng/L and 369.74 ng/L, respectively. Moreover, sulfonamides had the highest concentrations in rivers, with an average concentration of 191.11 ng/L. Antibiotics in aquatic tissues such as shrimp, fish, etc., can threaten the health of humans as consumers of these aquatic organisms. The average concentration of fluoroquinolone, as the most abundant antibiotic in marine organisms (fish, shrimp, oysters, fin whales, etc.), was reported as 145.77 ng/g. Among different countries around the world, the highest concentrations of antibiotics were reported in China. Furthermore, the results of the studies conducted in aquatic environments showed that the environments that are exposed to various effluents (urban, rural, veterinary, aquaculture, etc.) have higher concentrations and a greater variety of different antibiotics. Therefore, it is necessary to take the required approaches to reduce the entry of these pollutants into the environment. The following recommendations can be made for future investigations: • Detect the concentrations of antibiotics in more aquatic organisms.

•
The function of antibiotics is different in different organisms; therefore, it is necessary to study the concentrations of antibiotics in different organisms (fish, algae, etc.) simultaneously.
• Since antibiotics can accumulate in sediments of aquatic environments, it is recommended that future studies study the concentrations of antibiotics in sediments at different depths.

•
The widespread use of antibiotics and the lack of advanced sewage treatment systems in developing countries have caused the pollution of water sources, necessitating more detection of antibiotic contamination and the improvement of treatment systems to remove pollutants.