Bioaccumulation and Bioremediation of Heavy Metals in Fishes—A Review

Heavy metals, the most potent contaminants of the environment, are discharged into the aquatic ecosystems through the effluents of several industries, resulting in serious aquatic pollution. This type of severe heavy metal contamination in aquaculture systems has attracted great attention throughout the world. These toxic heavy metals are transmitted into the food chain through their bioaccumulation in different tissues of aquatic species and have aroused serious public health concerns. Heavy metal toxicity negatively affects the growth, reproduction, and physiology of fish, which is threatening the sustainable development of the aquaculture sector. Recently, several techniques, such as adsorption, physio-biochemical, molecular, and phytoremediation mechanisms have been successfully applied to reduce the toxicants in the environment. Microorganisms, especially several bacterial species, play a key role in this bioremediation process. In this context, the present review summarizes the bioaccumulation of different heavy metals into fishes, their toxic effects, and possible bioremediation techniques to protect the fishes from heavy metal contamination. Additionally, this paper discusses existing strategies to bioremediate heavy metals from aquatic ecosystems and the scope of genetic and molecular approaches for the effective bioremediation of heavy metals.


Introduction
Heavy metal contamination in aquatic water bodies is a major concern that has a serious impact on the associated organisms, especially fish [1][2][3]. Heavy metals naturally exist in the environment, but excessive application in different industries for several purposes has significantly altered the ecological system [4] by the excessive discharge of these metals into the soil and aquatic systems [5,6]. Generally, anthropogenic activities, such as the culture of crop foods, erosion from agricultural fields, and the discharge of industrial and household wastes, are considered main sources of heavy metals in aquatic systems [7,8].
the current review summarizes the recent information regarding bioaccumulation and developments in bioremediation techniques of heavy metals in fishes.

Bioaccumulation of Heavy Metals in Different Tissues of Fishes
Bioaccumulation assessment is one of the important indications for monitoring the geochemical cycle of heavy metals in the aquatic ecosystem. Toxic effects and oxidation of heavy metals vary with their forms and metal types, respectively. Chromium (Cr) generally exists in six different oxidative forms (+1 to +6), among which hexavalent Cr exerts destructive effects in fish [68]. Fish in heavy metal-contaminated aquatic systems pose a serious threat as fish accumulate metals through several important body tissues (gills, liver, kidney, skin, muscle, etc.), which are clearly illustrated in Figure 1. Fish require more energy, which is sourced from reserved nutrients, including protein, fats, and carbohydrates, to acclimate themselves in this stressed condition. Some of the metals (As, Cd, Cr, Cu, Fe, Hg, Ni, Pb, Zn) have redox potentiality, and they react to produce reactive oxygen species (ROS) that play an important role in maintaining a certain physiology in fish. ROS acts as an indicator of state oxidative stress that restricts the activity of cells through degrading protein, lipids, and DNA. Heavy metals bioaccumulate into different aquatic organisms through the food cycle and cause serious human health issues upon consumption of these contaminated fish [69][70][71][72][73]. Bioaccumulation of heavy metals in different fish organs is presented in Table 1, and different toxic effects of heavy metals in fish are demonstrated in Table 2.

As
As is one of the most toxic heavy metals which pollute aquatic water bodies by means of various natural as well as man-made actions [74]. It has been reported that inorganic As resulted in more toxic than organic forms [75,76]. As accumulates in different

As
As is one of the most toxic heavy metals which pollute aquatic water bodies by means of various natural as well as man-made actions [74]. It has been reported that inorganic As resulted in more toxic than organic forms [75,76]. As accumulates in different organs of fish (Table 1) at different rates [77,78]. It has been revealed that the highest amount of As (10.04 ± 2.99 µg/g) accumulated in the liver, whereas the lowest (3.74 ± 3.38 µg/g) was observed in muscle after 20 days of exposure by Oreochromis niloticus [57]. Several studies reported that As exposure caused various negative impacts on fish, such as growth and Toxics 2023, 11, 510 4 of 28 production reduction, hemato-biochemical changes, hormonal dysfunctions, histopathological anomalies, embryonic and larval development retardation, and other diseases [79][80][81][82][83]. Moreover, As toxicity significantly affected the hematology and immunology of several fish [84][85][86][87]. A high dose of As resulted in high mucus release, abnormal swimming, and loss of balance in Anabas testudineus and Danio rerio [88,89]. As stimulated the formation of apoptosis, micronuclei, and several cellular and nuclear abnormalities in erythrocytes of fish [82]. As induced several cytotoxicities and genotoxicities in medaka, Oryzias latipes [90]. Moreover, As contamination disrupted the reproductive activities of fish by inhibiting the gametogenesis process and, thus, negatively affecting sperm and ovum quality as well as quantity, fertilization, and hatching success [91][92][93].

Cd
Cd is very toxic and carcinogenic to humans and several animals, including fish. According to the Agency for Toxic Substances and Disease Registry of the United States, this metal ranks as the seventh most hazardous agent [94]. Several studies reported that the aquatic environment is significantly contaminated with Cd [95][96][97]. Assimilation and bioaccumulation of this toxic metal has occurred in a wide range of aquatic species (Table 1). Cd toxicity has resulted in the dysfunction of several important organs of fish, such as the liver, kidney, and gills, which affects the physiology of fish and hampers their growth [98][99][100]. Additionally, Cd alters the hematological indices by disturbing iron metabolism and creating anemic conditions [101,102]. Cd causes inhibition of antioxidant enzymes, inducing lipid peroxidation in animals [103,104]. Moreover, Cd toxicity negatively affects the reproductive performance of fish by shrinking the lobules of sperm, creating fibrosis in testis and lowering sperm motility and viability [19,76,105-107].

Cr
Cr is a ubiquitous metal that deteriorates the environment quality sourced from different types of industries [108,109]. Several studies reported the bioaccumulation of Cr in the different organs (Table 1) of Cyprinus carpio [110,111], Carassius auratus [112], O. aureus [113], and Cirrhinus mrigala [109]. Cr toxicity disturbs the physiological functions of fish and results in various allergic as well as organ-system failure [5,[114][115][116][117]. Additionally, Cr toxicity significantly alters the protein, lipid, and glycogen content in the muscle, liver, and gills of Labeo rohita [118] and C. mrigala [119], causes hepatic stress in C. auratus [120], disturbs the functions of important organs (liver, kidney) of Ctenopharyngodon idella [121], and causes abnormal functions of the endocrine system of several freshwater fish species [68]. Cr was found to alter the blood profile, resulting in cellular and nuclear abnormalities of Pangasianodon hypophthalmus [1,117]. Several studies reported that high Cr levels in fish diets significantly decreased the growth and feed utilization of different fish species [122]. Moreover, chronic Cr exposure resulted in complexities in the reproduction of fish by lowering spawning success [123,124], deforming testis [19], decreasing sperm motility [105], and hampering the formation of oocytes [125].

Cu
Cu is a major contaminant of aquatic systems that results in stressful conditions for the aquatic organisms and significantly hampers the growth and physiology of fish [126][127][128]. The bioaccumulation of Cu in different organs of fish species is exhibited in Table 1. Several studies revealed that the liver is the main site that accumulates a significant proportion of Cu in comparison with the other organs [129][130][131]. Excess Cu in the fish diet reduced the fish appetite, thus negatively affecting the feed utilization and growth of fish [132]. Moreover, Cu toxicity not only resulted in deformed reproductive organs but also drastically reduced the GSI, fecundity, fertilization, and hatching rate of several fish species [18,21,133].

Mn
Mn is commonly found in a wide range of environments [134]. Mn was found to dissolve into water bodies through various anthropogenic activities [135]. Several factors, including fish species, age, and water quality, may vary Mn toxicity in fish [136]. It has been revealed that Mn toxicity declines with the increase in water hardness [134]. The bioaccumulation of Mn in the liver, gills, and muscles of Argyrosomus japonicas disturbed the metabolic process of carbohydrates and altered the ionic profile of blood plasma [137]. Mn affects the physiology of fish and sometimes exhibits fatal and lethal effects [134]. Mn exposure results in oxidative stress in C. auratus [138]. Mn results in many neurogenetical disorders by inducing the formation of free radicals and the inactivation of several enzymes associated with an antioxidant capacity [139]. Additionally, Mn damages the liver and induces cell apoptosis of grouper [140].

Ni
Ni is extensively used in different industrial activities and is considered a dominant contaminant of aquatic systems. Basically, Ni in aquatic ecosystems combines with other chemical compounds to form soluble salts that have the ability to adsorb onto other substances and cause several synergistic and antagonistic effects [141]. The severity of Ni toxicity depends on various factors such as Ni concentration, water quality, and the physiological state of organisms [142]. Several studies revealed that Ni accumulated in different organs of fish, especially in gills, and resulted in complexities in respiratory functions [143][144][145][146][147]. In addition, Ni was found to accumulate in the intestine of fish and disrupt the function of the intestine [148,149]. Ni alters the normal physiology and causes the death of several freshwater fish species [150]. Ni contamination induces several histological deformations of gills, including hyperplasia, hypertrophy, and fusion of gill lamellae in Orechromis niloticus [151]. Additionally, Ni toxicity hampers ion regulation [152][153][154] and induces oxidative stress in fish [155][156][157][158][159]. Two studies observed no significant impacts on fish growth [160,161]; however, they showed significant effects on the growth of pulmonate snails [162] and zebrafish [163].

Pb
Pb is a potent hazardous element that is bioaccumulated in aquatic organisms through water and feed [164]. Pb is bioaccumulated in different fish organs, including the liver, kidney, gills, spleen, and even the digestive system [165][166][167][168][169][170][171]. Pb significantly changes the blood parameters of fish [172][173][174][175][176]. Additionally, Pb toxicity results in a significant alteration in enzyme activity in blood plasma and the liver of fish that causes several pathologies in the cell membrane and shreds the liver cell [175,[177][178][179]. Pb negatively affects the growth and feed utility of fish by reducing weight gain, specific growth rate, and feed intake [180][181][182][183]. Moreover, Pb results in poor reproductive performances such as low-quality sperm and ovum, reduced fertilization and hatching rate, low survival of embryo and larvae, etc. [17,20].

Zn
Zn is an essential micronutrient that plays a significant role in the growth and reproduction of fish [27, 184,185]; however, an excess amount of Zn has several hazardous effects on fish [186]. Zn contamination in aquatic ecosystems is well established [187,188]. Liver and kidney tissues are the main sites for Zn bioaccumulation [189]. Zn toxicity negatively affects the growth [190][191][192], reproduction [22], homeostasis [193], feed intake [194][195][196], and bone formation of fish [197]. Zn toxicity induces ammonia excretion that results in poor water quality and stressful conditions for fish [191]. In addition, Zn toxicity damages the fish liver by increasing the activity of ALT and AST [198][199][200]. Moreover, high Zn levels significantly reduce the body protein and lipids of fish, which might result in the oxidation of protein and lipids, as well as low protein intake [201][202][203][204].  Table 2. Heavy metals toxicity in fishes.

Species Toxicity References
As

Cyprinus carpio
Significantly reduced the growth and feed utilization indices [224] Channa gachua Liver: Vacuoles in the cytoplasm and stroma, degenerated nuclei [225] Poecilia Pb

Bioremediation of Heavy Metal Toxicity in Fishes
Bioremediation is a convenient and ecofriendly option that can be used to restore the contaminated environment by removing toxic metals from the environment [224,229]. Bioremediation of toxicants can be done by adsorption [230][231][232], physio-biochemical mechanisms [233][234][235][236], and molecular mechanisms [237][238][239]. Several enzymes (superoxide dismutase, SOD; catalase, CAT; glutathione S transferase, GST) and nonenzymatic compounds (reduced glutathione, GSH) play a key role in sustaining the ROS balance by detoxification (Figure 2). SOD has the capability to convert superoxide radicals to hydrogen peroxide radicals that transform into nontoxic oxygen and water through the activity of CAT enzymes [240]. On the other hand, GST detoxifies the toxicants by catalyzing electrophiles to GSH. Moreover, GSH converts into glutathione disulfide through the nonenzymatic oxidation of electrophilic compounds including free radicals and ROS [223].  Phytoremediation is a popular bioremediation technique in which various plants and microbes are used to reduce pollutants from the aquatic environment [4]. Microbial enzymes play a key role in converting toxic contaminants to safe ones by altering the chemical structure of contaminants. Some Lactobacillus spp. efficiently remediate heavy metals by converting the environment more acidic in nature [241,242] and through biosorption means or by forming bonds between heavy metals and their cell components [243]. Several mechanisms of microorganisms that make them resistant to heavy metals include extracellular sequestration [244,245], intracellular sequestration [246,247], reduction of heavy metal ions by the microbial cell [248], and extracellular barriers [249,250]. A wide range of microbes, such as bacteria, fungi, and algal species, have been used to detoxify heavy metals [251,252] and keep the environment clean; they are listed in Table 3. In addition to natural microbes, several genetically improved engineered microorganisms, especially surface-engineered microorganisms, were developed to use in the remediation process of target-specific heavy metals [253]. Several studies reported that the capabilities of genetically engineered microbes are greater than natural microbes for removing organic compounds, including heavy metals, under natural environmental systems [254,255]. Several engineering aspects, including single-gene edition, metabolic pathway modification, and alteration of gene sequences (coding and controlling), are successfully employed to modify the genetic makeup of microorganisms and transform them into engineered microorganisms [60], which more efficiently eliminates several heavy metals such as Ni, Hg, Cd, Fe, As, and Cu [256][257][258]. Additionally, the application of advanced engineering approaches (genomics, metagenomics, proteomics, metabolomics, and transcriptomics) has produced genetically modified microbes that play a crucial role in the bioremediation of several heavy metals [63,259]. The application of genetically modified Pseudomonas putida and Escherichia coli strain M109 has successfully removed Hg from contaminated sites [260]. The insertion of mer genes in Deinococcus geothemalis [4] and Cupriavidus metallidurans strain MSR33 [4,261] has been found to efficiently reduce Hg. Moreover, transporters in microbial membranes significantly improves the bioremediation of heavy metals from the environment [60,61]. It has been revealed that dietary Lactobacillus plantarum alleviated the toxicity caused by aluminum Phytoremediation is a popular bioremediation technique in which various plants and microbes are used to reduce pollutants from the aquatic environment [4]. Microbial enzymes play a key role in converting toxic contaminants to safe ones by altering the chemical structure of contaminants. Some Lactobacillus spp. efficiently remediate heavy metals by converting the environment more acidic in nature [241,242] and through biosorption means or by forming bonds between heavy metals and their cell components [243]. Several mechanisms of microorganisms that make them resistant to heavy metals include extracellular sequestration [244,245], intracellular sequestration [246,247], reduction of heavy metal ions by the microbial cell [248], and extracellular barriers [249,250]. A wide range of microbes, such as bacteria, fungi, and algal species, have been used to detoxify heavy metals [251,252] and keep the environment clean; they are listed in Table 3. In addition to natural microbes, several genetically improved engineered microorganisms, especially surface-engineered microorganisms, were developed to use in the remediation process of target-specific heavy metals [253]. Several studies reported that the capabilities of genetically engineered microbes are greater than natural microbes for removing organic compounds, including heavy metals, under natural environmental systems [254,255]. Several engineering aspects, including single-gene edition, metabolic pathway modification, and alteration of gene sequences (coding and controlling), are successfully employed to modify the genetic makeup of microorganisms and transform them into engineered microorganisms [60], which more efficiently eliminates several heavy metals such as Ni, Hg, Cd, Fe, As, and Cu [256][257][258]. Additionally, the application of advanced engineering approaches (genomics, metagenomics, proteomics, metabolomics, and transcriptomics) has produced genetically modified microbes that play a crucial role in the bioremediation of several heavy metals [63,259]. The application of genetically modified Pseudomonas putida and Escherichia coli strain M109 has successfully removed Hg from contaminated sites [260]. The insertion of mer genes in Deinococcus geothemalis [4] and Cupriavidus metallidurans strain MSR33 [4,261] has been found to efficiently reduce Hg. Moreover, transporters in microbial membranes significantly improves the bioremediation of heavy metals from the environment [60,61]. It has been revealed that dietary Lactobacillus plantarum alleviated the toxicity caused by aluminum (Al) in tilapia [262]. The application of Spirulina platensis significantly alters the negative effects of As toxicity in Oryzias latipes [90]. Additionally, the provision of probiotics in the diet has been found to reverse the negative effects of Cd on the growth and hematology of Oreochromis niloticus [263]. However, EDTA significantly reduced the body Cd level and, thus, improved the blood profile of Clarias gariepinus [264]. Pomegranate peel and Lactococcus lactis have shown positive results in remediating toxicity resulting from Hg in C. gariepinus [265,266]. Probiotic supplementation (L. reuteri) was found to effectively alter the negative effects of Pb in Cyprinus carpio [182,267].

Application of Bacteria as Bioremediator
Bacteria play an important role due to having some special features such as their size, distribution, and capability to grow in controlled and resilient environments [268]. It has been reported that 70% and 75% Cd are removed by the application of P. aeruginosa and Alcaligenes faecalis, respectively [269], while Bacillus pumilus and Brevibacterium iodinium are able to remove Pb up to 87% and 88%, respectively. Another study revealed that B. cereus had the ability to remove 72% Cr [251]. Micrococcus luteus can remove a significant amount of Pb [270]. It has been stated that the highest quantity of Pb, Cr, and Cd were removed by B. megaterium, Aspergillus niger, and B. subtilis, respectively [271]. Desulfovibrio desulfuricans was found to effectively reduce 99.8% Cr, 98.2% Cu, and 90.1% Ni [252]. It has been reported that mixtures of several bacterial species efficiently removed Cr, Zn, Cd, Pb, Cu, and Co [272].

Application of Fungi as Bioremediator
Several fungal species with great capability of metal uptake and recovery are widely used to remediate toxic metals [273,274]. Several studies reported that both live and dead cells of fungal species actively adsorb metals [275,276]. It has been reported that Aspergillus sp. efficiently removed 85% of Cr [277]. It has been revealed that dead cells of Aspergillus niger, Rhizopus oryzae, Saccharomyces cerevisiae, and Penicillium chrysogenum are suitable for transforming toxic Cr into a less toxic form [278]. Another fungal species, Coprinopsis atramentaria, is considered an important metal accumulator [279]. Candida sphaerica was found to significantly reduce metal loads by creating biosurfactants [280,281]. Moreover, Hansenula polymorpha, Saccharomyces cerevisiae, Yarrowia lipolytica, Rhodotorula pilimanae, Pichia guilliermondii, and Rhodotorula mucilage were found to effectively convert toxic Cr to a less toxic state [282][283][284].

Application of Algae as Bioremediator
Algae have a high biosorption capacity and, hence, are used as biosorbents to remove toxic heavy metals [285]. Several algae and cyanobacterial species have the capability to either remove or degrade toxic metals [208]. This degradation of toxic metals by algae may be attributed to the high photosynthetic capacity of algae, resulting in the availability of significant amounts of dissolved oxygen in aquatic systems that caused an aerobic breakdown of several organic compounds, including heavy metals. Heavy metals, as well as other toxic compounds, may be degraded, detoxified, and transformed through several enzymatic and metabolic activities of algae in their metabolism [286]. Moreover, the algal cell wall is composed of several essential functional groups (fucoidan, alginate) that play a significant role in the removal of toxic heavy metals through a biosorption mechanism [287,288]. Additionally, algae can bind heavy metals through the employment of several binding approaches (extracellular and intracellular), including chelation, complexation, and physical adsorption to lessen the associated toxicity [289]. In addition, algae can play a very significant role in the detoxification of metals through their ability to synthesize class III metallothioneins (phytochelatins) that are synthesized by phytochelatin synthase enzymes that require post-translational activation by heavy metals [290][291][292]. The algal surface contains various chemical substances such as hydroxyl, carboxyl, phosphate, and amide that act as binding sites for the metals [293]. Death cells of another important algal species, Chlorella vulgaris, have been established as an efficient remover of Cd, Cu, and Pb [294]. Table 3. Bioremediation of heavy metals toxicity in different fishes.

Species Doses Exposure Time (Days) Bioremediation References
Al

Cd
Oreochromis niloticus Reduced length of testicular cell size Reduced growth rate, altered hemato-biochemical parameters, increased mortality, and reduced gut microbial diversity [263] Cd (1 mg/L) + Lactobacillus plantarum (10 8 CFU/g) Improved growth performance, decreased mortality, and Cd level reversed alteration of hemato-biochemical parameters in blood      Caused lipid peroxidation and altered antioxidant enzymes SOD, CAT, proteins, glucose, glycogen, and amino acids in organs [306] Pb (0.017 mg/L) + Spirulina (500 mg/fish) Reduced Pb toxicity and enhanced SOD, and CAT activity in the liver and gills, thereby diminishing lipid peroxidation

Conclusions and Recommendations
The discharge of toxic heavy metals without proper treatment from various industries is adversely deteriorating aquatic ecosystems. As a result, toxic heavy metals from this contaminated environment have bioaccumulated in several important organs of fish and disturbed their normal functions. The bioaccumulation of these toxic metals has severely affected the normal physiology of fish, reducing the growth and reproduction of fish. Bioremediation has great potentiality to reshape the existing contaminations of aquatic systems in a sustainable approach. Additionally, bioremediation improves fish health by altering the toxic effects of several heavy metals. It is not only beneficial for aquatic organisms but also improves the productivity of aquatic ecosystems. By efficient application of this bioremediation process, we can significantly recycle the water that reduces wastage of water, and degradation of organic matter lowers the pathogenic organisms that enhance the biosecurity of our ecosystems. In parallel with the current practices of bioremediation, genetically engineered microorganisms (GEM) should be introduced in the future to increase the efficiency of bioremediation techniques to mitigate adverse heavy metal contamination. In this case, public acceptance of GEM and the safety of the environment should be taken into consideration.