Heavy metals are known to be a toxic pollutant that has always been existed, which may cause harmful effects on fish in aquatic ecosystems. The exposure of heavy metal toxic substances will have toxic effects on aquatic animals, but such toxic effects will vary with the biological conditions of the animals, the physical and chemical properties of the water, and the types, types and exposure time of toxic substances [20]. We also know that the type of fish, age and metabolic activity, the concentration of metals in food and sediments, as well as the temperature and salinity of environmental factors, may all affect the degree of heavy metal accumulation in their tissues [21]. Fish are usually at the top of the aquatic food chain, and exposure to heavy metals from bioaccumulation in fish can be considered as an assessment of metal pollution in the aquatic environment [22].In addition, the accumulation of Cr (VI) in fish may lead to potential health risks for those who consume them [23]. Therefore, it is necessary to explore the accumulation and purification of water. Cr (VI) in fish tissues, to evaluate the toxic effects of Cr (VI) as antioxidants in fish systems and relative eating risks. And the route of ingestion affects accumulation in aquatic animals, whether through respiration or through digestive organs.
In this study, through exposure to heavy metals in water, the highest accumulation of Cr (VI) in the tissue was observed in the liver of C. argus. In general, the accumulation of fish in the liver is exposed to a much greater amounts of tested metals than other organs. This is attributed to the higher tendency of the elements to be related to the sulfur Al-sulfhydryl groups in carboxylate, amino, nitrogen and metallothionein. Vutukuru et al. (2007) reported that there is a large accumulation of chromium in the liver of major carp in India, and Labeo rohita is exposed to Cr (VI), leading to metabolic processes and detoxification [24]. Intestines and gills are important in ingestion. The content of heavy metals and other toxic substances in fish [25]. Intestinal tract is very crucial for the nutrients digestion and absorption in fish, and the integrity of its organizational structure is crucial for the growth and development of fish. In general, the ingestion of metals in the intestine is mainly largely diet-related, because it will ingest metals in the case of water transmission. The high accumulation of metals in intestines may be related to enterohepatic circulation that bile could excrete metals into the intestines [24]. This research showed that the bioaccumulation of Cr (VI) aqueous exposed to different tissues is as follows: liver > gills > intestine. Because of the interpenetration between fish blood and the external environment, blood parameters have proven to be sensitive and reliable indexes of toxic metal exposure for aquatic animals [26–29].
The SOD, CAT, Gsh-Px and MDA evaluation are generally known as the important indicators to detect the antioxidant capacity of aquatilia [30]. Antioxidant enzymes such as SOD, CAT, and Gsh-Px are important in the cellular defense against xenobiotic exposure [31]. It is known that SOD can catalyze the search for superoxide anion radicals and can quickly turn oxygen-free radicals into hydrogen peroxide molecules, so as to balance the metabolism of free radicals and protect the cells [32–33]. Both CAT and Gsh-Px have the ability to eliminate H2O2 and transform it into H2O and O2, thereby helping to remove H2O2 and reduce tissue damage [34]. All of the enzymes mentioned can defense against the stress of exposure to toxins in the organism. Their proper function and activity are essential to prevent any cell damage or death [35]. The secondary product MDA is produced by lipids peroxidation, indicating that oxidant cell damage caused by binding to free protein amino acids and then crosslinking within and between protein molecules [36–37]. So, an increased MDA level is the main mechanism for oxidative damage of cells. Therefore, MDA accumulation is the sign of the risk of oxidant cell damages [38]. The results above indicated, compared with 0 mg/L, the CAT activity, Gsh-Px and the SOD levels in liver and gill markedly decreased (P < 0.05) when they were exposed to 2 mg/L Cr (VI) on the 28 day. Exposure of Cr (VI) in water can cause massive generation of free radicals and reduce the antioxidant capacity of aquatilia. Opposite trend, in liver and gill, the level of MDA increased significantly (P < 0.05) when exposed to 2 mg/L Cr (VI) compared with 0 mg/L, in the 28 day. The level of MDA which exposed to 2 mg/L Cr (VI) is the highest, especially in the liver at 28 day.
Because of the interpenetration between fish blood and the external environment, blood parameters have proven to be sensitive and reliable indexes of toxic metal exposure for aquatic animals [39–41]. In order to determine the degree of liver and gill toxicity caused by Cr (VI) exposure, the levels of several enzymes were estimated, including ALT, AST and LDH. ALT and AST are important transaminases in fish liver, and AST is an indicator of liver tissue. The serum biochemical parameters of injury are therefore considered to be important indicators for the diagnosis of liver diseases in aquatic animals [42–43]. Compared with the control, they were greatly improved at 28 day. This increase may be due to damage to liver cells and the leakage of these enzymes from damaged cells [44]. COR is synthesized by the adrenal cortex and is for regulating glucose metabolism and stress response. Meanwhile, the serum COR level is related to the stress process [45]. Srious metal pollution can cause abnormal COR metabolism of fish [46]. Previous studies have shown that the accumulation of Cr (VI) tends to lead to immunosuppression, thus increasing sensitivity of fish to diseases [47]. AKP is an important indicator of liver disease diagnosis. When the liver is damaged, liver cells overproduce AKP. LZM is an essential protective immune factor, hydrolyzing the peptidoglycan structure to remove all kinds of bacteria [48]. IgM is a large molecule involved in the specific immune response and is an important index to measure immune response [49]. This study shows that as the concentration of Cr (VI) increases, the level of LZM in the serum decreases significantly. It is suggested that immunosuppression may be caused because of inflammation by heavy metal stress [49]. At the same time, with the increase of lead concentration, the content of COR and AKP in this research increased significantly, which indicated that the lead accumulation caused an increase in the degree of stress. This could be due to the damage of the liver tissue structure caused by the accumulation of Cr (VI), and the COR and AKP in the serum also increase. However, IgM shows an upward trend with the prolonged exposure time in water. Some reports indicate that exposure to heavy metals and other toxic substances can cause changes in the body's immune system parameters [17]. Besides, higher levels of LZM and IgM have been observed in diseased fish [50]. Some studies have revealed that the environmental pressure has a negative impact on the immunity system, leading to increased sensitivity to diseases [51]. As the Cr (VI) content of water increases, the IgM level of C. argus also increases.