The Toxic Effects Of Cr(Ⅵ) On Marine Medaka (Oryzias Melastigma) Under Different Exposure Duration

The heavy metal chromium(Cr)is a biologically hazardous carcinogen that can seriously affect normal physiological functions of the organisms through bioaccumulation and biomagnification effects.As an transition metal,the biotoxicity of Cr(Ⅵ)may change over time.However,studies on the toxic effects of Cr(Ⅵ)over time are scarce and the underlying mechanisms are not fully clear.Therefore,it is necessary to investigate the differences in the toxicity of Cr(Ⅵ)under different exposure durations.In this study,we investigated the physiological and phenotypic changes of Cr(Ⅵ)exposure in marine medaka(Oryzias melastigma)at different exposure durations and the mechanisms of the toxic effects of Cr(Ⅵ)exposure on marine medaka.The specific results are as follows.(1)The antioxidant enzyme activity of the marine medaka is regulated by the duration of exposure.Among them,the antioxidant enzymes(T-SOD,CAT and GST)activity showed a trends of increasing first and then gradually decreasing,and reached the maximum induced levels in the 24d treatment group.However,the content of MDA was significantly up-regulated in both the 24d and 96d treatment group,indicating that Cr(Ⅵ)stress induced damage to the liver cell membrane of the marine medaka.In addition,the egg production,fertilization rate and hatching rate of marine medaka was not significantly affected at this exposure concentration.(2)When the parental marine medaka are stressed with Cr(Ⅵ),Cr(Ⅵ)will stimulate the heart rate of offspring embryos,and the embryos were more susceptible to Cr(Ⅵ)in early stages of embryonic development.As the embryos matured,the heart rate of all groups of F1 generation decreased to a normal rate.Notably,the heart rate of offspring after chronic Cr(Ⅵ)exposure was the earliest to return to normal levels relative to the other Cr(Ⅵ)-treated groups,which may suggest that the parental marine medaka gradually adapt to Cr(Ⅵ)environment and passed adaptability on to the offspring.In addition,the toxic effects of Cr(Ⅵ)gradually diminished during the the process of passage and did not cause persistent oxidative damage to the adult offspring,which we speculate may be the result of long-term purification recovery.(3)In the transcriptional analysis of the acute Cr(Ⅵ)exposure group,a total of 2988 DEGs were found after acute Cr(Ⅵ)stress.The upregulated DEGs were enriched to oxidoreductase activity.And KEGG pathway analyses showed that these DEGs were mainly involved in lipid metabolism,carbohydrate metabolism pathways.It suggesting that acute Cr(Ⅵ)exposure leads to oxidative stress in marine medaka,and the organism may provide more energy to the organism by activating carbohydrate metabolism to improve resistance.The down-regulated DEGs were mainly enriched to immune response,intracellular signal transduction and enzyme activity,and were mainly involved in immune response-related pathways.It can be seen that acute Cr(Ⅵ)stress inhibits various enzyme activities,interferes with lipid metabolism and thus affects the normal physiological activities of marine medaka.In further pathway analyses,we found that the PPAR signaling pathway was significantly activated in marine medaka,and Cr(Ⅵ)stress caused disruption of cholesterol metabolism by affecting the cholesterol utilization and transport pathways.In terms of reproductive toxicity of Cr(Ⅵ),the oocyte meiosis pathway and steroid hormone biosynthesis pathways were significantly inhibited.In addition,the expression levels of ZP gene family is suppressed,which encodes an important component of the chorion.It may lead to a decrease in the number of eggs and a weakening of embryonic resistance.In terms of immunotoxicity of Cr(Ⅵ),marine medaka can respond to stress by regulating the coagulation system,and many immune-related pathways are inhibited under Cr(Ⅵ)stress,suggesting that Cr(Ⅵ)is immunotoxic and causes significant effects on the immune system of the marine medaka.(4)In the transcriptional analysis of the chronic Cr(Ⅵ)exposure group,it was observed that many biological processes returned to a relatively stable state after chronic Cr(Ⅵ)stress,and only 42 DEGs were obtained,with most of which were up-regulated.It may be the result of the gradual adaptation of marine medaka to the stressful environment during chronic exposure.These DEGs were mainly enriched to primary metabolic processes,sulfate transport,and cytochrome complex assembly by GO analysis.And KEGG pathway analyses showed that these DEGs were mainly involved in pathways related to immune response and streptomycin biosynthesis.It suggests that chronic Cr(Ⅵ)stress affects the basal metabolic processes as well as biosynthetic processes in marine medaka.Notably,these DEGs are also enriched to cancer-related pathways,which deserve further study.(5)The acute and chronic Cr(Ⅵ)exposure induced biotoxicity and the molecular mechanisms involved different pathways.During acute exposure,marine medaka rapidly mobilize a large number of biological processes in response to the abruptly changing environment.In contrast,during chronic exposure,many biological processes return to a relatively stable state.Moreover,in this study,the acute toxicity of Cr(Ⅵ)was mainly in the reproductive toxicity as well as immunotoxicity,while the chronic toxicity of Cr(Ⅵ)was mainly in carcinogenic effects.Furthermore,we found 13 shared differential genes between these two groups,compared to 2975 and 29 unique genes in the acute-treated and chronic-treated groups,respectively.These shared DEGs were not affected by the exposure duration.They responded positively to Cr(Ⅵ)stress.In addition,several shared DEGs are involved in cell growth and differentiation,and had the potential to become Cr(Ⅵ)-specific molecular markers.This study reveals the toxic effects and mechanisms of Cr(Ⅵ)exposure on parental as well as offspring of marine medaka fish under different durations.It also provides an important reference for the ecological risk assessment of Cr(Ⅵ).