Effects Of Hypoxia On The Bioaccumulation Of Methylmercury In Fish

The study of the migration and transformation patterns of pollutants under different environmental conditions is a scientific basis and an important foundation for the development of accurate environmental risk management and environmental decisionmaking.Mercury(Hg)is a kind of persistent toxic pollutant that exists in many forms in the environment.Methylmercury(Me Hg)has attracted extensive attention due to its high bioaccumulative capability and toxicity.Consumption of fish and other aquatic products is one of the most significant sources of human exposure to mercury,and there are many strict requirements for Me Hg limits for aquatic foods from international organizations and countries.However,there are large differences in the migration and transformation patterns of mercury under different environmental conditions,so exploring the geographical differences in Me Hg enrichment in fish and its mechanisms can help to effectively evaluate the biotoxicity of mercury and provide important guidance for the development of more precise environmental standards.Me Hg in fish is mainly enriched through the food chain.Environmental Me Hg concentration,body length,weight,age,and trophic level of fish,assimilation rate are the main factors influencing Me Hg bioaccumulation in fish.Recent studies have indicated that concentrations of Me Hg in fish living in low oxygen condition,such as high altitude or deep sea,are significantly higher than the others.There is still a lack of knowledge on whether dissolved oxygenaffect the accumulation of Me Hg in fish which restraining our understanding of the mechanism of Me Hg fate and bioaccumulation in aquatic environment.To fulfill these,the objectives of this study were to 1)identify main factors controlling bioaccumulation of Me Hg in fish through analyzing total Hg(THg),Me Hg concentrations,and relavent parameters in different species of fish collected from Tibet and Wuhan;2)explore the effects of oxygen content on the bioaccumulation of Me Hg in fish by investigating distributions of Me Hg in zebrafish and common carp tissues exposured to Me Hg under hypoxic and normoxic conditions;3)elucidate the molecular mechanisms of dissolved oxygen affecting Me Hg bioaccumulation in fish using transcriptome sequencing technology.The major findings are as follows.1.Concentrations of THg and Me Hg in fish collected from Tibet and Wuhan and their controlling factors.Different species of fish were collected from Tibet and Wuhan,and concentrations of THg and Me Hg were determined.The results showed that the concentrations of THg and Me Hg in fish muscle sampled from Tibet were 0.65 ± 0.52 mg/kg and 0.51 ± 0.43 mg/kg,respectively,which were significantly higher than those from Wuhan(THg,0.11 ± 0.13mg/kg;Me Hg,0.04 ± 0.06 mg/kg).Correlation analysis results showed that body length and weight were significantly positive correlated with THg and Me Hg levels in fish(P <0.05),while age were only significantly positive relatedwith Me Hg concentration in fish(P< 0.05),suggesting that these biological parameters are important factors affecting Hg levels in fish from Tibet and Wuhan.The ratio of Me Hg to THg in fish muscle is often used to indicate the comprehensive ability of Me Hg enriching,allocating and accumulating in fish.No significant relationships between the above biological factors and Me Hg/THg ratios were found(P > 0.05)in our study,and comparable values of Me Hg/THg were observed in fish occupied different trophic levels.These results indicated that Me Hg level in fish was not only depended on biological fators,other factors also play important roles in controlling bioaccumulation of Me Hg in fish.2.Distributions of Me Hg in zebrafish and common carp tissues under different dissolved oxygen conditions and its effects on Me Hg bioaccumulation in fish.The results of Me Hg exposure experiments under different dissolved oxygen content showed that the concentration of Me Hg in zebrafish under hypoxic condition was significantly higher than that under normoxic conditions after short-term exposure(7 d),which may be caused by the fish being more sensitive to changes in dissolved oxygen and their stress response to hypoxic stress.At the end of the exposure experiments(78 d),Me Hg levels in zebrafish tissuse(especially brain and muscle)under hypoxic condition displayed rapidly increased trend.By calculating bioaccumulation factor(BAF)of Me Hg,higher BAF values in zebrafish tissues under hypoxic condition than that under normoxic conditions were found.These results indicated that oxygen content in water significantly affects Me Hg levels in tissues of zebrafish,and hypoxic condition enhances Me Hg bioaccumulation in fish.However,during the whole exposure period no significant variations in Me Hg concentration of carp tissues under different oxygen content conditions were observed,which may be due to different bioaccumulation ability of Me Hg in different fish and the strong hypoxic adaptation of common carp.3.Molecular mechanisms of variations of Me Hg bioaccumulation caused by changes in transcriptional levels of related genes in fish uncer hypoxia.Through comparative transcriptome analysis,it was found that at 7,60 and 78 days of exposure,the differential genes in the kidney were significantly enriched in the biological processes of oxygen binding,oxygen transport and heme binding.KEGG enrichment analysis showed that the common differential genes were significantly enriched in the porphyrin metabolic pathway,and the expressions of fech,cpox,hmbsa and ears2 related to erythropoiesis were up-regulated.The above results indicate that under the condition of low oxygen,the production of hemoglobin in fish kidney increases and the acceleration of hemoglobin circulation will not only maintain the normal physiological function of fish,but also lead to the allocation and accumulation of more Me Hg carried by hemoglobin in various tissues and organs,which may be one of the important reasons for the enrichment of Me Hg in fish under hypoxia.At the end of exposure,differentially expressed genes in kidney and muscle were significantly enriched in glutathione metabolic pathway,indicating that long-term hypoxia and Me Hg exposure affected glutathione detoxification in cells,resulting in slower metabolism of Me Hg in vivo and more enrichment in fish.