Evaluation of Dietary Organic and Inorganic Mercury Threshold Levels on Induced Mercury Toxicity in a Marine Fish Model

Effects of dietary and/or parental supplementation with selenium and mercury on their localisation in rainbow trout fry tissues (Oncorhynchus mykiss) by quantitative LA-ICP MS imaging

The increasing use of alternative feeds for sustainable aquaculture leads to a reduced selenium content requiring the use of supplements to maintain the antioxidant status of farmed fish. The introduction of tuna by-products in fish feed is attractive for their selenium content, but the presence of mercury limits their use as it may negatively impact fish health. Indeed, selenium and mercury metabolism, in relation to their interaction and biological effects in fish, is far from being fully understood. This study aims to assess the impact of parental and dietary supplementation with selenomethionine and methylmercury on selenium and mercury localisation and concentrations in tissues of 3-week rainbow trout fry. Six diets were used. Broodstock were fed a plant-based diet Bpc (with 0.2 μg g-1 selenium and 0 μg g-1 mercury) or a tuna by-product-based diet Bt (with 4 μg g-1 selenium and basal mercury at 0.2 μg g-1) during 6 months, supplemented or not with 4 μg g-1 of selenomethionine for the plant-based diet Bpo and/or with 2 μg g-1 of methylmercury for diets Bph, Bpho and Bth. Their offspring were fed the plant-based diet Fpc, supplemented or not with the same levels of selenomethionine and methylmercury, until their third week. Mercury and selenium concentrations were determined in fry whole-body by ICP MS and quantitative images of selenium and mercury were produced by LA-ICP MS from fry whole-body thin sections. Dietary supplementation of fry with selenium and/or mercury increased their concentration in all tissues, especially in liver, kidney, muscle and intestine. Parental selenomethionine supplementation had a more pronounced effect on fry selenium levels than tuna by-products diet. Mercury transfer from broodstock to their offspring was weak. Mercury concentrations decreased in the kidney of fry supplemented with both selenium and mercury, which could be related to the presence of selenium.

Genome-Scale Screening of Saccharomyces cerevisiae Deletion Mutants to Gain Molecular Insight into Tolerance to Mercury Ions

Mercury (Hg) is a global pollutant and a bioaccumulative toxin that seriously affects the environment. Though increasing information has been obtained on the mechanisms involved in mercury toxicity, there is still a knowledge gap between the adverse effects and action mechanisms, especially at the molecular level. In the current study, we screened a diploid library of Saccharomyces cerevisiae single-gene deletion mutants to identify the nonessential genes associated with increased sensitivity to mercury ions. By genome-scale screening, we identified 64 yeast single-gene deletion mutants. These genes are involved in metabolism, transcription, antioxidant activity, cellular transport, transport facilitation, transport routes, and the cell cycle, as well as in protein synthesis, folding, modification, and protein destination. The concentration of mercury ions was different in the cells of yeast deletion mutants. Moreover, the disruption of antioxidant systems may play a key role in the mercurial toxic effects. The related functions of sensitive genes and signal pathways were further analyzed using bioinformatics-related technologies. Among 64 sensitive genes, 37 genes have human homologous analogs. Our results may provide a meaningful reference for understanding the action mode, cellular detoxification, and molecular regulation mechanisms of mercury toxicity.

Prolonged exposure to mercuric chloride induces oxidative stress-mediated nephrotoxicity in freshwater food fish Channa punctatus

Mercury (Hg) is regarded as a serious hazard to aquatic life and is particularly prevalent in aquatic ecosystems. However, there is little evidence available regarding the toxicity of mercury chloride (HgCl2) in vital organs of fish. This study was conducted to assess the effects of HgCl2 (0.039 mg/L and 0.078 mg/L) on oxidative stress-mediated genotoxicity, poikilocytosis, apoptosis, and renal fibrosis after 15, 30, and 45 days of the exposure period. According to the findings, HgCl2 intoxication in fish resulted in a significantly (P < 0.05) elevated lipid peroxidation (LPO), protein carbonyls (PC), lactate dehydrogenase (LDH) activity levels in kidney tissues and significantly (P < 0.05) increased reactive oxygen species (ROS), poikilocytosis, DNA tail length, and the frequency of apoptotic cells (AC%) in blood cells. Kidney's ultra-structure and histopathology revealed its fibrosis, which was evident by mRNA expression of targeted genes KIM1, NOX4, TGFβ, and NFϏβ. Different indicators of oxidative stress, apoptosis, and genotoxicity were altered in a dose and time-dependent manner, according to a two-way ANOVA analysis. There was a considerable positive link between oxidative stress and kidney fibrosis in the fish Channa punctatus, and it is evident from regression correlation and PCA data analysis. The kidney's ultra-structure evaluation and histopathology both revealed a noticeable fibrosis state. Additionally, a significant (P < 0.05) downregulation in PPARδ reveals that fish body was unable to combat diseases such as kidney fibrosis induced by HgCl2. This study shed fresh light on the mechanisms underlying nephrotoxicity caused by HgCl2 exposure.

Coastal Mussel (Mytilus spp.) Soft Tissues as Bioindicators of Methylmercury: Exploring the Relationship Between Condition Index and Methylmercury Concentrations

We investigated total mercury (THg) and methylmercury (MeHg) concentrations in coastal mussels (Mytilus spp.) sampled from the Minas Basin, Bay of Fundy and evaluated the relationship with condition index (CI). THg concentrations were low in sediment (mean THg = 5.15 ± 2.11 ng/g dw; n = 6) and soft tissues (mean THg = 62.3 ± 13.7 ng/g; mean MeHg = 13.2 ± 6.3 ng/g; n = 57). The THg in tissues had no significant relationship with CI (Rs= -0.205, p = 0.126). MeHg in tissues were significantly and negatively correlated with condition index (Rs = -0.361, p = 0.006) indicating that healthier mussels (higher CI) have lower mercury content possibly due to elimination strategies or growth dilution.

Differential Cell Metabolic Pathways in Gills and Liver of Fish (White Seabream Diplodus sargus) Coping with Dietary Methylmercury Exposure

Mercury (Hg) is a dangerous and persistent trace element. Its organic and highly toxic form, methylmercury (MeHg), easily crosses biological membranes and accumulates in biota. Nevertheless, understanding the mechanisms of dietary MeHg toxicity in fish remains a challenge. A time-course experiment was conducted with juvenile white seabreams, Diplodus sargus (Linnaeus, 1758), exposed to realistic levels of MeHg in feed (8.7 μg g^-1, dry weight), comprising exposure (E; 7 and 14 days) and post-exposure (PE; 28 days) periods. Total Hg levels increased with time in gills and liver during E and decreased significantly in PE (though levels of control fish were reached only for gills), with liver exhibiting higher levels (2.7 times) than gills. Nuclear magnetic resonance (NMR)-based metabolomics revealed multiple and often differential metabolic changes between fish organs. Gills exhibited protein catabolism, disturbances in cholinergic neurotransmission, and changes in osmoregulation and lipid and energy metabolism. However, dietary MeHg exposure provoked altered protein metabolism in the liver with decreased amino acids, likely for activation of defensive strategies. PE allowed for the partial recovery of both organs, even if with occurrence of oxidative stress and changes of energy metabolism. Overall, these findings support organ-specific responses according to their sensitivity to Hg exposure, pointing out that indications obtained in biomonitoring studies may depend also on the selected organ.

Sub-lethal acute effects of environmental concentrations of inorganic mercury on hematological and biochemical parameters in walking catfish, Clarias batrachus

The acute toxicity of mercury to the air-breathing fish Clarias batrachus (Linn.) was determined in this study using hematological (including hemoglobin, hematocrit, total erythrocyte count, total leukocyte count, and mean corpuscular hemoglobin concentration) and biochemical (including total serum protein, serum glucose, triglyceride, cholesterol, albumin) biomarkers at predetermined exposure concentrations (0.069 and 0.139 mg/L). Although significant differences were observed between exposure groups for all hematological and biochemical variables, a distinctive reduction in hemoglobin levels, hematocrit, and total erythrocyte count in the mercury-exposed fish compared to the control was observed from 24 h-96 h. Similarly, marked differences in serum globulin, total serum protein and cholesterol levels were observed across exposure groups from 24 h-96 h. While the marked hematological responses strongly suggestive of toxicant-related anemia, the marked biochemical responses suggest immune-modulation and metabolic disruption. The magnitude of toxic effects under graded toxicant exposures for weighted scores of combined biomarker response index (IBR) indicated an approximately 3-fold deterioration in overall health of mercury-exposed fish compared to control group. Depictions of hematological and biochemical effects in hardy species like Clarias batrachus indicate an imminent onset of anemia, immune-modulation and metabolic disruption within 24hs of exposure to inorganic mercury. Such observations for portends greater deleterious effects to less hardy aquatic biota under acute inorganic mercury environmental exposures.

Investigation of Protein Biomarkers and Oxidative Stress in Pinirampus pirinampu Exposed to Mercury Species from the Madeira River, Amazon-Brazil

In recent decades, the scientific community has widely debated the contamination of fish in the Amazon region by mercury species. As the diet of riverside populations in the Amazon region is based mainly on fish, these populations are exposed to mercurial species that can cause serious and irreversible damage to their health. The risks of consuming fish exposed to mercurial species in the Amazon region have motivated toxicological investigations. However, the effect of mercurial species on protein and enzyme levels is still controversial. In this work, analytical and bioanalytical techniques Two-dimensional polyacrylamide gel electrophoresis [2D-PAGE] Graphite Furnace Atomic Absorption Spectrometry [GFAAS], and Mass Spectrometry in Sequence with Electrospray Ionization [ESI-MS/MS] were used to identify proteins associated with mercury (metal-binding protein) in muscle and liver tissues of the fish species Pinirampus pirinampu from the Madeira River, in the Brazilian Amazon. Enzymatic and lipid peroxidation analyses were also used to assess changes related to oxidative stress. Determinations of total mercury by GFAAS indicated higher concentrations in liver tissue (555 ± 19.0 µg kg^-1) when compared to muscle tissue (60 ± 2.0 µg kg^-1). The fractionation process of tissue proteomes by 2D-PAGE and subsequent mapping of mercury by GFAAS in the protein spots of the gels identified the presence of mercury in three spots of the liver tissue (concentrations in the range of 0.800 to 1.90 mg kg^-1). The characterization of protein spots associated with mercury by ESI-MS/MS identified the enzymes triosephosphate isomerase A, adenylate kinase 2 mitochondrial, and glyceraldehyde-3-phosphate dehydrogenase as possible candidates for mercury exposure biomarkers. The muscle tissue did not show protein spots associated with mercury. Enzymatic activity decreased proportionally to the increase in mercury concentrations in the tissues.

Effect of Methylmercury Exposure on Bioaccumulation and Nonspecific Immune Respsonses in Hybrid Grouper Epinephelus fuscoguttatus × Epinephelus lanceolatus

Simple Summary

The head kidney was primary organ that accumulated methylmercury in hybrid grouper. Muscle tissue had lower methylmercury content than the head kidney and liver. Nonspecific immune responses and bioaccumulation of methylmercury were linked to hybrid grouper health.

Abstract

Mercury (Hg) is a dangerous heavy metal that can accumulate in fish and is harmful when consumed by humans. This study investigated the bioaccumulation of mercury in the form of methylmercury (MeHg) and evaluated nonspecific immune responses such as phagocytic activity and superoxide anion (O₂⁻) production in hybrid grouper (Epinephelus fuscoguttatus × E. lanceolatus). The hybrid grouper leukocytes were incubated with methylmercury chloride (CH₃HgCl) at concentrations of 10–10,000 µg/L to determine cell viability, phagocytic activity, and O₂⁻ production in vitro. Subsequently, the grouper were exposed daily to CH₃HgCl mixed in the experimental diets at concentrations of 0, 1, 5, and 10 mg/kg for 28 days. The bioaccumulation of MeHg in the liver, head kidney, and muscle tissue was measured, and the phagocytic activity and O₂⁻ production were evaluated. In vitro results indicated that cell viability was significantly lower than that of the control group at concentrations > 500 µg/L. The phagocytic rate and O₂⁻ production at concentrations ˃ 500 and ˃ 200 µg/L, respectively, were significantly lower than those of the control group. The dietary exposure demonstrated that MeHg accumulated more substantially in the liver and head kidney compared with the muscle tissue in the treatment groups. Moreover, the cumulative concentration significantly increased with higher concentrations and more days of exposure. The phagocytic rate and O₂⁻ production in the treatment groups were significantly lower than those in the control group from days 2 and 1, respectively. In conclusion, hybrid grouper accumulated significant MeHg in the liver and head kidney compared with the muscle tissue, and higher concentrations and more exposure days resulted in decreased cell viability, phagocytic activity, and O₂⁻ production.

Tissue Distribution of Mercury and Its Relationship with Selenium in Atlantic Bluefin Tuna (Thunnus thynnus L.)

Mercury (Hg) is an important heavy metal to consider in marine predators, while selenium (Se) has a natural antagonistic effect on this metal in fish. The Atlantic bluefin tuna (ABFT, Thunnus thynnus) is a pelagic top-level predator of the trophic web and their Hg muscular content is an object of concern in food safety. Nevertheless, little is known about levels of this metal in remaining tissues, which may be important as by-product source, and its relationship with Se. Thus, concentration of both elements in liver, kidney, brain, gill and bone, in addition to muscle, of ABFT were determined. The kidney was the tissue with the highest concentration of Hg (Total-Hg, THg) and Se, and the Se/THg concentration ratio was similar in all tissues, except bone and muscle. The Selenium Health Benefit Value (HBV_Se) was positive in each specimen and tissue, indicating that the Se plays an important role against Hg not only in the muscle.

Evaluation of the PGPR Capacity of Four Bacterial Strains and Their Mixtures, Tested on Lupinus albus var. Dorado Seedlings, for the Bioremediation of Mercury-Polluted Soils

Soil contamination by mercury, which is one of the most toxic heavy metals due to its bioaccumulative capacity, poses a risk to the environment as well as health. The Almadén mining district in Ciudad Real, Spain is one of the most heavily-polluted sites in the world, making the soils unusable. Bioremediation, and more specifically phyto-rhizoremediation, based on the synergistic interaction established between plant and Plant Growth Promoting Rhizobacteria (PGPR), improves the plant’s ability to grow, mobilize, accumulate, and extract contaminants from the soil. The objective of this study is to evaluate the plant growth-promoting ability of four PGPR strains (and mixtures), isolated from the bulk soil and rhizosphere of naturally grown plants in the Almadén mining district, when they are inoculated in emerged seeds of Lupinus albus, var. Dorado in the presence of high concentrations of mercury. After 20 days of incubation and subsequent harvesting of the seedlings, biometric measurements were carried out at the root and aerial levels. The results obtained show that the seeds treatment with PGPR strains improves plants biometry in the presence of mercury. Specifically, strain B2 (Pseudomonas baetica) and B1 (Pseudomonas moraviensis) were those that contributed the most to plant growth, both individually and as part of mixtures (CS5 and CS3). Thus, these are postulated to be good candidates for further in situ phyto-rhizoremediation tests of mercury-contaminated soils.

Evaluation of dietary selenium, vitamin C and E as the multi-antioxidants on the methylmercury intoxicated mice based on mercury bioaccumulation, antioxidant enzyme activity, lipid peroxidation and mitochondrial oxidative stress

Mercury (Hg) in high exposures can be a potent life threatening heavy metal that bioaccumulate in aquatic food-chain mainly as organic methylmercury (MeHg). In this regard, fish and seafood consumptions could be the primary sources of MeHg exposure for human and fish-eating animals. The objective of the present study was to elucidate the effects of dietary supplementation of some antioxidants on induced mercury toxicity in mice model. In this study, a 30-day long investigation has been conducted to evaluate the dietary effect of selenium (Se) in combination with vitamin C and vitamin E on methylmercury induced toxicity in mice. Total 54 mice fed the diets with three levels of Hg (0, 50 or 500 μg kg^-1) and two levels of Se in combination with vitamin C and E (Se: 0, 2 mg kg^-1; vitamin C: 0, 400 mg kg^-1; vitamin E: 0, 200 mg kg^-1) in triplicates. The results show that Hg accumulated in blood and different tissues such as muscle, liver and kidney tissues of mice on dose dependent manner. The bioaccumulation pattern of dietary Hg, in decreasing order, kidney > liver > muscle > blood. Superoxide dismutase levels in blood serum showed no significant differences in mice fed the diets. However, dietary antioxidants significantly reduced the levels of thiobarbituric acid reactive substances in mice fed the mercury containing diets. Cytochrome c oxidase enzyme activities showed no significant differences as the mercury level increases in liver and kidney tissues of mice. Kaplan-Meier curve showed a dose- and time-dependent survivability of mice. Cumulative survival rate of Hg intoxicated mice fed the antioxidant supplemented diets were increased during the experimental period. Overall, the results showed that dietary Se, vitamin C and vitamin E had no effect on reducing the mercury bioaccumulation in tissues but reduced the serum lipid peroxidation as well as prolonged the cumulative survival rate in terms of high Hg exposures in mice.

A comparative study of effects of dietary mercuric chloride and methylmercury chloride on growth performance, tissue accumulation, stress and immune responses, and plasma measurements in Korean rockfish, Sebastes schlegeli

Mercury (Hg) is known as a highly toxic heavy metal, and its toxicity varies depending on its form due to different toxicokinetics between inorganic and organic Hg. Limited information on comparison of Hg toxicity concerning its chemical form by oral exposure is currently available in cultured fishes. Therefore, we conducted a comparative study to have a better understanding of distinct toxic effects between mercuric chloride (HgCl₂) and methylmercury chloride (CH₃HgCl) in Korean rockfish. The 12-weeks dietary exposure of HgCl₂ and CH₃HgCl with its graded levels (0.4-6.4 ppm) (2 × 5 factorial design) in the young-of-the-year rockfish (initial weight: 82 ± 0.3 g) resulted in neither interactive nor main effects on whole-organism responses, including growth, feed utilization, and survival. However, the distinct pattern of Hg accumulation between the two forms in dorsal muscle, brain, liver and kidney tissues was observed, showing that the rockfish fed the CH₃HgCl-contained diets exhibited the dose-dependent accumulation throughout the sampling points (1, 2, 4, 8, and 12 weeks post feeding), whereas those fed the HgCl₂-contained diets did not show such response. The CH₃HgCl exposure induced higher oxidative stress and immunotoxicity, reflected by the elevated plasma superoxide dismutase and lysozyme activities, respectively. In addition, the CH₃HgCl-induced alteration in plasma measurements, including the plasma aspartate transaminase activity and total protein level was found. Taken together, the dietary exposure of methylmercury chloride had more pronounced toxic effects than mercuric chloride in the young-of-year rockfish, needed to be taken into consideration for regulation of maximum allowed levels for Hg by its chemical form.