Toxicity of mercury during the embryonic development of Chasmagnathus granulatus (Brachyura, Varunidae)

Unveiling the molecular mechanisms and developmental consequences of mercury (Hg) toxicity in zebrafish embryo-larvae: A comprehensive approach

Mercury (Hg) is a global contaminant affecting aquatic ecosystems' health. Chronic exposure to Hg has shown that the normal development of zebrafish embryo-larvae is affected. However, the molecular mechanisms behind the toxicity of Hg on fish embryonic development are still poorly understood. This work aimed to investigate the effects of Hg exposure on zebrafish embryo-larvae using a combined approach at individual (mortality, embryo development and locomotor behavior) and biochemical (neurotoxicity and oxidative stress enzymatic activities and protein phosphatase expression) levels. The Fish Embryo Toxicity assay followed the Organization for Economic Cooperation and Development Guideline 236 and used a concentration range between 13 and 401 μg Hg/L. Lethal and developmental endpoints were examined at 24, 48, 72 and 96 hpf. Biochemical markers, including Acetylcholinesterase (AChE), Catalase (CAT), Glutathione Reductase (GR), and Glutathione-S-Transferase (GST) activities and, for the first time, the expression of the protein phosphatase 1 gamma (PP1γ) was assessed after 24, 48, 72 and 96 h of exposure to 10 and 100 μg Hg/L. The behavioral effects of a sublethal range of Hg (from 0.8 to 13 μg Hg/L) were assessed using an automated video tracking system at 120 hpf. Several developmental abnormalities on zebrafish embryos and larvae, including pericardial edema, spin and tail deformities and reduced rate of consumption of the yolk sac, were found after exposure to Hg (LC50 at 96 hpf of 139 μg Hg/L) with EC50 values for total malformations ranging from 22 to 264 μg Hg/L. After 96 hpf, no significant effects were observed in the CAT and GR activities. However, an increase in the GST activity in a concentration and time-dependent manner was found, denoting possible stress-related adaptation of zebrafish embryos to deleterious effects of Hg exposure. The AchE activity showed a response pattern in line with the behavioral responses. At the lowest concentration tested, no significant effects were found for the AChE activity, whereas a decrease in AChE activity was observed at 100 μg Hg/L, suggesting that exposure to Hg induced neurotoxic effects in zebrafish embryos which in turn may explain the lack of equilibrium found in this study (EC50 at 96 hpf of 83 μg Hg/L). Moreover, a decrease in the PP1γ expression was found after 96 h of exposure to 10 and 100 μg Hg/L. Thus, we suggest that Hg may be an inhibitor of PP1γ in zebrafish embryos-larvae and thus, along with the alterations in the enzymatic activity of GST, explain some of the developmental malformations observed, as well as the lack of equilibrium. Hence, in this study, we propose the use of PP1 expression, in combination with apical and biochemical endpoints, as a precursor for assessing Hg's toxic mechanism on embryonic development.

Planarian behavioural endpoints in ecotoxicology: A case study evaluating mercury and salinity effects

Freshwater planarians can be useful for the evaluation of contaminant stress on behavioral endpoints. In this work, we studied the sensitivity of the freshwater planarian Girardia tigrina in response to two model stressors (Hg and NaCl) by evaluating mortality, feeding rate and locomotion. A simple feeding assay with G. tigrina was devised, and an automated tracking system was used to evaluate locomotion. The estimated 96 h LC₅₀s were 176.8 μg L^-1 of Hg and 6.79 g L^-1 of NaCl. Acute effects of Hg also included the disintegration of tissues, and loss of pigmentation. Acute effects of NaCl included motionlessness and rupture of the tegument. Hg and NaCl sub-lethal exposures caused feeding inhibition and locomotion impairment. This study demonstrates the usefulness of planarians for ecotoxicological research and that sensitive behavioral endpoints can evaluate the sub-lethal impacts of stressors to freshwater invertebrates.

In vivo visualization assay to evaluate the effects of maternal exposure to mercury on offspring bioaccumulation in the oriental river prawn (Macrobrachium nipponense)

Mercury (Hg) is a persistent pollutant that accumulates in aquatic animals. However, studies related to understand how gonad tissue of this species responds to mercury exposure and elucidation of mercury bioaccumulation in crustacean offspring by cross-generational, are still sparse. The present study aimed to assess the bioaccumulation of Hg²⁺in vivo in prawn offspring by a specific aggregation-induced emission fluorogen (AIEgen). The 96 h median lethal concentration (LC₅₀) values of mercury to the juveniles were 0.072 mg/L. Hg²⁺ reduced growth performance, damaged oocyte quality, and inhibited ovary maturation, thus inhibiting gonadal maturation in intact prawns. F1 offspring were exposed to Hg²⁺ by direct transfer from their F0 parents, as shown by the distribution of mercury in gonads and fertilized eggs. In the medium containing oriental river prawn larvae, the Hg²⁺ concentration decreased rapidly, indicating fast initial larval uptake of Hg²⁺. Due to metal ion triggered AIE activity, analysis of fluorescence images showed that prawn offspring accumulated Hg²⁺ via maternal transfer, and there was a relationship among the photoluminescence intensity, the AIEgen concentration, and mercury levels. The quantitative detection of Hg²⁺ absorption from prawn larvae by the AIEgen represents a novel analytical technique to understand the dynamics of Hg²⁺ between maternal and offspring.

Transgenerational Effects of Hexavalent Chromium on Marine Medaka (Oryzias melastigma) Reveal Complex Transgenerational Adaptation in Offspring

Hexavalent chromium [Cr(VI)] pollution is one of most serious heavy metal pollutants in the coastal area and posed serious threats to marine organisms and human beings. Many studies have been conducted on its toxicological effects on living organisms from morphological to physiological aspects. However, there are few studies about the transgenerational toxicological of Cr(VI). In this study, we exposed adult marine medaka fishes with Cr(VI) and their offspring with Cr(VI) to examine transgenerational effects of Cr(VI). We found that there were mechanisms such as changing reproduction modes in males to compensate for impacts on the reproduction. There were differences and similarities between the parental effect and the environmental effect, with the former one causing more serious adverse effects on the offspring of Cr(VI)-exposed fish. It was noteworthy that there was an interaction between the parental and offspring treatment which leads to the attenuation of the parental effects on offspring when the offspring also underwent the same treatment. In addition, physiological adaptation has also been observed in fish to improve their fitness. Overall, effects of Cr(VI) on fish and their offspring were studied to pave a way to study the of mechanisms of adaptation.

Exposure to mercuric chloride induces developmental damage, oxidative stress and immunotoxicity in zebrafish embryos-larvae

Mercury (Hg) is a widespread environmental pollutant that can produce severe negative effects on fish even at very low concentrations. However, the mechanisms underlying inorganic Hg-induced oxidative stress and immunotoxicity in the early development stage of fish still need to be clarified. In the present study, zebrafish (Danio rerio) embryos were exposed to different concentrations of Hg²⁺ (0, 1, 4 and 16μg/L; added as mercuric chloride, HgCl₂) from 2h post-fertilization (hpf) to 168hpf. Developmental parameters and total Hg accumulation were monitored during the exposure period, and antioxidant status and the mRNA expression of genes related to the innate immune system were examined at 168hpf. The results showed that increasing Hg²⁺ concentration and time significantly increased total Hg accumulation in zebrafish embryos-larvae. Exposure to 16μg/L Hg²⁺ caused developmental damage, including increased mortality and malformation, decreased body length, and delayed hatching period. Meanwhile, HgCl₂ exposure (especially in the 16μg/L Hg²⁺ group) induced oxidative stress affecting antioxidant enzyme (CAT, GST and GPX) activities, endogenous GSH and MDA contents, as well as the mRNA levels of genes (cat1, sod1, gstr, gpx1a, nrf2, keap1, hsp70 and mt) encoding antioxidant proteins. Moreover, the transcription levels of several representative genes (il-1β, il-8, il-10, tnfα2, lyz and c3) involved in innate immunity were up-regulated by HgCl₂ exposure, suggesting that inorganic Hg had the potential to induce immunotoxicity. Taken together, the present study provides evidence that waterborne HgCl₂ exposure can induce developmental impairment, oxidative stress and immunotoxicity in the early development stage of fish, which brings insights into the toxicity mechanisms of inorganic Hg in fish.

Effects of glyphosate on egg incubation, larvae hatching, and ovarian rematuration in the estuarine crab Neohelice granulata

Ovigerous females of the estuarine crab (Neohelice granulate) were exposed to both pure glyphosate (2.5 mg/L and 5 mg/L) and a glyphosate formulation (Roundup Ultramax, containing glyphosate at 2.5 mg/L acid equivalent). At the end of the egg incubation period, a significant reduction in the number of hatched larvae was seen as a result of Roundup exposure. Additionally, several larvae abnormalities were seen in both pure glyphosate (2.5 mg/L) and Roundup treatments, such as hydropsy and hypopigmented eyes, and atrophied eyes were observed in the Roundup treatment. To evaluate the effect of the herbicide on ovarian rematuration, females remained exposed for 32 d. Pure glyphosate at 2.5 mg/L stimulated ovarian maturation over control levels, mainly in terms of a higher gonadosomatic index and a higher percentage of vitellogenic oocytes. A plausible hypothesis to be tested in further experiments is that exposure to glyphosate disrupts the hormonal system controlling reproduction.

Modulation of immune-associated parameters and antioxidant responses in the crab (Scylla serrata) exposed to mercury

Organic and inorganic contaminants can suppress immune function in molluscs and crustaceans. It was postulated that metals could modulate immune function in marine crabs. To test this hypothesis, sublethal effects of mercury (Hg) on cellular immune and biochemical responses of crabs were determined. When crabs were exposed for 14 d to environmentally-relevant concentrations of Hg, changes in immune-associated parameters including, total haemocyte count, lysosomal membrane stability, phenoloxidase, super oxide generation and phagocytosis were observed. Oxidative stress, as measured by lipid peroxidation, antioxidant responses, including superoxide dismutase and catalase activities and glutathione-mediated antioxidant enzymes in serum, haemocyte lysate, gills, hepatopancreas and muscle were assessed in crabs exposed to Hg. Exposure to Hg resulted in significantly lesser immune-associated parameters in haemolymph and antioxidants in all tissues studied. Conversely, GST and phenoloxidase activity, were greater in crabs exposed to Hg. Responses of antioxidant parameters (SOD, CAT and GP(x)) were positively correlated with immune responses, including THC, superoxide and phagocytosis. These results were postulated to be due to an immediate response of antioxidant defense to oxygen radicals generated. Overall, the results suggest that 14 d exposure to environmentally realistic concentrations of Hg causes immunomodulation and potentially harmful lessened antioxidant defenses of crabs.

Effect of mercury and cadmium on early life stages of Java medaka (Oryzias javanicus): a potential tropical test fish

Several organisms have been used as indicators, bio-monitoring agents or test organisms in ecotoxicological studies. A close relative of the well established Japanese medaka, the Java medaka (Oryzias javanicus), has the potential to be a test organism. The fish is native to the estuaries of the Malaysian Peninsula, Thailand, Indonesia and Singapore. In this study, newly fertilised eggs were exposed to different concentrations of Cd and Hg. Observations were done on the development of the embryos. Exposure to low levels of Cd and Hg (0.01-0.05 ppm) resulted in several developmental disorders that led to death. Exposure to ≥1.0 ppm Cd resulted in immediate developmental arrest. The embryos of Java medaka showed tolerance to a certain extent when exposed to ≥1.0 ppm Hg compared to Cd. Based on the sensitivity of the embryos, Java medaka is a suitable test organism for ecotoxicology in the tropical region.

Biokinetics of Hg and Pb accumulation in the encapsulated egg of the common cuttlefish Sepia officinalis: radiotracer experiments

Uptake and depuration kinetics of dissolved (203)Hg and (210)Pb were determined during the entire embryonic development of the eggs of the cuttlefish, Sepia officinalis (50d at 17 degrees C). (203)Hg and (210)Pb were accumulated continuously by the eggs all along the development time reaching load/concentration ratio (LCR) of 467+/-43 and 1301+/-126g, respectively. During the first month, most of the (203)Hg and (210)Pb remained associated with the eggshell indicating that the latter acted as an efficient shield against metal penetration. From this time onwards, (203)Hg accumulated in the embryo, indicating that it passed through the eggshell, whereas (210)Pb did not cross the chorion during the whole exposure time. It also demonstrated that translocation of Hg associated with the inner layers of the eggshell is a significant source of exposure for the embryo. This study highlighted that the maturing embryo could be subjected to the toxic effects of Hg in the coastal waters where the embryonic development is taking place.

Responses of a free-living marine nematode community to mercury contamination: results from microcosm experiments

A microcosm experiment was used to examine the effects of mercury contamination on a free-living nematode community in a Tunisian lagoon. Sediments were contaminated with three mercury concentrations (low, 0.084 ppm; medium, 0.167 ppm; and high, 0.334 ppm), and effects were examined after 60 days. Results from multiple-comparison tests showed significant differences between nematode assemblages from undisturbed control and those from mercury treatments. Most univariate measures decreased significantly with increasing level of Hg contamination. Results from multivariate analyses of the species abundance data demonstrated that responses of nematode species to mercury contamination were varied: Araeolaimus bioculatus was eliminated at all the mercury doses tested and seemed to be an intolerant species to mercury contamination; Marylynnia stekhoveni increased at low and medium concentrations and appeared to be an "opportunistic" species at these doses, whereas Prochromadorella neapolitana, which increased at all doses tested (0.084, 0.167, and 0.334 ppm), seemed to be a "mercury-resistant" species.

Sulfur-Containing Hetero-Calix[4]pyrroles as Mercury(II) Cation-Selective Receptors: Thermodynamic Aspects

Two sulfur-containing hybrid calix[4]pyrrole derivatives (III and IV) have been synthesized and fully characterized. Several analytical techniques (1H NMR, conductance measurements, UV-vis spectrophotometry, titration potentiometry, and titration calorimetry) have been used to assess the interaction between these hybrid calixpyrrole receptors and metal cations in acetonitrile and dimethylsulfoxide. The partition constants of calix[4]pyrrole, I, II, and IV in the acetonitrile-hexane solvent system and the solubilities of the ligands in various solvents at 298.15 K were determined. 1H NMR measurements reveal the sites of interaction of calixpyrrole ligands with metal cations in CD3CN. Conductance and UV-vis spectrophotometric measurements were performed to establish the composition of mercury(II) calixpyrrole complexes in acetonitrile at 298.15 K. Titration calorimetry was used to quantitatively assess Hg(II)-calixpyrrole interactions. Thus the thermodynamics of complexation of calixpyrrole ligands with the mercury(II) cation in acetonitrile at 298.15 K are reported. Potentiometric titrations were also used to establish the stepwise stability constants for the complexation of calix[3]thieno[1]pyrrole with the Hg(II) cation in acetonitrile at 298.15 K. The results show that replacement of one or more pyrrole units by thiophene rings in calix[4]pyrrole has tuned significantly the discrimination ability of these ligands between anions and enables the produced hybrid calixpyrroles to bind selectively with Hg(II) in acetonitrile. No interaction was observed between these ligands and other metal cations in acetonitrile.

Enzymatic biomarkers in the crab Carcinus maenas from the Minho River estuary (NW Portugal) exposed to zinc and mercury

Zinc and mercury are common contaminants of estuaries and coastal ecosystems where they may induce adverse effects on the biota. Carcinus maenas is a key-species in several European estuaries, living in close association with the sediment where a considerable number of environmental contaminants, including zinc and mercury may accumulate. In the present study, the acute effects of zinc and mercury on C. maenas from the Minho River Estuary (NW Portugal) were investigated by using the activity of the enzymes cholinesterase (ChE), lactate dehydrogenase (LDH) and glutathione S-transferases (GST) as effects' criteria. Crabs were exposed for 96h to several concentrations of Zn(2+) (0, 1.84, 3.71, 7.39 and 14.79mg/l) or Hg(2+) (0, 0.09, 0.19, 0.37 and 0.74mg/l). The choice of Zn(2+) test concentrations was based on the LC50 value (14.86mg/l) determined in a first part of the study, while the choice of Hg(2+) concentrations was based on values from the literature. At the end of the bioassays, eye, muscle and hepatopancreas tissues were isolated and used for ChE, LDH and GST determinations, respectively. Zinc significantly inhibited ChE activity (p<0.05, EC50=14.68mg/l), caused significant alterations of GST activity (p<0.05) and induced LDH activity (p<0.05) at the exposure of 14.79mg/l. Mercury significantly inhibited ChE activity (p<0.001, LOEC=0.09mg/l, EC50=0.235mg/l) and increased both GST (p<0.05, LOEC=0.774mg/l) and LDH activities (p<0.05, LOEC=0.119mg/l). These results suggest that both metals interfere with cholinergic neurotransmission in C. maenas by inhibiting ChE activity. In addition, they also interfere with GST activity and this may reduce the capacity of detoxification of some chemicals and/or to increase the probability of oxidative stress to occur. Furthermore, both metals increase LDH activity, suggesting an interference with energy production pathways. Therefore, the presence of zinc and mercury in estuaries at concentrations in the mg/l or mug/l range, respectively, may have a negative impact on C. maenas.