Water chemistry alters gene expression and physiological end points of chronic waterborne copper exposure in zebrafish, Danio rerio

Trace metals in surface water of the Pechora River and its tributaries: Content, water quality and risks assessment (Arctic Ocean basin)

The article is devoted to the study of trace metals content in the Pechora River and its tributaries, which is the largest river in the Arctic Ocean basin. The study of physico-chemical properties of water showed aerobic conditions of low-mineralised water with pH values suitable for drinking water. Analysis of the content of trace metals has revealed that the main flow of Al, Mn, Co, Cu, Zn, V, As, Mg and Fe into the Pechora River comes from its tributaries. The average content of Cr, Mn, Cu, Zn, Pb, V, As, Mg and Fe in the water of the Pechora River delta was higher than for the main river channel, which indicates the transfer of a large mass of trace metals to the Barents Sea and the Arctic water ecosystem. For the studied river and tributaries, chronic pollution of the Pechora River with Al, Mn, Ba, Fe, Cu, U and Pb metals has adverse effects on waterborne organisms. The degree of contamination of natural waters by metals has been assessed using water quality indices (TMPI, TMEI, TMTI) and public health risks (HI and CR). Low contamination and weak toxicity of the Pechora River water has been determined, but serious contamination, toxicity and high non-carcinogenic risk of some of its tributaries have been observed. A high carcinogenic risk has been established for the consumption of all water sampled. Al, Ni, As, Fe, Mn, Cr, Cd, and Ba were found to be the highest priority contaminating trace metals. At present, contamination of the Pechora River and its tributaries with trace metals is formed under the influence of dissolution processes of rocks of the Ural Mountains, the Taminsky Ridge, underlying mineral deposits, permafrost degradation processes and glacier melting. Anthropogenic factors include oil and gas production and transportation.

Sex-biased response of pollution biomarkers in fish: Insights from the killifish Poecilia vivipara

Copper ions (Cu) are one of the most frequent trace-contaminants found in Brazilian waters and, although considered as an essential element, in high concentrations can accumulate and induce toxicity. Biomarkers are important tools that can be used to assess these impacts, but to be considered trustworthy, they have to be previously tested in target organisms through laboratory studies under controlled conditions. However, many of these experiments are conducted using only males, as it is believed that the hormonal variation of females can bias the results, increasing data variability. Notwithstanding, few studies have actually tested this hypothesis, highlighting the importance of considering and measuring the role of sex in ecotoxicological studies. The aim this study was to evaluate the influence of sex on biomarkers classically used in environmental monitoring programs using the fish Poecilia vivipara as model. For this, females and males were exposed for 96 h to two Cu concentrations (9 and 20 μg/L) and a control group. In liver and gills, Cu accumulation, total antioxidant capacity (TAC) and lipid peroxidation (LPO) were evaluated. In addition, samples of peripheral blood were used for neutrophil to lymphocyte ratio determination, a measure of the onset of secondary stress. Results show that Cu hepatic accumulation did not differ between females and males, but higher levels of this metal were observed in exposed animals compared to control fish. Additionally, interactive effects were observed for hepatic LPO, as males showed elevated oxidative damage in comparison to females. Moreover, Cu exposure elevated hepatic LPO relative to control only in males, but this increase in oxidative damage was not accompanied by changes in liver TAC. On the other hand, differences in branchial Cu accumulation and LPO were not observed. Conversely, control females showed elevated TAC in comparison to control males, but Cu exposure eliminated this difference. Cu exposure also induced an increase in the N:L ratio, indicating the presence of a secondary stress response unrelated to sex. Ultimately, the findings of this study demonstrate that sex can influence the response of biomarkers that are typically used in ecotoxicological investigations in a multifaceted manner. As a result, using animals from a singular sex in such studies may result in consequential outcomes, potentially leading to underestimation or overestimation of results.

Toxicity mechanisms and bioavailability of copper to fish based on an adverse outcome pathway analysis

Copper (Cu) exists in a variety of forms in different aquatic environments, and affects their bioavailability. In this study we provide a systematic review on toxicity of Cu which focuses on identifying evidence in the mechanisms of Cu toxicity, and apply an adverse outcome pathway (AOP) analysis to identify multiple potential mechanisms and their interactions of Cu toxicity to fish. This analysis process included the mechanisms of behavior toxicant, oxidative toxicant, ion regulation disruption toxicity, as well as endocrine disruption toxicity. It was found that at low levels of Cu exposure, swimming, avoid predators, locating prey and other sensory functions will be impaired, and the organism will suffer from metabolic alkalosis and respiratory acidosis following the inhibition of the carbonic anhydrase active. The main pathway of acute toxicity of Cu to fish is the inhibition of the Na⁺/K⁺-ATPase enzyme, and lead to reduced intracellular sodium absorption, as well as Cu-induced increased cell permeability, in turn resulting in increased sodium ion loss, leading to cardiovascular collapse and respiratory insufficiency. The endocrine disruption toxicity of Cu to fish caused growth inhibition and reproductive reduction. In addition, there are several key pathways of Cu toxicity that are affected by hardness (e.g., Ca²⁺) and intracellular DOC concentrations, including inhibiting Cu-induction, improving branchial gas exchange, altering membrane transport functions, decreasing Na⁺ loss, and increasing Na⁺ uptake. The results of the AOP analysis will provide a robust framework for future directed research on the mechanisms of Cu toxicity.

Identifying Behavioral Response Profiles of Two Common Larval Fish Models to a Salinity Gradient

Salinization of aquatic systems is an emerging global issue projected to increase in magnitude, frequency, and duration with climate change and landscape modifications. To consider influences of salinity on locomotor activity of common fish models, we examined behavioral response profiles of two species, zebrafish (Danio rerio) and fathead minnow (Pimephales promelas), across a gradient of sodium chloride. Following each experiment, behavior was recorded with automated tracking software and then behavioral response variables, including locomotor (e.g., distance traveled, number of movements, duration of movements) and photolocomotor changes, were examined at several speed thresholds (bursting, cruising, freezing) to identify potential salinity responses. Zebrafish responses were significantly (p < 0.05) reduced at the highest treatment level (5.78 g/L) for multiple behavioral endpoints during both dark and light phases; however, fathead minnow responses were more variable and not consistently concentration dependent. Future efforts are needed to understand behavioral response profiles in combination with anthropogenic contaminants and natural toxins across the freshwater to marine continuum, considering salinization of inland waters, sea level rise, and transport of anthropogenic contaminants and algal toxins from inland waters to coastal systems.

Antagonistic bioaccumulation of waterborne Cu(II) and Cd(II) in common carp (Cyprinus carpio) and effects on ion-homeostasis and defensive mechanisms

In the aquatic environment, metals are present as mixtures, therefore studies on mixture toxicity are crucial to thoroughly understand their toxic effects on aquatic organisms. Common carp (Cyprinus carpio) were used to assess the effects of short-term Cu(II) and Cd(II) mixtures, using a fixed concentration of one of the metals, representing 25 % of its individual 96h-LC₅₀ (concentration lethal for 50 % of the population) combined with a variable concentration of the other metal corresponding to 10, 25 or 50 % of its 96h-LC₅₀, and vice versa. Our results showed a fast Cu and Cd bioaccumulation, with the percentage of increase in the order gill > liver > carcass. An inhibitory effect of Cu on Cd uptake was observed; higher Cu concentrations at fixed Cd levels resulted in a decreased accumulation of Cd. The presence of the two metal ions resulted in losses of total Na, K and Ca. Fish tried to compensate for the Na loss through the induction of the genes coding for Na⁺/K⁺-ATPase and H⁺-ATPase. Additionally, a counterintuitive induction of the gene encoding the high affinity copper transporter (CTR1) occurred, while a downregulation was expected to prevent further metal ion uptake. An induction of defensive mechanisms, both metal ion binding protein and anti-oxidant defences, was observed. Despite the metal accumulation and electrolyte loss, the low mortality suggest that common carp is able to cope with these metal levels, at least during a one-week exposure.

Combining elevated temperature with waterborne copper: Impacts on the energy metabolism of the killifish Poecilia vivipara

We have previously demonstrated in a companion work that acclimation to 28 °C potentiated waterborne copper (Cu) toxic effects in Poecilia vivipara through oxidative stress-related processes. In the present study, we hypothesized that these results were related to kinetic metabolic adjustments in enzymes from aerobic and anaerobic pathways. To test this, P. vivipara was acclimated to two temperatures (22 °C or 28 °C) for three weeks and then exposed to Cu (control, 9 or 20 μg/L) for 96 h. The activity of enzymes from glycolysis (pyruvate kinase [PK] and lactate dehydrogenase [LDH]), Krebs cycle (citrate synthase [CS]) and the electron transport chain system (ETS) were assessed in gills, liver and muscle. Interactive effects were only seen for hepatic LDH activity, as both metal exposure and heat stress, combined or not, inhibited this enzyme, showing a suppression in anaerobic pathways. Conversely, a Cu main effect was present in the liver, expressed as an elevation in ETS activity, showing an enhancement in hepatic aerobic metabolism likely related with the very energy-demanding process of metal detoxification. Moreover, this study shows that P. vivipara has a remarkable ability to compensate heat stress in terms of energy metabolism, as we could not observe acclimation temperature effects for most of the cases. Nonetheless, a tissue-dependent effect of elevated temperature was observed, as we could observe an inhibition in muscular CS activity. Finally, it is concluded that kinetic adjustments in terms of the energy metabolism are not related with the temperature-dependent elevation of Cu toxicity in P. vivipara as we previously hypothesized.

Regulation of metal homeostasis and zinc transporters in early-life stage zebrafish following sublethal waterborne zinc exposure

In the present research, the effects of exposure to a sublethal concentration of zinc (Zn) on metal and ion homeostasis, and the regulation and the localization of various Zn transporters (i.e., the Zrt-Irt Like Protein (ZIP) family of Zn transporters), were investigated in zebrafish (Danio rerio) during early development. Exposure to an elevated level of Zn [4 μM (high) vs. 0.25 μM (control)] from 0 day post-fertilization (dpf) resulted in a significant increase in the whole body content of Zn at 5 dpf. A transient decrease in the whole body calcium (Ca) level was observed in 3 dpf larvae exposed to high Zn. Similarly, whole body nickel (Ni) and copper (Cu) contents were also reduced in 3 dpf larvae exposed to high Zn. Importantly, the magnitude of reduction in whole body Ni and Cu contents following Zn exposure was markedly higher than that in Ca content, suggesting that internal Ni and Cu balance were likely more sensitive to Zn exposure in developing zebrafish. Exposure to high Zn altered the mRNA expression levels of specific zip transporters, with an increase in zip1 (at 3 dpf) and zip8 (at 5 dpf), and a decrease in zip4 (at 5 dpf). The expression levels of most zip transporters tended to decrease from 3 dpf to 5 dpf with the exception of zip4 and zip8. Results from in situ hybridization revealed that several zip transporters exhibited distinct spatial distribution (e.g., zip8 in the intestinal tract, zip14 in the pronephric tubules). Overall, our findings suggested that exposure to sublethal concentrations of Zn disrupts the homeostasis of essential metals during early development and that different ZIP transporters may play unique roles in regulating Zn homeostasis in various organs in developing zebrafish.

Influence of 96h sub-lethal copper exposure on aerobic scope and recovery from exhaustive exercise in killifish (Fundulus heteroclitus)

Production of industrial effluents have led to increased copper (Cu) pollution of aquatic ecosystems, impacting the physiology of aquatic vertebrates. Past work has shown that Cu exerts its toxicity by disruption ion regulation and/ or increasing oxidative stress. However, it remains unclear how Cu may influence aerobic metabolism and hypoxia tolerance, two possible targets of its toxicity. To address this issue, we exposed freshwater acclimated killifish (F. heteroclitus) to a 96 h Cu exposure at a target concentration of 100 μg L^-1. We determined resting oxygen consumption (ṀO₂), ṀO_2max after exhaustive exercise, and followed ṀO₂ for 3 h in post-exercise recovery in water with either no Cu or 100 μg L^-1 Cu. We assessed hypoxia tolerance by determining the critical oxygen tension (P_crit). It was found that killifish exposed to combined 96 h Cu exposure and Cu present during metabolic measurements, showed a significant decrease in ṀO_2max and in aerobic scope (ṀO_2max - ṀO_2rest), compared to control fish. However, changes in blood and muscle lactate and muscle glycogen were not consistent with an upregulation of anaerobic metabolism as compensation for reduced aerobic performance in Cu exposed fish. Hypoxia tolerance was not influenced by the 96 h Cu exposure or by presence or absence of Cu during the P_crit test. This study suggests that Cu differentially influences responses to changes in oxygen demand and oxygen availability.

Investigating the effects of a sub-lethal metal mixture of Cu, Zn and Cd on bioaccumulation and ionoregulation in common carp, Cyprinus carpio

The aquatic environment is continuously under threat because it is the final receptor and sink of waste streams. The development of industry, mining activities and agriculture gave rise to an increase in metal pollution in the aquatic system. Thus a wide occurrence of metal mixtures exists in the aquatic environment. The assessment of mixture stress remains a challenge considering that we can not predict the toxicity of a mixture on the basis of single compounds. Therefore the analysis of the effects of environmentally relevant waterborne mixtures is needed to improve our understanding of the impact of metal pollution in aquatic ecosystems. Our aim was to assess whether 10 % of the concentration of the 96 h LC₅₀ (the concentration that is lethal to 50 % of the population in 96 h) of individual metal exposures can be considered as a "safe" concentration when applied in a trinomial mixture. Therefore, common carp were exposed to a sublethal mixture of Cu 0.07 ± 0.001 μM (4.3 ± 0.6 μg/L), Zn 2.71 ± 0.81 μM (176.9 ± 52.8 μg/L) and Cd 0.03 ± 0.0004 μM (3.0 ± 0.4 μg/L) at 20 °C for a period of one week. Parameters assessed included survival rate, bioaccumulation and physiological biomarkers related to ionoregulation and defensive mechanisms such as MT induction. Our results showed a sharp increase in Cu and Cd concentration in gills within the first day of exposure while Zn levels remained stable. The accumulation of these metals led to a Na drop in gills, liver and muscle as well as a decreased K content in the liver. Biomarkers related to Na uptake were also affected: on the first day gene expression for H⁺-ATPase was transiently increased while a concomitant decreased gene expression of the Na⁺/H⁺ exchanger occurred. A fivefold induction of metallothionein gene expression was reported during the entire duration of the experiment. Despite the adverse effects on ionoregulation all fish survived, indicating that common carp are able to cope with these low metal concentrations, at least during a one week exposure.

Influence of Selective Conditions on Various Composite Sorbents for Enhanced Removal of Copper (II) Ions from Aqueous Environments

Numerous pollutants, including dyes, heavy metals, pesticides, and microorganisms, are found in wastewater and have great consequences when discharged onto natural freshwater sources. Heavy metals are predominantly reported in wastewater. Heavy metals are persistent, non-biodegradable and toxic, transforming from a less toxic form to more toxic forms in environmental media under favourable conditions. Among heavy metals, copper is dominantly found in wastewater effluent. In this review, the effects of high concentration of copper in plants and living tissues of both aquatic animals and humans are identified. The performance of different polymer adsorbents and the established optimum conditions to assess the resultant remediation effect as well as the amount of copper removed are presented. This procedure allows the establishment of a valid conclusion of reduced time and improved Cu (II) ion removal in association with recent nano-polymer adsorbents. Nano-polymer composites are therefore seen as good candidates for remediation of Cu ions while pH range 5-6 and room temperature were mostly reported for optimum performance. The optimum conditions reported can be applied for other metal remediation and development of potent novel adsorbents and process conditions.

FTIR study of the binary effect of titanium dioxide nanoparticles (nTiO2) and copper (Cu2+) on the biochemical constituents of liver tissues of catfish (Clarias gariepinus)

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The mixture of nanomaterial and essential metal is of concern for freshwater.

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Co-exposure of titanium dioxide nanoparticles (nTiO₂) and copper (Cu²⁺) revealed.

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Notable alteration of biochemical constituents and histopathology of liver tissues.

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FTIR and histological techniques are adequate for the monitoring of changes in fish tissues.

The increasing demand for nanomaterials and essential metals leads to their discharge into the aquatic ecosystems through water run-off and this is of great concern for the aquatic biodiversity.

In the present study titanium dioxide nanoparticles (nTiO₂) and copper (Cu²⁺) effects in binary mixture on the biochemical constituents of the liver of Clarias gariepinus by using Fourrier- transform infrared (FT-IR) techniques was examined. The FT-IR revealed significant differences in absorbance intensities between the control and exposed liver tissues demonstrating changes on the critical biochemical constituents such as proteins, lipids and carbohydrates. This result further reveals the binary mixture responsible for the absence of CH₃ bending lipids in liver tissues due to their toxic effect. The observed synergistic decreasing ratio of integrated area (1545/3296) for binary mixture exposed liver tissues suggests that lipid degradation predominates over amide formation. In addition, binary mixture causes an alteration in protein secondary structures by decreasing the β turns of liver tissues. Histology in liver showed marked damages. The frequent alteration in the biochemical constituents in the liver tissues of C. gariepinus could be an indication of alteration of existing biochemical components or the expression of new components.

Metabolic implications of exposure to wastewater effluent in bluegill sunfish

Effluent from wastewater treatment plants (WWTP) contains a complex mixture of contaminants and is a major worldwide source of aquatic pollution. We examined the effects of exposure to treated effluent from a municipal WWTP on the metabolic physiology of bluegill sunfish (Lepomis macrochirus). We studied fish that were wild-caught or experimentally caged (28 d) downstream of the WWTP, and compared them to fish that were caught or caged at clean reference sites. Survival was reduced in fish caged at the effluent-contaminated site compared to those caged at the reference site. Resting rates of O₂ consumption (MO₂) were higher in fish from the contaminated site, reflecting a metabolic cost of wastewater exposure. The increases in routine MO₂ did not reduce aerobic scope (difference or quotient of maximal MO₂ and resting MO₂), suggesting that physiological compensations accompanied the metabolic costs of wastewater exposure. Fish exposed to wastewater also had larger hearts and livers. The activity of mitochondrial enzymes (cytochrome c oxidase, citrate synthase) per liver mass was unaltered across treatments, so the increased mass of this organ increased its cumulative oxidative capacity in the fish. Wastewater exposure also reduced glycogen content per liver mass. The effects of caging itself, based on comparisons between fish that were wild-caught or caged at clean sites, were generally subtle and not statistically significant. We conclude that exposure to wastewater effluent invokes a metabolic cost that leads to compensatory physiological adjustments that partially offset the detrimental metabolic impacts of exposure.

Waterborne copper is more toxic to the killifish Poecilia vivipara in elevated temperatures: Linking oxidative stress in the liver with reduced organismal thermal performance

In this study, we measured the interactive effect of temperature (22 °C and 28 °C) and waterborne copper (Cu) contamination (9 μg/L and 20 μg/L) on the killifish Poecilia vivipara. Endpoints analyzed included parameters involved in Cu-accumulation, antioxidant capacity (antioxidant capacity against peroxyl radicals [ACAP] and total antioxidant capacity [TAC]), oxidative damage (lipid peroxidation [LPO]) and upper thermal tolerance (critical thermal maximum [CTMax]). Results show that Cu hepatic accumulation was elevated in 28 °C in comparison to 22 °C in both exposure groups. For gills, this was true only in 20 μg/L. Moreover, hepatic and brachial accumulation were concentration-dependent in both acclimation temperatures. Additionally, Hepatic ACAP and TAC were elevated in animals acclimated to 28 °C and only the animals kept at this temperature had reduced ACAP and TAC levels facing metal exposure (9 and 20 μg/L). Similarly, the combination of elevated temperature and Cu exposure raised hepatic LPO levels. Finally, animals acclimated to 28 °C had higher CTMax levels in comparison to fish acclimated to 22 °C both in control and exposed animals, however, CTMax of contaminated fish were only reduced in comparison to control in animals kept at 28 °C. Concluding, we show that the physiological mechanism besides the potentiating effect of elevated temperature in Cu toxicity is related to higher hepatic and branchial metal accumulation and elevated oxidative stress in the liver, outlined by reduced antioxidant capacity and elevated oxidative damage. We also show that these outcomes lead to compromised organismal performance, characterized by reduced CTMax. Finally, it is concluded that Cu exposure in warmer periods of the year or within global warming predictions may be more hazardous to fish populations.

Influence of interspecific interactions on avoidance response to contamination

An increasing number of studies have shown the ability of organisms to escape from toxic effects due to contamination, by moving spatially towards less contaminated habitats. However, this issue has been investigated in monospecific scenarios, without considering possible interactions between species during the contamination avoidance process. It is widely known that the spatial distribution of one species can be affected by another one, in different ways. Therefore, the main question addressed in the present study was as follows: Might interspecific interaction between the freshwater fish Danio rerio (zebrafish) and Poecilia reticulata (guppy) change their behavior patterns in terms of avoidance in the presence of a copper gradient? Zebrafish and guppies exposed to a copper gradient were tested for avoidance responses in a free-choice, non-forced, static, multi-compartmented exposure system, using two distinct approaches: (1) monospecific tests, in which only one species was exposed to the copper gradient, at two different population densities; and (2) multispecific tests, in which both species were tested simultaneously. In the control (with no copper) monospecific tests, both species were randomly distributed; however, in the control multispecific test, P. reticulata tended to aggregate. In the monospecific tests with a copper gradient, both species avoided copper in a similar way, with AC₅₀ (concentration triggering avoidance in 50% of the exposed population) values between 15 and 18 μg·L^-1, irrespective of the population density. However, in the multispecific tests, P. reticulata displaced D. rerio to previously avoided copper levels, consequently increasing the AC₅₀ of D. rerio to 75 μg·L^-1. This study shows the importance of understanding the interactions among species in contaminated areas, and the way that one species can prevent the avoidance behavior of another.

Effects of life-time exposure to waterborne copper on the somatotropic axis of the viviparous fish Poecilia vivipara

Reduced fish growth following chronic exposure to dissolved copper (Cu) is well reported in the literature. Nevertheless, information on the mechanism(s) involved in this process is scarce. Therefore, we evaluated growth, gene expression and concentrations of proteins related to growth regulation in the viviparous guppy Poecilia vivipara chronically exposed to dissolved Cu. Newborn (<24 h after birth) fish were kept under control conditions or exposed to environmentally relevant concentrations of Cu (5 and 9 μg/L) in salt water (24 ppt) for 345 days. After exposure, fish growth was evaluated based on body weight and length. Also, growth hormone (gh) mRNA expression was evaluated in brain, while growth hormone receptor 1 (ghr1) and 2 (ghr2) mRNA expressions were analyzed in brain, skeletal muscle and liver. In turn, insulin-like growth factor 1 (igf1) and 2 (igf2) mRNA expressions were evaluated in skeletal muscle and liver. Additionally, Gh concentration was assessed in brain, while Ghr concentration was evaluated in skeletal muscle and liver. Exposure to 9 μg/L Cu reduced fish body weigh and length. Metal exposure affected mRNA expression only in skeletal muscle. Reduced ghr2 mRNA expression was observed in guppies exposed to 5 and 9 μg/L Cu. Additionally, reduced igf1 and igf2 mRNA expressions were observed in guppies exposed to 9 μg/L Cu. However, no significant change in Ghr concentration was observed. The reduced ghr2 mRNA expression suggests that chronic Cu exposure induced an insensitivity of the skeletal muscle to Gh, thus resulting in reduced igf1 and igf2 mRNA expression which lead to reduced fish growth. These findings indicate that chronic exposure to dissolved Cu disrupts the somatotropic axis regulation, thus helping to elucidate the mechanism underlying the Cu-dependent inhibition of growth observed in the viviparous fish P. vivipara.

Distribution and speciation of zinc in the gills of rainbow trout (Oncorhynchus mykiss) during acute waterborne zinc exposure: Interactions with cadmium or copper

We utilized micro X-ray fluorescence imaging (μ-XFI) and micro X-ray absorption near-edge spectroscopy (μ-XANES), which are both synchrotron-based techniques to investigate Zn distribution profile, its co-localization patterns with Ca, S, and Fe and speciation in the gills of rainbow trout (RBT). Fish (~100 g) were exposed to acutely toxic levels of waterborne Zn alone and in combination with waterborne Cd or Cu for 24 h (each at 1 × 96 h LC₅₀). Gill sections were prepared and analyzed at the VESPERS beamline of the Canadian Light Source. The primary lamellae of the fish gill were found to be the primary area of Zn accumulation. These regions also correspond to the zones of mitochondria rich cells localization in fish gills, supporting the putative roles of these cells in metal uptake. Zn was also found to predominantly co-localize with Ca and S, but not with Fe, indicating the roles of Ca and S in intracellular Zn handling. Zn distribution in the gill was markedly reduced during co-exposure to Cd, but not to Cu, suggesting a competitive interaction between Zn and Cd for uptake. The speciation of Zn in the gill was dominated by Zn-phosphate, Zn-histidine and Zn-cysteine species; however, the interactions of Zn with Cd or Cu resulted in the loss of Zn-cysteine. Overall, our findings provide important novel insights into the interactions of Zn, Cd and Cu in the fish gill, which may ultimately help to explain the mechanisms underlying the acute toxicity of these metals in binary mixture to fish.

Cellular and molecular responses of adult zebrafish after exposure to CuO nanoparticles or ionic copper

Due to their antimicrobial, electrical and magnetic properties, copper nanoparticles (NPs) are suitable for a vast array of applications. Copper can be toxic to biota, making it necessary to assess the potential hazard of copper nanomaterials. Zebrafish (Danio rerio) were exposed to 10 µg Cu/L of CuO NPs of ≈100 nm (CuO-poly) or ionic copper to compare the effects provoked after 3 and 21 days of exposure and at 6 months post-exposure (mpe). At 21 days, significant copper accumulation was only detected in fish exposed to ionic copper. Exposure to both copper forms caused histopathological alterations that could reduce gill functionality, more markedly in the case of ionic copper. Nevertheless, at 6 mpe higher prevalences of gill lesions were detected in fish previously exposed to CuO-poly NPs. No relevant histological alterations were detected in liver, but the lysosomal membrane stability test showed significantly impaired general health status after exposure to both metal forms that lasted up to 6 mpe. 69 transcripts appeared regulated after 3 days of exposure to CuO-poly NPs, suggesting that NPs could produce oxidative stress and reduce metabolism and transport processes. Thirty transcripts were regulated after 21 days of exposure to ionic copper, indicating possible DNA damage. Genes of the circadian clock were identified as the key genes involved in time-dependent differences between the two copper forms. In conclusion, each copper form showed a distinct pattern of liver transcriptome regulation, but both caused gill histopathological alterations and long lasting impaired health status in adult zebrafish.

Juvenile roach (Rutilus rutilus) increase their anaerobic metabolism in response to copper exposure in laboratory conditions

This study aims to determine the potential impairment of cell energy synthesis processes (glycolysis and respiratory chain pathways) by copper in juvenile roach at different regulation levels by using a multi-marker approach. Juvenile roach were exposed to 0, 10, 50, and 100 µg/L of copper for 7 days in laboratory conditions. The glycolysis pathway was assessed by measuring the relative expression levels of 4 genes encoding glycolysis enzymes. The respiratory chain was studied by assessing the electron transport system and cytochrome c oxidase gene expression. Muscle mitochondria ultrastructure was studied, and antioxidant responses were measured. Furthermore, the main energy reserves-carbohydrates, lipids, and proteins-were measured, and cellular energy was evaluated by measuring ATP, ADP, AMP and IMP concentrations. This study revealed a disturbance of the cell energy metabolism due to copper exposure, with a significant decrease in adenylate energy charge in roach exposed to 10 μg/L of copper after 1 day. Moreover, ATP concentrations significantly decreased in roach exposed to 10 μg/L of copper after 1 day. This significant decrease persisted in roach exposed to 50 µg/L of copper after 7 days. AMP concentrations increased in all contaminated fish after 1 day of exposure. In parallel, the relative expression of 3 genes encoding for glycolysis enzymes increased in all contaminated fish after 1 day of copper exposure. Focusing on the respiratory chain, cytochrome c oxidase gene expression also increased in all contaminated fish at the two time-points. The activity of the electron transport system was not disturbed by copper, except in roach exposed to 100 µg/L of copper after 1 day. Copper induced a metabolic stress. Juvenile roach seemed to respond to the ensuing high energy demand by increasing their anaerobic metabolism, but the energy produced by the anaerobic metabolism is unable to compensate for the stress induced by copper after 7 days. This multi-marker approach allows us to reach a greater understanding of the effects of copper on the physiological responses of juvenile roach.

Common barbel (Barbus barbus) as a bioindicator of surface river sediment pollution with Cu and Zn in three rivers of the Danube River Basin in Serbia

River sediments are a major source of metal contamination in aquatic food webs. Due to the ability of metals to move up the food chain, fishes, occupying higher trophic levels, are considered to be good environmental indicators of metal pollution. The aim of this study was to analyze the metal content in tissues of the common barbel (Barbus barbus), a rheophilous cyprinid fish widely distributed in the Danube Basin, in order to find out if it can be used as a bioindicator of the metal content in the river sediment. We analyzed bioavailable concentrations of 15 elements (Al, As, B, Ba, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Se, Sr, and Zn) in sediments of the Danube (D), the Zapadna Morava (ZM), and the Južna Morava (JM) using the inductively coupled plasma spectroscopy (ICP-OES). The barbel specimens were collected in the proximity of sediment sampling sites for the analysis of metals in four tissues, gills, muscle, intestine, and liver. The sediment analysis indicated that the ZM is the most polluted with Cu, Ni, and Zn compared to other two rivers. The JM had the lowest concentrations of almost all observed elements, while the Danube sediments were mainly characterized by higher concentrations of Pb. The fish from the ZM had the highest concentration of Cu and Ni in the liver and intestine, and of Zn in the muscle tissue, which was in accordance with the concentrations of these metals in the sediment. Scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM-EDS) was used for further analyses of metal interactions with fish tissues. The results suggest that the barbel can potentially be used as a bioindicator of sediment quality with respect to metal contamination.

The oxidative stress response in freshwater-acclimated killifish (Fundulus heteroclitus) to acute copper and hypoxia exposure

Aquatic organisms face multiple stressors in natural ecosystems. Here we examine the effects of moderate hypoxia and low-level copper (Cu) on freshwater (FW)-acclimated killifish. Both Cu and hypoxia can affect oxidative stress in fish, but it is unclear if in combination these two stressors would act synergistically. We exposed killifish for 96h to Cu in normoxia (total 23.4±0.9μg CuL(-1)), or either no Cu (2.33±0.01mg O2 L(-1)) or with Cu in hypoxia (23.6±0.8μg Cu L(-1); 2.51±0.04mg O2 L(-1)), and compared them to normoxic controls with no added Cu (0.7±0.1μg Cu L(-1); 9.10±0.00mg O2 L(-1)) at a hardness of 140mgL(-1) as CaCO3 equivalents. Gills showed significant Cu accumulation with both excess waterborne Cu in normoxia and in hypoxia. This was accompanied by increases in gill catalase (CAT) activity but with no significant changes in either protein carbonyls or lipid peroxidation (TBARS). Hypoxia alone decreased gill protein carbonyls. Liver showed no change in Cu load, but a significant decline in CAT activity occurred with Cu in normoxia. Liver showed an increase in TBARS with Cu in normoxia. Cu when combined with hypoxia caused a significant decline in cytochrome c oxidase (COX) and citrate synthase (CS) activity in gill and liver. Thus, low waterborne levels of Cu and moderate hypoxia both affected gill and liver phenotypes. However, killifish are tolerant of Cu and hypoxia, and there was no evidence of a synergistic response to exposure to the two stressors combined compared to each stressor alone.

Evidence of common cadmium and copper uptake routes in zebrafish Danio rerio

Cadmium and copper accumulations in gills of zebrafish were measured during a 48 h exposure to 0.025 μM 106Cd and 0.05 or 0.5 μM 65Cu as a single metal or their mixtures. The gill transcript levels of genes involved in the transport of Cu (CTR1 and ATP7A), Na (NHE-2), Ca (ECaC), divalent metals (DMT1), and Zn (ZIP8) were also compared between treatments at 24 and 48 h. Cd uptake was significantly suppressed in the presence of Cu, indicating interaction between Cu and Cd at uptake sites, but Cu uptake was unaffected by Cd. The decrease in Cd accumulation rates in the presence of Cu was associated with an increase in transcript abundance of ECaC at 24 h and DMT1 at 48 h and a decrease in Zip8 transcript levels, all known as routes for Cd uptake. Fish exposed to 0.5 μM 65Cu show an increase in gill ATP7a transcript abundance, suggesting that Cu is removed from the gill and is transferred to other organs for detoxification. A reduction in gill CTR1 transcript abundance was observed during the Cu-Cd exposure; this may be a regulatory mechanism to reduce Cu loading if Cu is entering the gills by other uptake routes, such as ECaC and DMT1.

Chemosensory mediated behaviors and gene transcription profiles in wild yellow perch (Perca flavescens) from metal contaminated lakes

The olfactory system of fish is sensitive to the toxic effects of low concentrations of contaminants. To investigate the effects of long-term metal exposure on olfaction in wild yellow perch (Perca flavescens), fish from one clean (Geneva Lake) and two metal-contaminated lakes (Ramsey and Hannah lakes) were collected in and around the metal-mining district of Sudbury, ON. Two different techniques were used to measure the effects of exposure to environmental contamination: (i) behavioral responses were recorded in response to conspecific skin extract and (ii) gene transcription differences in olfactory rosettes were characterized using a novel, 1000-candidate gene yellow perch microarray. Behavioral assays performed on fish from the clean lake demonstrated avoidance of a conspecific skin extract, while fish from metal contaminated lakes showed no avoidance response. A total of 109 out of the 1000 genes were differentially transcribed among the lakes. Most of the differentially transcribed genes were between the two metal contaminated lakes relative to either of the contaminated lakes and the reference lake. No genes were differentially expressed between Geneva Lake (clean) and Hannah Lake (metal contaminated). These results demonstrated that even though the different populations of fish from both Hannah and Ramey lakes were affected at the behavioral level, the impairment of olfaction was not measurable using gene transcriptional changes in olfactory rosettes.

Salinity-dependent copper accumulation in the guppy Poecilia vivipara is associated with CTR1 and ATP7B transcriptional regulation

Copper (Cu) accumulation and regulation of key-genes involved in Cu homeostasis were evaluated in freshwater- and saltwater-acclimated guppies Poecilia vivipara. Fish were exposed (96h) to environmentally relevant concentrations of dissolved Cu (0, 5.0, 9.0 and 20.0μg/L). In freshwater guppies, gill and liver Cu accumulation was dependent on Cu concentration in the exposure medium. In saltwater guppies, this dependence was observed only in the gut. These findings indicate that Cu accumulation was salinity- and tissue-dependent. Key genes involved in Cu metabolism were sequenced for the first time in P. vivipara. Transcripts coding for the high-affinity copper transporter (CTR1) and copper-transporting ATPase (ATP7B) were identified using polymerase chain reaction (PCR) and gene sequencing. The full-length CTR1 open reading frame (1560bp) and a partial ATP7B (690bp) were discovered. Predicted amino acid sequences shared high identities with the CTR1 of Fundulus heteroclitus (81%) and the ATP7B of Sparus aurata (87%). Basal transcriptional levels addressed by RT-qPCR in control fish indicate that CTR1 and ATP7B was highly transcribed in liver of freshwater guppies while CTR1 was highly transcribed in gut of saltwater guppies. This could explain the higher Cu accumulation observed in liver of freshwater guppies and in gut of saltwater guppies, because CTR1 is involved in Cu uptake. Reduced gill mRNA expression of CTR1 was observed in freshwater guppies exposed to 20.0μg/L Cu and in saltwater guppies exposed to 5.0μg/L Cu. In turn, reduced mRNA expression of gut ATP7B was observed in freshwater and salt water guppies exposed to 9.0 and 20.0μg/L Cu. Liver CTR1 and ATP7B transcription were not affected by Cu exposure. These findings suggest that gill CTR1 and gut ATP7B are down-regulated to limit Cu absorption after exposure to dissolved Cu, while liver CTR1 and ATP7B levels are maintained to allow Cu storage and detoxification. In conclusion, findings reported here indicate that Cu accumulation in the euryhaline guppy P. vivipara is tissue specific and dependent on water salinity. They also suggest that Cu homeostasis involves a differential transcriptional regulation of the newly identified Cu transporters, CTR1 and ATP7B.

Physiological response to a metal-contaminated invertebrate diet in zebrafish: importance of metal speciation and regulation of metal transport pathways

Dietary metal uptake in fish is determined by metal bioavailability in prey and the metal requirements of the fish. In this study zebrafish were fed the intertidal polychaete worm Nereis diversicolor (3% wet weight day(-1)) collected from Ag, Cd and Cu-impacted Restronguet Creek (RC) or a reference site, Blackwater estuary (BW), for 21 days. On days 0, 7, 14 and 21 fish were fed a single meal of RC or BW N. diversicolor labeled with (110m)Ag or (109)Cd for measurements of metal assimilation efficiency (AE). Zebrafish intestines were also taken for mRNA expression analysis of copper transporter 1 (ctr1), divalent metal transporter 1 (dmt1) and metallothionein 2 (mt2). No significant difference was observed in the AE of (109)Cd in metal naïve fish at day 0 between RC and BW worms, 11.8±2.1 and 15.3±2.8%, respectively. However, AE of (110m)Ag was significantly greater in fish fed worms from BW compared to RC, 5±1.2% and 1.6±0.5%, respectively at day 0. Fractionation analysis of radiolabeled metal partitioned in N. diversicolor from RC revealed a greater proportion of Ag (40±1.1%) in a fraction containing protein and organelle bound metal, associated with high trophic availability, compared to BW polychaetes (24±2.5%). Lower AE of (110m)Ag from RC polychaetes is therefore unlikely due to speciation of (110m)Ag in N. diversicolor from RC, but to the high concentration of Cu, a potential Ag antagonist. Exposure to RC polychaetes significantly increased the AE of (110m)Ag (6.2±1%), but not (109)Cd, from RC worms, after 21 days. AE of (110m)Ag and (109)Cd was unaffected by pre-exposure to BW. Elevated concentration of intestinal Cu and increased expression of ctr1, dmt1 and mt2 after 14 days exposure in fish fed worms from RC suggest altered Cu handling strategy of these fish which may increase AE of Ag via shared Ag and Cu transport pathways. These data suggest metal exposure history of invertebrates may affect metal bioavailability to fish, and fish may alter intestinal uptake physiology during chronic dietary exposure with implications for the assimilation and toxicity of dietary metals.