Oxidative stress and metabolic responses to copper in freshwater- and seawater-acclimated killifish, Fundulus heteroclitus

pCO2-driven seawater acidification affects aqueous-phase copper toxicity in juvenile flounder Paralichthys olivaceus: Metal accumulation, antioxidant defenses and detoxification in livers

Ocean acidification potentially influences the biotoxicity of metals and the antioxidant defense systems of marine organisms. This study investigated how pCO₂-driven seawater acidification (SA) affected aqueous-phase copper (Cu) toxicity in the juvenile flounder Paralichthys olivaceus from the perspective of hepatic oxidative stress and damage to better understand the mechanisms underlying the biological effects produced by the two stressors. Fish were exposed to aqueous-phase Cu at relevant ambient and polluted concentrations (0, 5, 10, 50, 100 and 200 μg L^-1) at different pH levels (no SA: pH 8.10; moderate SA: pH 7.70, pCO₂ ∼1353.89 μatm; extreme SA: pH 7.30, pCO₂ ∼3471.27 μatm) for 28 days. A battery of biomarkers in the livers was examined to investigate their roles in antioxidant defense and detoxification in response to coexposure. Hepatic Cu accumulation (30.22-184.90 mg kg^-1) was positively correlated with Cu concentrations. The biomarkers responded adaptively to different redox states following SA and Cu exposure. In unacidified seawater, increases in Cu concentrations significantly induced hepatic lipid peroxidation (LPO, by up to 27.03 %), although compensatory responses in antioxidant defenses and detoxification were activated. Moderate SA helped maintain hepatic redox homeostasis and alleviated LPO through different defense strategies, depending on Cu concentrations. Under extreme SA, antioxidant-based defenses were activated to cope with oxidative stress at ambient-low Cu concentrations but failed to defend against Cu toxicity at polluted Cu levels, and LPO (by up to 63.90 %) was significantly induced. Additionally, thiols (GSH and MT) responded actively to cope with Cu toxicity under SA. SOD, CAT, EROD, and GST were also sensitively involved in defending against hepatic oxidative stress during coexposure. These findings highlight the notable interactive effects of SA and Cu and provide a basis for understanding antioxidant-based defenses in marine fish confronting environmental challenges.

Copper-induced oxidative stress, transcriptome changes, intestinal microbiota, and histopathology of common carp (Cyprinus carpio)

Copper (Cu) is a common contaminant in aquatic environments, which could cause physiological dysfunction in aquatic organisms. However, few studies have comprehensively examined the impact of copper toxicity in freshwater fish over the past decade. In this research, the oxidative stress, liver transcriptome, intestinal microbiota, and histopathology of common carp (C. carpio) in response to Cu exposure were studied, by exposing juvenile carp to 0.2 mg/ml Cu²⁺ for 30 days. The results revealed that Cu²⁺ could induce significant changes in malondialdehyde (MDA) content and antioxidant enzyme (superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx)) activity. The changes in antioxidant enzyme activities indicate that Cu can induce oxidative stress by generating reactive oxygen species (ROS) content. RNA-seq analysis of the liver identified 1069 differentially expressed genes (DEGs) after treatment with 2.0 mg/L Cu²⁺. Among the DEGs, 490 genes were upregulated and 579 genes were downregulated. GO functional enrichment analysis revealed that Cu could affect the fatty acid biosynthetic process, carnitine biosynthetic process, and activity of carboxylic acid transmembrane transporter. Meanwhile, the most significantly enriched KEGG pathway also included the lipid metabolism pathway. In addition, Cu²⁺ exposure increased bacterial richness and changed bacterial composition. At the phylum level, we found that the ratio of Bacteroidetes to Firmicutes was increased in the treatment carps, which can regulate intestinal epithelium function and reduce inflammation and immune responses. At the genus level, the abundances of 11 genera were significantly altered after exposure to Cu²⁺. The altered composition of the microbial community caused by Cu exposure may play a useful role in compensation of the intestinal lesions by Cu exposure. Furthermore, we found that Cu²⁺ exposure could cause histological alterations such as structural damage to the liver and intestines. The results of this research contribute to a better understanding of mechanisms related to Cu toxicity in fish.

Impacts of heavy metal pollution on the ionomes and transcriptomes of Western mosquitofish (Gambusia affinis)

Our understanding of the mechanisms mediating the resilience of organisms to environmental change remains lacking. Heavy metals negatively affect processes at all biological scales, yet organisms inhabiting contaminated environments must maintain homeostasis to survive. Tar Creek in Oklahoma, USA, contains high concentrations of heavy metals and an abundance of Western mosquitofish (Gambusia affinis), though several fish species persist at lower frequency. To test hypotheses about the mechanisms mediating the persistence and abundance of mosquitofish in Tar Creek, we integrated ionomic data from seven resident fish species and transcriptomic data from mosquitofish to test hypotheses about the mechanisms mediating the persistence of mosquitofish in Tar Creek. We predicted that mosquitofish minimize uptake of heavy metals more than other Tar Creek fish inhabitants and induce transcriptional responses to detoxify metals that enter the body, allowing them to persist in Tar Creek at higher density than species that may lack these responses. Tar Creek populations of all seven fish species accumulated heavy metals, suggesting mosquitofish cannot block uptake more efficiently than other species. We found population-level gene expression changes between mosquitofish in Tar Creek and nearby unpolluted sites. Gene expression differences primarily occurred in the gill, where we found upregulation of genes involved with lowering transfer of metal ions from the blood into cells and mitigating free radicals. However, many differentially expressed genes were not in known metal response pathways, suggesting multifarious selective regimes and/or previously undocumented pathways could impact tolerance in mosquitofish. Our systems-level study identified well characterized and putatively new mechanisms that enable mosquitofish to inhabit heavy metal-contaminated environments.

Influence of salinity on the toxicity of copper and cadmium to Zebrafish embryos

Salinization has become a serious worldwide environmental perturbation in freshwater ecosystems. Concomitantly, many of such ecosystems are already impacted by other toxicants, which together with increased salinity may result in synergistic, antagonistic or additive toxic effects to biota. This work intended to assess the influence of increasing salinity (by using NaCl) on the lethal and sublethal toxicity of two metallic elements (copper and cadmium) in embryos of the fish species Danio rerio. This goal was achieved by exposing zebrafish embryos to seven concentrations of NaCl, individually or combined with each metal, using a full factorial design. The following endpoints were monitored in the test organisms: mortality, hatching, malformations and the enzymatic activity of glutathione S-transferase (GST) and cholinesterase (ChE). Overall, moderate salinity levels alleviated the lethal toxicity of both copper and cadmium although this effect was stronger in the copper assay. This effect was also influenced, as expected, by the concentrations of the metals indicating that the protective effect of salt only reaches some levels, after what is overwhelmed by the high metal toxicity, especially with the non-essential metal cadmium. At sub-lethal concentrations, the interactive effect resulting from NaCl and metals was not consistent and varied with the endpoint analyzed and the metal tested. Overall, the interactions between the salt and metals seem complex and with more drastic effects (positive or negative) on lethal endpoints than sub-lethal.

Toxic effects of dietary copper and EGCG on bioaccumulation, antioxidant enzyme and immune response of Korean bullhead, Pseudobagrus fulvidraco

There are few reports of dietary Cu (copper) toxicity to Korean bullhead, Pseudobagrus fulvidraco, and little is known about recovery from dietary Cu exposure. In this study, P. fulvidraco (mean length 16.9 ± 1.38 cm, and mean weight 53.2 ± 1.22 g) were exposed for 4 weeks to dietary Cu concentration of 0 (control), 700, 900, and 1100 mg Cu kg^-1 dry feed to establish maximum tolerable levels of dietary Cu. All fish were then fed the dietary EGCG (Epigallocatechin gallate) concentration of 100 and 500 mg EGCG kg^-1 dry feed for a further 2 weeks to assess recovery. We were measured bioaccumulation (in the intestine, liver, and gill tissue), antioxidant enzymes (SOD and CAT) and immune responses (lysozyme and phagocytosis). The Cu exposure induced a significant accumulation in the intestine, liver, and gill tissues and the highest accumulation was observed in intestinal tissues (17-34 fold), but dietary EGCG exposure decreased (about 0.8-fold) Cu concentration in each tissue (ANOVA, P < 0.05). In antioxidant enzymes, SOD and CAT significantly increased by approximately 1.6-fold by dietary Cu exposure in the liver and gill tissue, respectively, but dietary EGCG exposure decreased SOD and CAT by about 1.1-fold, respectively (ANOVA, P < 0.05). For immune responses, lysozyme and phagocytosis in the blood significantly were decreased by approximately 1.5-fold, respectively, by dietary Cu exposure, but dietary EGCG exposure increased lysozyme and phagocytosis by about 1.1-fold, respectively (ANOVA, P < 0.05). During recovery period, bioaccumulation, antioxidant enzymes (SOD and CAT activity), and immune response (lysozyme and phagocytosis activity) tended to alleviate the significant changes by Cu exposure, and the tendency to return normal state was observed in high level of EGCG. The result of this study indicate that Cu exposure to P. fulvidraco affects bioaccumulation, antioxidant enzymes, and immune responses, and high level of EGCG were effective to alleviate the toxic effects of Cu exposure.

Modulation of physiological oxidative stress and antioxidant status by abiotic factors especially salinity in aquatic organisms

Exposure to a variety of environmental factors such as temperature, pH, oxygen and salinity may influence the oxidative status in aquatic organisms. The present review article focuses on the modulation of oxidative stress with reference to the generation of reactive oxygen species (ROS) in aquatic animals from different phyla. The focus of the review article is to explore the plausible mechanisms of physiological changes occurring in aquatic animals due to altered salinity in terms of oxidative stress. Apart from the seasonal variations in salinity, global warming and anthropogenic activities have also been found to influence oxidative health status of aquatic organisms. These effects are discussed with an objective to develop precautionary measures to protect the diversity of aquatic species with sustainable conservation. Comparative analyses among different aquatic species suggest that salinity alone or in combination with other abiotic factors are intricately associated with modulation in oxidative stress in a species-specific manner in aquatic animals. Osmoregulation under salinity stress in relation to energy demand and supply are also discussed. The literature survey of >50 years (1960-2020) indicates that oxidative stress status and comparative analysis of redox modulation have evolved from the analysis of various biotic and/or abiotic factors to the study of cellular signalling pathways in these aquatic organisms.

Dietary uptake and effects of copper in Sticklebacks at environmentally relevant exposures utilizing stable isotope-labeled 65CuCl2 and 65CuO NPs

Copper oxide nanoparticles (CuO NPs) accumulating in sediment can be taken up by invertebrates that serve as prey for fish. Thus, it is likely that the latter are exposed to CuO NPs via the gut. However, to this day it is unknown if CuO NPs can be taken up via the gastrointestinal tract and if and in which tissues/organs they accumulate. To address this knowledge gap, we synthesized CuO NPs enriched in the stable isotope ⁶⁵Cu and incorporated them at low concentration (5 μg ⁶⁵Cu g^-1 ww food) into a practical diet prepared from worm homogenate, which was then fed to Three-spined Stickleback (Gasterosteus aculeatus) for 16 days. For comparison, fish were exposed to a diet spiked with a ⁶⁵CuCl₂ solution. Background Cu and newly taken up ⁶⁵Cu in fish tissues/organs including gill, stomach, intestine, liver, spleen, gonad and carcass and feces were quantified by ICP-MS. In addition, expression levels of genes encoding for proteins related to Cu uptake, detoxification and toxicity (ctr-1, gcl, gr, gpx, sod-1, cat, mta and zo-1) were measured in selected tissues using RT-qPCR. The obtained results showed that feces of fish fed ⁶⁵CuO NP-spiked diet contained important amounts of ⁶⁵Cu. Furthermore, there was no significant accumulation of ⁶⁵Cu in any of the analyzed internal organs, though ⁶⁵Cu levels were slightly elevated in liver. No significant modulation in gene expression was measured in fish exposed to ⁶⁵CuO NP-spiked diet, except for metallothionein, which was significantly upregulated in intestinal tissue compared to control fish. Altogether, our results suggests that dietary absorption efficiency of CuO NPs, their uptake across the gastrointestinal barrier into the organism, and effects on Cu-related genes is limited at low, environmentally relevant exposure doses (0.2 μg ⁶⁵Cu ^-1 fish ww day^-1).

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.

Influence of Thermal Pollution on the Physiological Conditions and Bioaccumulation of Metals, Metalloids, and Trace Metals in Whitefish (Coregonus lavaretus L.)

The Kola nuclear power plant, which discharges warm water into one of the bays of subarctic Lake Imandra, significantly changes fish habitats. The temperature gradient of the lake is between 2 and 8 °C, which makes it significantly different from the natural temperature of the lake water. The stenothermal cold-water native species (lake whitefish (Coregonus lavaretus L.)), living for more than 40 years under conditions of thermal pollution, has adapted to this stressor. Moreover, this population differs favorably from the population in the natural-temperature environment in terms of its physiological state. Firstly, the hemoglobin concentrations in the fish blood are in the range of the ecological optimum, and secondly, it has a higher somatic growth, as estimated by Fulton’s condition factor. One of its main adaptive mechanisms of ion regulation is an intense metabolism of Na due to the high respiratory activity of the whitefish in warmer water. An increased accumulation of Rb and excretion of Se, Mo, and Si are associated more or less with that feature. Under conditions of an increased water temperature, the main metabolic need is due to a deficiency of Se in fish. The intensive metabolism of selenoproteins may involve risks of toxic effects and the bioaccumulation of Hg, As, and Cu in cases of increased existing stressors or the appearance of new ones.

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.

Sensitivity of white sturgeon and rainbow trout to waterborne copper exposure: A comparative study of copper-induced disruption of sodium homeostasis

Recent studies have demonstrated that white sturgeon are more sensitive to acute exposure to Cu than rainbow trout (Oncorhynchus mykiss), especially during early life-stages. However, the physiological mechanisms underlying this difference in sensitivity to Cu is not known. In the present study, we first confirmed the higher sensitivity (lower 96 h LC₅₀ values) of white sturgeon to Cu at three different life stages (larva, swim-up, and juvenile) relative to their counterparts in rainbow trout. We also demonstrated that acute exposure to Cu (50 μg/L for 4.5 h) caused a significantly greater reduction in the rate of waterborne Na uptake in white sturgeon relative to that in rainbow trout across all three life-stages. In agreement with this observation, we also found that acute exposure to Cu (20 μg/L for 48 h) elicits a significantly greater decrease in whole body Na level in all life stages of white sturgeon compared to rainbow trout. In contrast, white sturgeon demonstrated a higher or similar level of Cu body burden relative to rainbow trout during acute Cu exposure (20 μg/L for 24 h), thereby indicating that Cu bioaccumulation is not a good indicator of its toxicity in these species. Overall, our study demonstrated that the differences in sensitivity to acute Cu exposure between white sturgeon and rainbow trout can be explained on the basis of differential effects of Cu on Na homeostasis.

Sub-lethal effects of waterborne copper in early developmental stages of rainbow trout (Oncorhynchus mykiss)

The aim of this work was to study the impact of copper during a sub-chronic exposure to environmental concentrations in the early life stages of rainbow trout (Oncorhynchus mykiss). Eyed-stage embryos of rainbow trout, at 265 °D, were exposed in semi-static conditions to sub-lethal concentrations of CuSO₄ up to the larval stage (528 °D) under laboratory-controlled conditions. During 3 weeks, they were exposed to the environmentally-realistic concentration of 2 µg/L Cu and to a 10-fold higher concentration, 20 µg/L Cu. Several biological (survival, hatching success, malformation, growth) and behavioral (swimming activity) and molecular endpoints (genotoxicity and gene transcription) were studied. Exposure to 20 µg/L Cu had an inhibitory effect on hatching and increased half-hatched embryos (25%). At the end of the exposure, no significant differences were observed in growth of the larvae exposed to the highest Cu concentration. However, larvae exposed to 2 µg/L Cu exhibited increased growth in comparison with non-exposed larvae. The percentage of malformed larvae was significantly higher for both copper conditions, with skeletal malformations being the most observed. Expression of several genes was evaluated in whole larvae using quantitative real-time PCR. Genes involved in detoxification (gst, mt1 and mt2) and in cell cycle arrest (p53) were significantly repressed in both copper conditions when compared to control. In addition, potential genotoxic effects on larvae were investigated by the comet assay on blood cells, but this test did not demonstrate any significant DNA damage on larvae exposed to copper. This study confirms the adverse effects of copper on early life stages of rainbow trout even at the lowest environmentally relevant tested concentration.

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.

Zonation of Ca2+ transport and enzyme activity in the caeca of rainbow trout - a simple structure with complex functions

Trout caeca are vermiform structures projecting from the anterior intestine of the gastrointestinal tract. Despite their simple gross morphology, these appendages are physically distinct along the anterior-posterior axis, and ultrastructural evidence suggests zonation of function within the structures. Individual caeca from three sections (anterior, middle and posterior) were removed from the intestine of freshwater rainbow trout and investigated for ion transport and enzyme activity. Ca²⁺ absorption appeared as a combination of active and passive movement, with Michaelis-Menten kinetics observable under symmetrical conditions, and was inhibited by several pharmacological agents (ouabain, La³⁺ and a calmodulin antagonist). There was a decrease in ion transport function from adjacent to the intestine (proximal) to the distal tip of each caecum, along with decreasing transport from anterior to posterior for the proximal portion alone. Feeding increased the J _Max and K _M for Ca²⁺ absorption within all sections, whereas ion-poor water (IPW) exposure further increased the J _Max and K _M for Ca²⁺ transport in the anterior and middle sections. Increased Na⁺/K⁺-ATPase (NKA) and citrate synthase (CS) activity rates paralleled trends seen in Ca²⁺ transport. Feeding in freshwater and IPW exposure increased the glycolytic capacity of the caeca via increased pyruvate kinase (PK) and decreased lactate dehydrogenase (LDH) activity, while amino acid metabolism increased with IPW exposure through increased glutamate dehydrogenase (GDH) activity. Overall, feeding and IPW exposure each altered ionoregulation within the caeca of freshwater rainbow trout in a zone-specific pattern, with the anterior and proximal portions of the caeca being most affected. Increased carbohydrate and protein metabolism fueled the increased ATP demand of NKA through CS.

Acute exposure to copper induces variable intensity of oxidative stress in goldfish tissues

Copper is an essential element, but at high concentrations, it is toxic for living organisms. The present study investigated the responses of goldfish, Carassius auratus, to 96 h exposure to 30, 300, or 700 μg L^-1 of copper II chloride (Cu²⁺). The content of protein carbonyls was higher in kidney (by 158%) after exposure to 700 mg L^-1 copper, whereas in gills, liver, and brain, we observed lower content of protein carbonyls after exposure to copper compared with control values. Exposure to copper resulted in increased levels of lipid peroxides in gills (76%) and liver (95-110%) after exposure to 300 and 700 μg L^-1 Cu²⁺. Low molecular mass thiols were depleted by 23-40% in liver and by 29-67% in kidney in response to copper treatment and can be used as biomarkers toxicity of copper. The activities of primary antioxidant enzymes, superoxide dismutase and catalase, were increased in liver as a result of Cu²⁺ exposure, whereas in kidney catalase activity was decreased. The activities of glutathione-related enzymes, glutathione peroxidase, glutathione-S-transferase, and glutathione reductase were decreased as a result of copper exposure, but glutathione reductase activity increased by 25-40% in liver. Taken together, these data show that exposure of fish to Cu²⁺ ions results in the development of low/high intensity oxidative stress reflected in enhanced activities of antioxidant and associated enzymes in different goldfish tissues.

A protocol for identifying suitable biomarkers to assess fish health: A systematic review

Background

Biomarkers have been used extensively to provide the connection between external levels of contaminant exposure, internal levels of tissue contamination, and early adverse effects in organisms.

Objectives

To present a three-step protocol for identifying suitable biomarkers to assess fish health in coastal and marine ecosystems, using Gladstone Harbour (Australia) as a case study.

Methods

Prior to applying our protocol, clear working definitions for biomarkers were developed to ensure consistency with the global literature on fish health assessment. First, contaminants of concern were identified based on the presence of point and diffuse sources of pollution and available monitoring data for the ecosystem of interest. Second, suitable fish species were identified using fisheries dependent and independent data, and prioritised based on potential pathways of exposure to the contaminants of concern. Finally, a systematic and critical literature review was conducted on the use of biomarkers to assess the health of fish exposed to the contaminants of concern.

Results/Discussion

We present clear working definitions for bioaccumulation markers, biomarkers of exposure, biomarkers of effect and biomarkers of susceptibility. Based on emission and concentration information, seven metals were identified as contaminants of concern for Gladstone Harbour. Twenty out of 232 fish species were abundant enough to be potentially suitable for biomarker studies; five of these were prioritised based on potential pathways of exposure and susceptibility to metals. The literature search on biomarkers yielded 5,035 articles, of which 151met the inclusion criteria. Based on our review, the most suitable biomarkers include bioaccumulation markers, biomarkers of exposure (CYP1A, EROD, SOD, LPOX, HSP, MT, DNA strand breaks, micronuclei, apoptosis), and biomarkers of effect (histopathology, TAG:ST).

Conclusion

Our protocol outlines a clear pathway to identify suitable biomarkers to assess fish health in coastal and marine ecosystems, which can be applied to biomarker studies in aquatic ecosystems around the world.

A broader look at ammonia production, excretion, and transport in fish: a review of impacts of feeding and the environment

For nearly a century, researchers have studied ammonia production and excretion in teleost fish. Stemming from past investigations a body of knowledge now exists on various aspects including biochemical mechanisms of ammonia formation and specific routes and tissues used for ammonia transport, culminating in a current detailed theoretical model of branchial transport, including the molecular identities of the moieties involved. However, typical studies examining ammonia balance use routine laboratory conditions and fasted fish. While avoiding additional variables that influence nitrogen balance, these studies are arguably idealistic and do not reflect the natural variety of conditions that fish encounter. Further studies have revealed the impacts of extrinsic factors (e.g. salinity, pH, temperature) on ammonia handling in fasted fish whereas others have explored intrinsic factors, such as life history and developmental impacts. One routine challenge for ammonia balance that fish encounter is feeding and digestion. Fortunately, many new studies have revealed the impact of feeding and digestion on several aspects of ammonia balance; from production to excretion and to transport, and several have done so incorporating supplemental extrinsic and/or intrinsic factors. Together, these complex studies reveal realistic responses to multifactorial challenges encountered by animals in the wild and begin to provide a holistic view of ammonia balance in freshwater teleost fish.

Effects of salinity change on two superoxide dismutases (SODs) in juvenile marbled eel Anguilla marmorata

Salinity is one of the most important factors that affect the fish growth and survival. Superoxide dismutases (SODs), as the primary antioxidant enzymes, play a first role in the process of preventing oxidative stress caused by excessive superoxide anion (O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}${}_{2}^{-}$\end{document}2−) in living organisms. In the present study, we investigated the effects of salinity on the gene expressions as well as enzymatic activities of MnSOD and Cu/ZnSOD in gill, intestine, kidney, liver and muscle tissues of the marbled eel Anguilla marmorata. We found that the liver might possess stronger redox capacity compared with other tissues. Furthermore, the gene expressions and enzymatic activities of SODs in juvenile marbled eels could be effectively enhanced by low salinity but inhibited when the salinity was higher than the body tolerance. Our findings indicated that MnSOD and Cu/ZnSOD played vital roles in the adaptation of marbled eels to salinity variation, which contributed to the elucidation of physiological adaptation and regulatory mechanism of SODs in eels.

In vivo effects of Aphanizomenon flos-aquae DC-1 aphantoxins on gas exchange and ion equilibrium in the zebrafish gill

Aphantoxins, neurotoxins or paralytic shellfish poisons (PSPs) generated by Aphanizomenon flos-aquae, are a threat to environmental safety and human health in eutrophic waters worldwide. The molecular mechanisms of neurotoxin function have been studied; however, the effects of these neurotoxins on oxidative stress, ion transport, gas exchange, and branchial ultrastructure in fish gills are not fully understood. Aphantoxins extracted from A. flos-aquae DC-1 were detected by high-performance liquid chromatography. The major ingredients were gonyautoxins 1 and 5 and neosaxitoxin, which comprised 34.04%, 21.28%, and 12.77% of the total, respectively. Zebrafish (Danio rerio) were administered A. flos-aquae DC-1 aphantoxins at 5.3 or 7.61μg saxitoxin equivalents (eq)/kg (low and high doses, respectively) by intraperitoneal injection. The activities of Na(+)-K(+)-ATPase (NKA), carbonic anhydrase (CA), and lactate dehydrogenase (LDH), ultrastructural alterations in chloride and epithelial cells, and reactive oxygen species (ROS) and total antioxidative capacity (T-AOC) were investigated in the gills during the first 24h after exposure. Aphantoxins significantly increased the level of ROS and decreased the T-AOC in zebrafish gills from 3 to 12h post-exposure, suggesting an induction of oxidative stress and inhibition of antioxidant capacity. Reduced activities of NKA and CA demonstrated abnormal ion transport and gas exchange in the gills of aphantoxin-treated fish. Toxin administration also resulted in increased LDH activity and ultrastructural alterations in chloride and epithelial cells, suggesting a disruption of function and structure in zebrafish gills. The observed abnormalities in zebrafish gills occurred in a time- and dose-dependent manner. These findings demonstrate that aphantoxins or PSPs may inhibit ion transport and gas exchange, increase LDH activity, and result in ultrastructural damage to the gills through elevations in oxidative stress and reduced antioxidant capacity. These effects of aphantoxins in the gills of zebrafish suggest an induction of respiratory toxicity. The parameters investigated in this study may be also considered as biomarkers for studying aphantoxin/PSP exposure and cyanobacterial blooms in nature.

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.

Glutamate ameliorates copper-induced oxidative injury by regulating antioxidant defences in fish intestine

The objective of this study was to determine the protective effect of glutamate (Glu) in Cu-induced oxidative injury in fish intestine in vivo and enterocytes in vitro. The results indicated that exposure to 6 mg/l Cu for 72 h induced the production of reactive oxygen species, thereby increasing protein oxidation and lipid peroxidation in enterocytes of grass carp in vitro. Cells exposed to Cu alone resulted in a significant increase in lactate dehydrogenase release, which is accompanied by depletions of antioxidants, including total superoxide dismutase (T-SOD), glutathione S-transferase (GST), glutathione reductase (GR), anti-superoxide anion (ASA), anti-hydroxy radical (AHR) activities and GSH content. Pre-treatment with Glu remarkably prevented the toxic effects of Cu on the T-SOD, GST, GR, AHR, and ASA activities and GSH content in enterocytes. However, Cu induced an adaptive increase in the activities of catalase and glutathione peroxidase (GPx). Glu supplementation further increased GPx activity in enterocytes. Interestingly, the experiment in vivo showed that Glu pre-supplementation significantly elevated SOD, GPx, GST, GR, ASA and AHR activities, as well as GSH content. Further results showed that pre-treatment with Glu could alleviate Cu-induced oxidative injury by elevating antioxidant enzyme activities through regulating the expression of NF-E2-related nuclear factor 2 (Nrf2) mRNA. Together, these results indicated that Glu could attenuate Cu-induced cellular oxidative damage in fish intestine, likely mediated through Nrf2 signalling pathways regulating mRNA expressions of antioxidant enzyme genes and synthesis of GSH.

Salinity-dependent mechanisms of copper toxicity in the galaxiid fish, Galaxias maculatus

The euryhaline galaxiid fish, inanga (Galaxias maculatus) is widely spread throughout the Southern hemisphere occupying near-coastal streams that may be elevated in trace elements such as copper (Cu). Despite this, nothing is known regarding their sensitivity to Cu contamination. The mechanisms of Cu toxicity in inanga, and the ameliorating role of salinity, were investigated by acclimating fish to freshwater (FW), 50% seawater (SW), or 100% SW and exposing them to a graded series of Cu concentrations (0-200μgL(-1)) for 48h. Mortality, whole body Cu accumulation, measures of ionoregulatory disturbance (whole body ions, sodium (Na) influx, sodium/potassium ATPase activity) and ammonia excretion were monitored. Toxicity of Cu was greatest in FW, with mortality likely resulting from impaired Na influx. In both FW and 100% SW, ammonia excretion was significantly elevated, an effect opposite to that observed in previous studies, suggesting fundamental differences in the effect of Cu in this species relative to other studied fish. Salinity was protective against Cu toxicity, and physiology seemed to play a more important role than water chemistry in this protection. Inanga are sensitive to waterborne Cu through a conserved impairment of Na ion homeostasis, but some effects of Cu exposure in this species are distinct. Based on effect concentrations, current regulatory tools and limits are likely protective of this species in New Zealand waters.

Contaminant-induced oxidative stress in fish: a mechanistic approach

The presence of reactive oxygen species (ROS) in living organisms was described more than 60 years ago and virtually immediately it was suggested that ROS were involved in various pathological processes and aging. The state when ROS generation exceeds elimination leading to an increased steady-state ROS level has been called "oxidative stress." Although ROS association with many pathological states in animals is well established, the question of ROS responsibility for the development of these states is still open. Fish represent the largest group of vertebrates and they inhabit a broad range of ecosystems where they are subjected to many different aquatic contaminants. In many cases, the deleterious effects of contaminants have been connected to induction of oxidative stress. Therefore, deciphering of molecular mechanisms leading to such contaminant effects and organisms' response may let prevent or minimize deleterious impacts of oxidative stress. This review describes general aspects of ROS homeostasis, in particular highlighting its basic aspects, modification of cellular constituents, operation of defense systems and ROS-based signaling with an emphasis on fish systems. A brief introduction to oxidative stress theory is accompanied by the description of a recently developed classification system for oxidative stress based on its intensity and time course. Specific information on contaminant-induced oxidative stress in fish is covered in sections devoted to such pollutants as metal ions (particularly iron, copper, chromium, mercury, arsenic, nickel, etc.), pesticides (insecticides, herbicides, and fungicides) and oil with accompanying pollutants. In the last section, certain problems and perspectives in studies of oxidative stress in fish are described.

Copper and hypoxia modulate transcriptional and mitochondrial functional-biochemical responses in warm acclimated rainbow trout (Oncorhynchus mykiss)

To survive in changing environments fish utilize a wide range of biological responses that require energy. We examined the effect of warm acclimation on the electron transport system (ETS) enzymes and transcriptional responses to hypoxia and copper (Cu) exposure in fish. Rainbow trout (Oncorhynchus mykiss) were acclimated to cold (11 °C; control) and warm (20 °C) temperatures for 3 weeks followed by exposure to Cu, hypoxia or both for 24 h. Activities of ETS enzyme complexes I-IV (CI-CIV) were measured in liver and gill mitochondria. Analyses of transcripts encoding for proteins involved in mitochondrial respiration (cytochrome c oxidase subunits 4-1 and 2: COX4-1 and COX4-2), metal detoxification/stress response (metallothioneins A and B: MT-A and MT-B) and energy sensing (AMP-activated protein kinase α1: AMPKα1) were done in liver mitochondria, and in whole liver and gill tissues by RT-qPCR. Warm acclimation inhibited activities of ETS enzymes while effects of Cu and hypoxia depended on the enzyme and thermal acclimation status. The genes encoding for COX4-1, COX4-2, MT-A, MT-B and AMPKα1 were strongly and tissue-dependently altered by warm acclimation. While Cu and hypoxia clearly increased MT-A and MT-B transcript levels in all tissues, their effects on COX4-1, COX4-2 and AMPKα1 mRNA levels were less pronounced. Importantly, warm acclimation differentially altered COX4-2/COX4-1 ratio in liver mitochondria and gill tissue. The three stressors showed both independent and joint actions on activities of ETS enzymes and transcription of genes involved in energy metabolism, stress response and metals homeostasis. Overall, we unveiled novel interactive effects that should not be overlooked in real world situations wherein fish normally encounter multiple stress factors.

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.

Nickel has biochemical, physiological, and structural effects on the green microalga Ankistrodesmus falcatus: An integrative study

In recent years, the release of chemical pollutants to water bodies has increased due to anthropogenic activities. Ni(2+) is an essential metal that causes damage to aquatic biota at high concentrations. Phytoplankton are photosynthesizing microscopic organisms that constitute a fundamental community in aquatic environments because they are primary producers that sustain the aquatic food web. Nickel toxicity has not been characterized in all of the affected levels of biological organization. For this reason, the present study evaluated the toxic effects of nickel on the growth of a primary producer, the green microalga Ankistrodesmus falcatus, and on its biochemical, enzymatic, and structural levels. The IC50 (96h) was determined for Ni(2+). Based on this result, five concentrations were determined for additional tests, in which cell density was evaluated daily. At the end of the assay, pigments and six biomarkers, including antioxidant enzymes (catalase [CAT], glutathione peroxidase [GPx], superoxide dismutase [SOD]), and macromolecules (proteins, carbohydrates and lipids), were quantified; the integrated biomarker response (IBR) was determined also. The microalgae were observed by SEM and TEM. Population growth was affected starting at 7.5 μg L(-1) (0.028 μM), and at 120 μg L(-1) (0.450 μM), growth was inhibited completely; the determined IC50 was 17 μg L(-1). Exposure to nickel reduced the concentration of pigments, decreased the content of all of the macromolecules, inhibited of SOD activity, and increased CAT and GPx activities. The IBR revealed that Ni(2+) increased the antioxidant response and diminished the macromolecules concentration. A. falcatus was affected by nickel at very low concentrations; negative effects were observed at the macromolecular, enzymatic, cytoplasmic, and morphological levels, as well as in population growth. Ni(2+) toxicity could result in environmental impacts with consequences on the entire aquatic community. Current regulations should be revised to protect primary producers.

Effects of copper, hypoxia and acute temperature shifts on mitochondrial oxidation in rainbow trout (Oncorhynchus mykiss) acclimated to warm temperature

Temperature fluctuations, hypoxia and metals pollution frequently occur simultaneously or sequentially in aquatic systems and their interactions may confound interpretation of their biological impacts. With a focus on energy homeostasis, the present study examined how warm acclimation influences the responses and interactions of acute temperature shift, hypoxia and copper (Cu) exposure in fish. Rainbow trout (Oncorhynchus mykiss) were acclimated to cold (11°C; control) and warm (20°C) temperature for 3 weeks followed by exposure to environmentally realistic levels of Cu and hypoxia for 24h. Subsequently, mitochondrial electron transport system (ETS) respiratory activity supported by complexes I-IV (CI-IV), plasma metabolites and condition indices were measured. Warm acclimation reduced fish condition, induced aerobic metabolism and altered the responses of fish to acute temperature shift, hypoxia and Cu. Whereas warm acclimation decelerated the ETS and increased the sensitivity of maximal oxidation rates of the proximal (CI and II) complexes to acute temperature shift, it reduced the thermal sensitivity of state 4 (proton leak). Effects of Cu with and without hypoxia were variable depending on the acclimation status and functional index. Notably, Cu stimulated respiratory activity in the proximal ETS segments, while hypoxia was mostly inhibitory and minimized the stimulatory effect of Cu. The effects of Cu and hypoxia were modified by temperature and showed reciprocal antagonistic interaction on the ETS and plasma metabolites, with modest additive actions limited to CII and IV state 4. Overall, our results indicate that warm acclimation came at a cost of reduced ETS efficiency and increased sensitivity to added stressors.

Copper levels and changes in pH induce oxidative stress in the tissue of curimbata (Prochilodus lineatus)

We analyzed the effect of exposure to 25% 96 h-LC50 of copper at low (24.5 μg L(-1) Cu, pH 4.5), neutral (7.25 μg L(-1) Cu, pH 7.0) and high pH (4.0 μg L(-1) Cu, pH 8.0) at 20 °C on antioxidant defenses and oxidative stress in the liver, gills and white muscle of the fish Prochilodus lineatus. Water at pH 4.5 and 8.0 affected the enzymatic and non-enzymatic antioxidant systems of the liver and gills, but not of the white muscles of P. lineatus, when compared to water at pH 7.0. After Cu exposure, SOD (superoxide dismutase), GPx (glutathione peroxidase), GR (glutathione reductase) and GST (glutathione S-transferase) activities increased and CAT (catalase) activity decreased in the liver at water at pH 4.5 and 8.0. Meanwhile, the activities of SOD, CAT, GPx, GR and GST increased in the gills at these pHs. SOD and CAT activities increased in the white muscle after Cu exposure at pH 8.0 and GPx, GR and GST activities decreased after Cu exposure at pH 4.5 and 8.0. LPO levels decreased in the liver and gills of fish that were exposed to water at pH 4.5 and 8.0 and, after Cu exposure, the LPO level increased in the liver, gills and white muscle of fish that were exposed to water at pH 4.5 and 8.0, when compared to the control group at pH 7.0. The metallothionein (MT) concentration increased in the liver of fish in water at pH 4.5 and 8.0 and the gill of fish in water at pH 8.0. After Cu exposure, MT in the liver and gills was significantly elevated in fish exposed to water at pH 4.5 and 8.0, but remained at levels similar to the control group in the white muscle. These results indicate a differing sensitivity of fish organs and tissues to essential metals, such as copper, and that toxicity may be relevant at environmental concentrations. These results indicate that the effect of Cu on the response of antioxidant defense systems is determined by water pH.