Metal subcellular partitioning determines excretion pathways and sensitivity to cadmium toxicity in two marine fish species

Trophic dilution of rare earth elements along the food chain of the Seine estuary (France)

Society's interest in rare earth elements (REEs) and their increasing use in many fields is leading to enrichments in aquatic environments, such as estuaries. This study of the Seine estuary assessed the distribution of REEs along the food web, including different species from 5 phyla representing different trophic levels. Total REE concentrations, which were higher in algae, mollusks, crustaceans and annelids (4.85-156; 1.59-4.08; 2.48 ± 1.80 and 0.14 ± 0.11 μg/g dw, respectively) than in vertebrates (0.03-0.15 μg/g dw), correlated with δ15N indicated a trophic dilution. REE contributions in the studied species were higher for light REEs than for heavy and medium REEs. Positives anomalies for Eu, Gd, Tb and Lu were highlighted particularly in vertebrates, possibly due to species-dependent bioaccumulation/detoxification or related to anthropogenic inputs. The calculated BAF and BSAF indicated an important partitioning of REEs in organisms compared to the dissolved phase and a limited transfer from sediment to organisms.

Trophic Transfer of Cd, Cu, Pb, Zn, P and Se in Dutch Storage Water Reservoirs

Heavy metals are naturally omnipresent in aquatic systems. Excess amounts of heavy metals can accumulate in organisms of pollution impacted systems and transfer across a food web. Analysing the food web structure and metal contents of the organisms can help unravel the pathways of biomagnification or biodilution and gain insight in trophic linkages. We measured heavy metals and other elements in mussel bank detritus and organisms of the Biesbosch reservoirs (the Netherlands) and linked those to stable isotopic signatures. The heavy metal contents (cadmium, copper, lead, and zinc) were often lowest in benthivorous, omnivorous and piscivorous species (mainly fish); whereas, phosphorus contents were lower in the autotrophs. Mussel bank detritus contained the highest amounts of heavy metals. The heavy metals were negatively correlated with δ15N values. For selenium no clear trend was observed. Furthermore, there was a negative correlation between fish length and some heavy metals. Based on all 20 analysed elemental contents, similarities between species became apparent, related to niche or habitat. This study confirms that elemental contents of species can differ between feeding guilds and/or species, which can be attributed to metabolic and physiological processes. The organisms in higher trophic levels have adaptations preventing metal accumulation, resulting in lower contents. Within the fish species biodilution occurs, as most metal contents were lowest in bigger fish. Overall, the metals did not seem to biomagnify, but biodilute in the food web. Metal analyses combined with isotopic signatures could thus provide insights in metal transfer and possible trophic linkages within a system.

Contrasting trophic transfer patterns of cadmium and mercury in the Arctic marine food web of east Hudson Bay, Canada

We investigated trophic transfer of cadmium (Cd) through an Arctic marine food web in Hudson Bay and compared it with mercury (Hg), a metal known to strongly biomagnify. We evaluated blue mussel, sea urchin, common eider, sculpin, Arctic cod, and ringed seal for the influence of dietary and biological variables on variation in Cd and Hg concentrations. Age and size influenced metal concentrations among individuals within a vertebrate species. Consumer carbon and sulfur isotope values were correlated with their Cd and Hg concentrations, indicating habitat-specific feeding influenced metal bioaccumulation. Trophic transfer patterns for Cd depended on the vertebrate tissue, with food web biodilution observed for the muscle but not the liver. Liver Cd concentrations were higher in ringed seal and some common eider relative to prey. In contrast, we observed mercury biomagnification for both tissues. Tissue- and species-specific physiology can explain discrepancies of Cd trophic transfer in this Arctic marine food web.

Trophic transfer of rare earth elements in the food web of the Loire estuary (France)

The increasing use of rare earth elements (REEs) in many industrial sectors and in medecine, causes discharges into the environment and particularly in estuarine areas subjected to strong anthropogenic pressures. Here, we assessed the distribution of REEs along the food web of the Loire estuary. Several species representative of different trophic levels were sampled: 8 vertebrates, 3 crustaceans, 2 mollusks, 3 annelids and 4 algae, as well as Haploops sp. tubes rather related to sediment. The total REE concentrations measured by ICP-MS were the highest in Haploops sp. tubes (141.1 ± 4.7 μg/g dw), algae (1.5 to 34.5 μg/g dw), mollusks (9.9 to 12.0 μg/g dw), annelids (0.7 to 19.9 μg/g dw) and crustaceans (1.4 to 6.3 μg/g dw) and the lowest in vetebrates (0.1 to 1.6 μg/g dw). The individual contribution of REEs was, however, similar between most studied species with a higher contribution of light REEs (76.7 ± 7.6 %) compared to heavy REEs (14.1 ± 3.7 %) or medium REEs (9.2 ± 5.8 %). Trophic relations were estimated by stable isotope analysis of C and N and the linear regression of δ15N with total REE concentrations highlighted a trophic dilution with a corresponding TMS of -2.0. The tissue-specific bioaccumulation investigated for vertebrates demonstrated a slightly higher REE accumulation in gonads than in the muscle. Finally, positive Eu, Gd, Tb and Lu anomalies were highlighted in the normalized REE patterns of most studied species (especially in fish and crustaceans), which is consistent with results in the dissolved phase for Eu and Gd. These anomalies could either be due to anthropogenic inputs or to various bioaccumulation/elimination processes according to the specific species physiology. This study, including most of the trophic levels of the Loire estuary food web provides new insights on the bioaccumulation and trophic transfer of REEs in natural ecosystems.

Hg and Se in Muscle and Liver of Blue Shark (Prionace glauca) from the Entrance of the Gulf of California: An Insight to the Potential Risk to Human Health

The blue shark (Prionace glauca) is the most commonly caught species of Elasmobranchii at the entrance to the Gulf of California. Although fins are the primary target commodity, the entire organism is consumed. This study examined the concentration of Hg and Se in muscle and liver to understand the antagonistic process that occurs between these two elements within the organism. Twenty-two individuals were captured at the Gulf of California inlet between September 2019 and March 2021. Hg was measured by cold vapor atomic absorption, and Se by atomic absorption spectrophotometry in a graphite furnace. All individuals studied showed higher concentrations (µg g-1 wet weight) of Hg (0.69) and Se (2.49) in liver than in muscle (Hg 0.63 and Se 0.08). Although the mean Hg values were below the maximum allowable limits (Hg 1.0 µg g-1 wet weight), the molar ratio (< 1.0) and the negative health benefit value of selenium (HBVSe) in muscle show that additional caution should be taken when consuming this species. We recommend a more thorough study of the antagonistic interaction between Hg and Se to accurately assess the health risk for consumers of blue shark.

The Role of Glutathione and Sulfhydryl Groups in Cadmium Uptake by Cultures of the Rainbow Trout RTG-2 Cell Line

The aim of this study is to investigate the role of cellular sulfhydryl and glutathione (GSH) status in cellular cadmium (Cd) accumulation using cultures of the rainbow trout cell line RTG-2. In a first set of experiments, the time course of Cd accumulation in RTG-2 cells exposed to a non-cytotoxic CdCl2 concentration (25 μM) was determined, as were the associated changes in the cellular sulfhydryl status. The cellular levels of total GSH, oxidized glutathione (GSSG), and cysteine were determined with fluorometric high-performance liquid chromatography (HPLC), and the intracellular Cd concentrations were determined with inductively coupled plasma mass spectrometry (ICP-MS). The Cd uptake during the first 24 h of exposure was linear before it approached a plateau at 48 h. The metal accumulation did not cause an alteration in cellular GSH, GSSG, or cysteine levels. In a second set of experiments, we examined whether the cellular sulfhydryl status modulates Cd accumulation. To this end, the following approaches were used: (a) untreated RTG-2 cells as controls, and (b) RTG-2 cells that were either depleted of GSH through pre-exposure to 1 mM L-buthionine-SR-sulfoximine (BSO), an inhibitor of glutathione synthesis, or the cellular sulfhydryl groups were blocked through treatment with 2.5 μM N-ethylmaleimide (NEM). Compared to the control cells, the cells depleted of intracellular GSH showed a 25% reduction in Cd accumulation. Likewise, the Cd accumulation was reduced by 25% in the RTG-2 cells with blocked sulfhydryl groups. However, the 25% decrease in cellular Cd accumulation in the sulfhydryl-manipulated cells was statistically not significantly different from the Cd accumulation in the control cells. The findings of this study suggest that the intracellular sulfhydryl and GSH status, in contrast to their importance for Cd toxicodynamics, is of limited importance for the toxicokinetics of Cd in fish cells.

Do marine protected areas influence mercury exposure? Insights from a shark community in the tropical Northeast Pacific

Biomass depletion caused by overfishing is likely to alter the structure of food webs and impact mercury transfer to marine predators. Although marine protected areas (MPAs) are spared from fishing pressure, their influence on biota mercury levels is poorly understood. Here, we used carbon and nitrogen stable isotope compositions as well as mercury concentrations in fin clips to characterize foraging habitat and mercury exposure of a shark community composed of migratory and resident species of the Revillagigedo archipelago, an offshore MPA in the Northeast Pacific off Mexico. We found that the probability of finding migratory sharks in the isotopic niche of Revillagigedo-resident sharks was low, likely reflecting the use of habitats outside the archipelago by highly mobile species. Community-wide variations in mercury were primarily explained by shark length, revealing that bioaccumulation was the main driver of Hg concentrations. We failed to detect a clear effect of foraging habitat on shark mercury exposure, which may be related to migratory species using both exploited and protected areas when moving outside the Revillagigedo MPA. More similar studies on the potential mitigation of Hg contamination by MPAs are needed in the future if fishing pressure increases to satisfy the growing global human population.

Effect of cooking temperature on metal concentrations and speciation in fish muscle and seal liver

Fish and marine mammals constitute a significant part of the country food diet of many Indigenous communities in Canada. These animals sometimes accumulate essential elements as well as elevated levels of toxic metals. We experimentally assessed how changes in cooking temperature (23-99 °C by boiling) modified elemental concentrations in whitefish muscle and grey seal liver (two organs commonly consumed in some northern communities). Wet and dry elemental concentrations changed linearly as a function of temperature, and two patterns were observed: methylmercury, selenium, and rare earth elements tended to remain associated with the food during cooking, whereas alkali, alkaline-earth metals, and arsenic were significantly transferred to cooking juices. Mass balances indicated that speciation of mercury was stable during cooking. Because elements generally behaved similarly as those of their periodic table group or their ecotoxicological classes (A, B, intermediate), we propose that elemental behavior during cooking is partly a function of chemical affinity, and this relationship can be used to predict the behavior of data-poor elements of emerging concern, such as technology-critical elements. Furthermore, the marked increases and decreases in elemental concentrations during cooking (e.g., -14% As and +39% Se in whitefish; -22% Cd and +55% Hg in seal liver, on a wet weight basis) should be considered when assessing risk because current exposure models usually only consider elemental concentrations in raw food.

Fish liver damage related to the wastewater treatment plant effluents

Wastewater treatment plants (WWTPs) continuously release a complex mixture of municipal, hospital, industrial, and runoff chemicals into the aquatic environment. These contaminants are both legacy contaminants and emerging-concern contaminants, affecting all tissues in a fish body, particularly the liver. The fish liver is the principal detoxifying organ and effects of consistent pollutant exposure can be evident on its cellular and tissue level. The objective of this paper is thus to provide an in-depth analysis of the WWTP contaminants' impact on the fish liver structure, physiology, and metabolism. The paper also gives an overview of the fish liver biotransformation enzymes, antioxidant enzymes, and non-enzymatic antioxidants, their role in metabolizing xenobiotic compounds and coping with oxidative damage. Emphasis has been placed on highlighting the vulnerability of fish to xenobiotic compounds, and on biomonitoring of exposed fish, generally involving observation of biomarkers in caged or native fish. Furthermore, the paper systematically assesses the most common contaminants with the potential to affect fish liver tissue.

Cerebellar injury induced by cadmium via disrupting the heat-shock response

Cadmium (Cd) is a food contaminant that poses serious threats to animal health, including birds. It is also an air pollutant with well-known neurotoxic effects on humans. However, knowledge on the neurotoxic effects of chronic Cd exposure on chicken is limited. Thus, this study assessed the neurotoxic effects of chronic Cd on chicken cerebellum. Chicks were exposed to 0 (control), 35 (low), and 70 (high) mg/kg of Cd for 90 days, and the expression of genes related to the heat-shock response was investigated. The chickens showed clinical symptoms of ataxia, and histopathology revealed that Cd exposure decreased the number of Purkinje cells and induced degeneration of Purkinje cells with pyknosis, and some dendrites were missing. Moreover, Cd exposure increased the expression of heat-shock factors, HSF1, HSF2, and HSF3, and heat-shock proteins, HSP60, HSP70, HSP90, and HSP110. These changes indicate that HSPs improve the tolerance of the cerebellum to Cd. Conversely, the expressions of HSP10, HSP25, and HSP40 were decreased significantly, which indicated that Cd inhibits the expression of small heat-shock proteins. However, HSP27 and HSP47 were upregulated following low-dose Cd exposure, but downregulated under high-dose Cd exposure. This work sheds light on the toxic effects of Cd on the cerebellum, and it may provide evidence for health risks posed by Cd. Additionally, this work also identified a novel target of Cd exposure in that Cd induces cerebellar injury by disrupting the heat-shock response. Cd can be absorbed into chicken's cerebellum through the food chain, which eventually caused cerebellar injury. This study provided a new insight that chronic Cd-induced neurotoxicity in the cerebellum is associated with alterations in heat-shock response-related genes, which indicated that Cd through disturbing heat-shock response induced cerebellar injury.

First metal and metalloid study assessment for the Black Triggerfish Melichthys niger (Bloch, 1786): Baseline data from a pristine South Atlantic oceanic island

Metal and metalloid concentrations in Black Triggerfish, Melichthys niger (Tetraodontiformes, Balistidae), and sediments from Trindade Island, an isolated, pristine, and understudied environment in the Southwestern Atlantic Ocean were determined. Several elements were detected in sediment, including Rare Earth Elements (REE). Hg and the REE Nd were, however, not detected. Elements determined in triggerfish are indicative of systemic circulation and the presence of toxic elements indicates an exposure source. No significant associations were detected between elements in fish organs and total length, suggesting no bioaccumulation with age, nor between organs and sediment, indicating probable dietary origin. Biliary excretion indicated a local chronic source of As and Cd and non-quantifiable REE elimination. Hg concentrations were over thresholds that affect fish biochemistry processes. These baseline data are valuable concerning elemental contamination in remote oceanic islands, with the potential to be applied to future biomonitoring efforts and conservation measures for reef ecosystems worldwide.

From geochemistry to ecotoxicology of rare earth elements in aquatic environments: Diversity and uses of normalization reference materials and anomaly calculation methods

The geochemistry of rare earth elements (REEs) has been studied for a long time and has allowed us to highlight enrichments or depletions of REEs in aquatic ecosystems and to estimate anthropogenic inputs through normalization of data to different reference materials. This review of current literature on REE normalization highlighted the large number of different reference materials (a total of 12), as well as different anomaly calculation methods. This statement showed a real need for method harmonization to simplify the comparison between studies, which is currently very difficult. Normalization to Post-Archean Australian Shale (PAAS) emerged as being the most used (33 % of reported studies) regardless of the location and the nature of the studied samples and seem to be of higher quality. The interest of other reference materials was nevertheless underlined, as they could better represent the geographical situation or the nature of samples. Two main anomaly calculation methods have been highlighted: the linear interpolation/extrapolation and the geometric extrapolation using logarithmic modeling. However, due to variations in the estimation of neighbors' values, these two methods produce many different equations for the anomaly calculation of a single element. Current normalization practices based on shales and chondrites are suitable for abiotic samples but are questionable for biota. Indeed, normalization is increasingly used in studies addressing ecotoxicological issues which focus on biota and often aim to estimate the anthropogenic origin of bioaccumulated REEs. Due to the interspecific variability, as well as the complexity of mechanisms occurring in organisms when exposed to contaminants, new reference materials need to be established to consider the bioaccumulation/metabolization processes and the anthropogenic inputs of REEs based on the results of biotic samples.

Significance of chemical affinity on metal subcellular distribution in yellow perch (Perca flavescens) livers from Lake Saint-Pierre (QUEBEC, Canada)

The subcellular partitioning approach provides useful information on the location of metals within cells and is often used on organisms with high levels of bioaccumulation to establish relationships between the internal concentration and the potential toxicity of metals. Relatively little is known about the subcellular partitioning of metals in wild fish with low bioaccumulation levels in comparison with those from higher contaminated areas. This study aims to examine the subcellular partitioning of various metals considering their chemical affinity and essentiality at relatively low contamination levels. Class A (Y, Sr), class B (Cu, Cd, MeHg), and borderline (Fe, Mn) metal concentrations were measured in livers and subcellular fractions of yellow perch (n = 21) collected in Lake Saint-Pierre, QC, Canada. The results showed that all metals, apart from MeHg, were distributed among subcellular fractions according to their chemical affinity. More than 60% of Y, Sr, Fe, and Mn were found in the metal-sensitive fractions. Cd and Cu were largely associated with the metallothionein-like proteins and peptides (60% and 67% respectively) whereas MeHg was found mainly in the metal-sensitive fractions (86%). In addition, the difference between the subcellular distribution of Cu and other essential metals like Fe and Mn denotes that, although the essentiality of some metals is a determinant of their subcellular distribution, the chemical affinity of metals is also a key driver. The similarity of the subcellular partitioning results with previous studies on yellow perch and other fish species from higher contaminated areas supports the idea that metals are distributed in the cellular environment according to their chemical properties regardless of the bioaccumulation gradient.

Elucidating the Differences in Metal Toxicity by Quantitative Adverse Outcome Pathways

Numerous studies have reported that the toxicity differences among metals are widespread; however, little is known about the mechanism of differences in metal toxicity to aquatic organisms due to the lack of quantitative understanding of their adverse outcome pathway. Here, we investigated the effects of Cd and Cu on bioaccumulation, gene expression, physiological responses, and apical effects in zebrafish larvae. RNA sequencing was conducted to provide supplementary mechanistic information for the effects of Cd and Cu exposure. On this basis, we proposed a quantitative adverse outcome pathway (qAOP) suitable for metal risk assessment of aquatic organisms. Our work provides a mechanistic explanation for the differences in metal toxicity where the strong bioaccumulation of Cu enables the newly accumulated Cu to reach the threshold that causes different adverse effects faster than Cd in zebrafish larvae, resulting in a higher toxicity of Cu than that of Cd. Furthermore, we proposed a parameter C_IT/BCF (the ratio of internal threshold concentration and bioaccumulation factor) that helps to understand the toxicity differences by combining the information of bioaccumulation and internal threshold of adverse effects. This work demonstrated that qAOP is an effective quantitative tool for understanding the toxicity mechanism and highlight the importance of toxicokinetics and toxicodynamics at different biological levels in determining the metal toxicity.

Effects of waterborne cadmium exposure on Spinibarbus sinensis hepatopancreas and kidney: Mitochondrial cadmium accumulation and respiratory metabolism

To examine the relationship between heavy metal accumulation in mitochondria and their respiration function in fish during in vivo exposure, juvenile Spinibarbus sinensis were exposed to different waterborne cadmium (Cd) concentrations for up to 28 days. We measured the state III respiration rate and cytochrome c oxidase (CCO) activity of mitochondria in hepatopancreas and kidney and the accumulated Cd concentrations in mitochondria and heat-stable protein (HSP) fractions. Dose- and time-dependent Cd accumulation occurred at different levels in both organs, but was lower in hepatopancreas. When hepatopancreas mitochondrial Cd concentrations in Cd-exposed groups were > 5.5 μg/g dwt, their state III respiration rates were significantly lower than the control. CCO activity of hepatopancreas mitochondria exhibited decreasing dose- and time-dependent trends. However, kidney mitochondria respiratory activities were not affected significantly by Cd exposure. Cd concentrations in kidney HSP fraction were 2-5 times higher than in hepatopancreas under all exposure conditions, and were mainly present as non-deleterious metallothionein (MT)-Cd complexes. These results suggest that Cd accumulation occurred in hepatopancreas and kidney mitochondria of S. sinensis following waterborne Cd exposure, which significantly inhibited the respiration function of hepatopancreas mitochondria but did not have a deleterious effect on kidney mitochondria. The inhibitory pattern of hepatopancreas mitochondrial Cd concentrations related to function exhibited threshold and saturation effects, suggesting the capacity of S. sinensis to manage Cd toxicity. The difference in the relative proportion of Cd occurring as MT-Cd complexes in organs likely causes the organ-specific effects of Cd on hepatopancreas and kidney mitochondrial function.

Role of Toxicokinetic and Toxicodynamic Parameters in Explaining the Sensitivity of Zebrafish Larvae to Four Metals

Given the persistence and toxic potencies of metal contaminants in ecosystems, animals, and human beings, they are considered to be hazardous global pollutants. While the lethality of metal toxicities (e.g., LC₅₀) can significantly vary, even within the same species, the underlying mechanisms are less well-understood. In this study, we developed a subcellular two-compartment toxicokinetic-toxicodynamic (TK-TD) model for zebrafish larvae when exposed to four metals (cadmium, lead, copper, and zinc) to reveal whether differences in metal toxicity (LC₅₀ values) were dominated by the TK or TD processes. Results showed that the subcellular TK and TD parameters of the four metals were significantly different, and the bioconcentration factor (BCF) value of copper was higher than those of the other metals. We also found that the TD parameter internal threshold concentration (C_IT) was significantly positively correlated to the LC₅₀ values (R² = 0.7), suggesting a dominant role of TD processes in metal toxicity. Furthermore, the combined parameter C_IT/BCF for a metal-sensitive fraction (BCF_MSF), which linked exposure to effects through the TK-TD approach, explained up to 89% of the variation in toxicity to the four metals. The present study suggests that the observed variation in toxicity of these four metals was mainly determined by TD processes but that TK processes should not be ignored, especially for copper.

Differences in the subcellular distribution of cadmium and copper in the gills and liver of white sturgeon (Acipenser transmontanus) and rainbow trout (Oncorhynchus mykiss)

Recent studies have shown that white sturgeon (Acipenser transmontanus) are more resistant to cadmium (Cd) compared to rainbow trout (Oncorhynchus mykiss), whereas they are more sensitive than rainbow trout when exposed to copper (Cu). Differences in the subcellular distribution of metals among species could be one of the factors responsible for the differences in the sensitivity to metals. Although, subcellular distribution has been studied extensively in many species with many metals, its direct role in species-specific differences in the sensitivity has not been well studied. The objective of this study was to evaluate the role of subcellular distribution of metals in species-specific differences in the sensitivity to metals between sturgeon and trout. We compared the subcellular distribution of metals Cd and Cu in the cellular debris, heat-stable proteins, heat-denatured fraction, metal-rich granules, and organelles fractions from the gills and liver after exposure of juveniles of both species to 1.25 and 20 μg/L Cd and Cu for 8 days, respectively. Sturgeon diverted a higher amount of Cd towards biologically inactive metal pool (BIM) and a lower amount towards the biologically active metal pool (BAM) compared to trout in both tissues. This explained why sturgeon are able to tolerate a relatively higher exposure level to Cd compared to trout. For Cu, there was no statistically significant species-specific differences in the amounts diverted towards either BAM or BIM; hence, white sturgeon's greater sensitivity to Cu was not explained by its subcellular distribution strategies.

Distinct dietary cadmium toxic effects and defense strategies in two strains of gibel carp (Carassius gibelio) revealed by a comprehensive perspective

Previous studies demonstrated that gibel carp A strain was more susceptible to herpesvirus infection than other strains. Thus, we hypothesized that F strain might display better defense responses than the A strain against cadmium (Cd) exposure. To test our hypothesis, gibel carp A strain and F strain were exposed to three diets comprising of different concentrations of Cd for 8 weeks to compare their resistances to Cd. Comprehensive evaluations on biochemical, physiological and histological responses were conducted post-exposure. Results showed that no adverse effects and differences were observed on growth in two strains of gibel carp, compromising of the remarkable hepatoxicity-caused liver damage as shown by histological observations. Dietary Cd exposure stimulated antioxidant defense in the liver to counteract the Cd hepatoxicity, especially in the F strain. Activation of ER stress response positively stimulated the autophagy, then triggering apoptosis in fish after dietary Cd exposure. Thus, Cd-induced autophagy served as a protective strategy to alleviate hepatoxicity, but overaction of ER stress also triggered irreparable cell death via apoptosis. Cd induced dysregulation of lipid accumulation, which might be a common mechanism in response to hepatoxicity. Last but not least, the F strain showed stronger response on antioxidant, ER stress and autophagy, but apoptosis were remitted compared with the A strain, implying the F strain showed stronger response but better defense strategies to dietary Cd exposure. Our finding provides useful information for genetic breeding in aquaculture, and ultimately contribute to the safety assessment of aquatic products for human consumption.

Mercury isotopes as tracers of ecology and metabolism in two sympatric shark species

In coastal ecosystems, top predators are exposed to a wide variety of nutrient and contaminant sources due to the diversity of trophic webs within inshore marine habitats. Mercury contamination could represent an additional threat to shark populations that are declining worldwide. Here we measured total mercury, carbon and nitrogen isotopes, as well as mercury isotopes, in two co-occurring shark species (the bull shark Carcharhinus leucas and the tiger shark Galeocerdo cuvier) and their potential prey from a coastal ecosystem of the western Indian Ocean (La Réunion Island). Our primary goals were to (i) determine the main trophic Hg sources for sharks and (ii) better characterize their diet composition and foraging habitat. Hg isotope signatures (Δ¹⁹⁹Hg and δ²⁰²Hg) of shark prey suggested that bull sharks were exposed to methylmercury (MeHg) produced in offshore epipelagic waters, while tiger sharks were exposed to offshore mesopelagic MeHg with additional microbial transformation in slope sediments. Δ¹⁹⁹Hg values efficiently traced the ecology of the two predators, demonstrating that bull sharks targeted coastal prey in shallow waters while tiger sharks were mainly foraging on mesopelagic species in the deeper waters of the island slope. Unexpectedly, we found a positive shift in δ²⁰²Hg (>1‰) between sharks and their prey, leading to high δ²⁰²Hg values in the two shark species (e.g. 1.91 ± 0.52‰ in bull sharks). This large shift in δ²⁰²Hg indicates that sharks may display strong MeHg demethylation abilities, possibly reflecting evolutionary pathways for mitigating their MeHg contamination.

Integrated assessment of the acclimation capacity of the marine bivalve Crenomytilus grayanus under naturally highly contaminated conditions: Subcellular distribution of trace metals and structural alterations of nephrocytes

The aim of our work was to assess whether the cellular processes in the nephrocytes of the long-lived mussel Crenomytilus grayanus tend to acclimation or destruction under trace metal contamination. Mussels were collected from three sites in the north-western Pacific Ocean: reference site, upwelling site, and a site highly contaminated with trace metals. Concentration, subcellular distribution of trace metals (Cd, Cu, Zn, and Pb) in the mussel kidneys, and ultrastructural alterations of the nephrocytes were studied. To assess the total load of accumulated trace metals, the total concentration coefficient (∑СС) was determined. In the kidneys of the reference C. grayanus, trace metals were eliminated from cell metabolism mainly by lysosomal granules or residue bodies. Under high levels of contamination, the defense mechanisms of C. grayanus are practically suppressed (no metallothionein-like protein peak, decreased content of granules) by the total effect of accumulated pollutants that leads to the destruction of cellular structures. Under natural conditions (upwelling site), increased accumulation of trace metals in the mussel kidneys did not lead to an increase in the number or size of lysosomal granules. However, abnormal high Cd accumulation in the kidneys caused the synthesis of high levels of metallothionein-like proteins that sequester most of the studied trace metals. To quickly lower the metal levels in nephrocytes under these conditions, a unique long-term acclimatory response - apocrine-like secretion in nephrocytes, which provides rapid elimination of me-MTLP complexes from the cell arose. Thus, our integrated study of the subcellular distribution of trace metals and ultrastructural alterations in nephrocytes allowed us to characterize the features of the structural and functional alterations in mussel cells under the field conditions tested.

Differential Bioaccumulation of Trace Elements and Rare Earth Elements in the Muscle, Kidneys, and Liver of the Invasive Indo-Pacific Lionfish (Pterois spp.) from Cuba

The Indo-Pacific lionfish is a saltwater fish that inhabits the Red Sea waters and the Indian and Pacific oceans; it is an invasive species in the western Atlantic and was recently introduced into the local diet in the USA, Central and South America, and the Caribbean with the aim of controlling the invasion of this species. Due to its predatory nature, it tends to bioaccumulate metals and other contaminants via the marine food web and could thus constitute a suitable species for monitoring aquatic ecosystems. The presence and distribution of 23 trace elements and 16 rare earth elements (REEs) were investigated by inductively coupled plasma-mass spectrometry (ICP-MS) in the muscle, liver, and kidneys of lionfish from Cuba. Significant differences in metal concentrations were found in the different fish organs. The liver and kidneys registered the highest concentrations for most trace elements and for ΣREE, thus demonstrating that they are effective bioindicators of possible pollution on the environment in which fish live, and assuming great importance in the choice of early biomonitoring. Trace element concentrations in the muscle are instead of crucial interest for consumer safety. The limits set by EU regulations and Cuban guidelines for Cd and Pb in fish muscle were never exceeded, suggesting that lionfish from Cuba could therefore represent a good source of minerals and proteins for the local population. Graphical abstract.