Different transcriptomic responses of two marine copepods, Tigriopus japonicus and Pseudodiaptomus annandalei, to a low dose of mercury chloride (HgCl2)

Mercury-induced toxicity: Mechanisms, molecular pathways, and gene regulation

Mercury is a well-known neurotoxicant for humans and wildlife. The epidemic of mercury poisoning in Japan has clearly demonstrated that chronic exposure to methylmercury (MeHg) results in serious neurological damage to the cerebral and cerebellar cortex, leading to the dysfunction of the central nervous system (CNS), especially in infants exposed to MeHg in utero. The occurrences of poisoning have caused a wide public concern regarding the health risk emanating from MeHg exposure; particularly those eating large amounts of fish may experience the low-level and long-term exposure. There is growing evidence that MeHg at environmentally relevant concentrations can affect the health of biota in the ecosystem. Although extensive in vivo and in vitro studies have demonstrated that the disruption of redox homeostasis and microtube assembly is mainly responsible for mercurial toxicity leading to adverse health outcomes, it is still unclear whether we could quantitively determine the occurrence of interaction between mercurial and thiols and/or selenols groups of proteins linked directly to outcomes, especially at very low levels of exposure. Furthermore, intracellular calcium homeostasis, cytoskeleton, mitochondrial function, oxidative stress, neurotransmitter release, and DNA methylation may be the targets of mercury compounds; however, the primary targets associated with the adverse outcomes remain to be elucidated. Considering these knowledge gaps, in this article, we conducted a comprehensive review of mercurial toxicity, focusing mainly on the mechanism, and genes/proteins expression. We speculated that comprehensive analyses of transcriptomics, proteomics, and metabolomics could enhance interpretation of "omics" profiles, which may reveal specific biomarkers obviously correlated with specific pathways that mediate selective neurotoxicity.

Arsenic bioaccumulation and biotransformation in the marine copepod Tigriopus japonicus under chronic dietborne and waterborne exposure

Arsenic (As) can be transferred along the food chain, while little is known about the toxic effects of dietborne As on marine copepods. In this study, we investigated the short-term and long-term effects of waterborne and dietborne As exposure on the bioaccumulation and biotransformation, as well as developmental toxicity of Tigriopus japonicus. Under acute As exposure, As bioaccumulation increased and reached a plateau with increasing exposure concentration. Moreover, As accumulation at dietborne exposure was 4.3 and 5.7 times greater than that at control group for AsIII and AsV, respectively. At chronic As exposure, As accumulation increased continuously with exposure time, with a 2.8-day extension of development time and a 45% reduction in 10-d fecundity under dietborne exposure compared to control, whereas 2.3-day extension of development time and a 20% reduction in 10-d fecundity were observed under waterborne exposure. Among As species, inorganic As had the highest concentrations, but the proportion of inorganic As decreased from 89% to 63% during 4 to 21 d of exposure, suggesting the conversion of inorganic As to organic As. The organic As was dominated by arsenobetaine (AsB, 13-25%), followed by monomethylarsenic (MMA, 8-25%). These results suggest that dietborne exposure has more pronounced toxic effects on T. japonicus, but the toxicity of As could be reduced through biotransformation under chronic exposure. Therefore, the arsenic species should be considered when assessing As toxicity.

Multigenerational effects of glyphosate-based herbicide and emamectin benzoate insecticide on the reproduction and gene expression of the copepod Pseudodiaptomus annandalei (Sewell, 1919)

This study investigates the effects of glyphosate-based herbicide (GLY) and pure emamectin benzoate (EB) insecticide on the brackish copepod Pseudodiaptomus annandalei. The 96h median lethal concentration (96 h LC50) was higher in the GLY exposure (male: 3420.96 ± 394.67 μg/L; female: 3093.46 ± 240.67 μg/L) than in the EB (male: 79.10 ± 7.30 μg/L; female: 6.38 ± 0.72 μg/L). Based on the result of 96h LC50, we further examined the effects of GLY and EB exposures at sub-lethal concentrations on the naupliar production of P. annandalei. Subsequently, a multigenerational experiment was conducted to assess the long-term impact of GLY and EB at concentrations 375 μg/L, and 0.025 μg/L respectively determined by sub-lethal exposure testing. During four consecutive generations, population growth, clutch size, prosome length and width, and sex ratio were measured. The copepods exposed to GLY and EB showed lower population growth but higher clutch size than the control group in most generations. Gene expression analysis indicated that GLY and EB exposures resulted in the downregulation of reproduction-related (vitellogenin) and growth-related (myosin heavy chain) genes, whereas a stress-related gene (heat shock protein 70) was upregulated after multigenerational exposure. The results of the toxicity test after post-multigenerational exposure indicated that the long-term GLY-exposed P. annandalei displayed greater vulnerability towards GLY toxicity compared to newly-exposed individuals. Whereas, the tolerance of EB was significantly higher in the long-term exposed copepod than in newly-exposed individuals. This suggests that P. annandalei might have greater adaptability towards EB toxicity than towards GLY toxicity. This study reports for the first time the impacts of common pesticides on the copepod P. annandalei, which have implications for environmental risk assessment and contributes to a better understanding of copepod physiological responses towards pesticide contaminations.

Multistress Interplay: Time and Duration of Ocean Acidification Modulate the Toxicity of Mercury and Other Metals

The current understanding of multistress interplay assumes stresses occur in perfect synchrony, but this assumption is rarely met in the natural marine ecosystem. To understand the interplay between nonperfectly overlapped stresses in the ocean, we manipulated a multigenerational experiment (F0-F3) to explore how different temporal scenarios of ocean acidification will affect mercury toxicity in a marine copepod Pseudodiaptomus annandalei. We found that the scenario of past acidification aggravated mercury toxicity but current and persistent acidification mitigated its toxicity. We specifically performed a proteomics analysis for the copepods of F3. The results indicated that current and persistent acidification initiated the energy compensation for development and mercury efflux, whereas past acidification lacked the barrier of H+ and had dysfunction in the detoxification and efflux system, providing a mechanistic understanding of mercury toxicity under different acidification scenarios. Furthermore, we conducted a meta-analysis on marine animals, demonstrating that different acidification scenarios could alter the toxicity of several other metals, despite evidence from nonsynchronous scenarios remaining limited. Our study thus demonstrates that time and duration of ocean acidification modulate mercury toxicity in marine copepods and suggests that future studies should move beyond the oversimplified scenario of perfect synchrony in understanding multistress interaction.

Single and combined effects of increased temperature and methylmercury on different stages of the marine rotifer Brachionus plicatilis

Rapid, anthropogenic activity-induced global warming is a severe problem that not only raises water temperatures but also shifts aquatic environments by increasing the bioavailability of heavy metals (HMs), with potentially complicated effects on aquatic organisms, including small aquatic invertebrates. For this paper, we investigated the combined effects of temperature (23 and 28 °C) and methylmercury (MeHg) by measuring physiological changes, bioaccumulation, oxidative stress, antioxidants, and the mitogen-activated protein kinase signaling pathway in the marine rotifer Brachionus plicatilis. High temperature and MeHg adversely affected the survival rate, lifespan, and population of rotifers, and bioaccumulation, oxidative stress, and biochemical reactions depended on the developmental stage, with neonates showing higher susceptibility than adults. These findings demonstrate that increased temperature enhances potentially toxic effects from MeHg, and susceptibility differs with the developmental stage. This study provides a comprehensive understanding of the combined effects of elevated temperature and MeHg on rotifers. ENVIRONMENTAL IMPLICATION: Methylmercury (MeHg) is a widespread and harmful heavy metal that can induce lethal effects on aquatic organisms in even trace amounts. The toxicity of metals can vary depending on various environmental conditions. In particular, rising temperatures are considered a major factor affecting bioavailability and toxicity by changing the sensitivity of organisms. However, there are few studies on the combinational effects of high temperatures and MeHg on aquatic animals, especially invertebrates. Our research would contribute to understanding the actual responses of aquatic organisms to complex aquatic environments.

A Meta-analysis Reveals Global Change Stressors Potentially Aggravate Mercury Toxicity in Marine Biota

Growing evidence demonstrates that global change can modulate mercury (Hg) toxicity in marine organisms; however, the consensus on such effect is lacking. Here, we conducted a meta-analysis to evaluate the effects of global change stressors on Hg biotoxicity according to the IPCC projections (RCP 8.5) for 2100, including ocean acidification (-0.4 units), warming (+4 °C), and their combination (acidification-warming). The results indicated an overall aggravating effect (ln RRΔ = -0.219) of global change on Hg toxicity in marine organisms, while the effect varied with different stressors; namely, acidification potentially alleviates Hg biotoxicity (ln RRΔ = 0.117) while warming and acidification-warming have an aggravating effect (ln RRΔ = -0.328 and -0.097, respectively). Moreover, warming increases Hg toxicity in different trophic levels, i.e., primary producers (ln RRΔ = -0.198) < herbivores (ln RRΔ = -0.320) < carnivores (ln RRΔ = -0.379), implying increasing trends of Hg biomagnification through the food web. Notably, ocean hypoxia appears to boost Hg biotoxicity, although it was not considered in our meta-analysis because of the small sample size. Given the persistent global change and combined effects of these stressors in marine environments, multigeneration and multistressor research is urgently needed to fully disclose the impacts of global change on Hg pollution and its risk.

Multigenerational effects of arsenate on development and reproduction in marine copepod Tigriopus japonicus

Arsenic (As) is a persistent toxic substance, however, its toxicity to marine zooplankton remains unclear. In this study, copepods were exposed to a series of dissolved arsenate (As(V)) for four generations (F0-F3) and subsequently depurated in clean seawater for two generations (F4-F5) to assess multigenerational toxicity of As(V). As(V) exposure prolonged copepod development. The development time were 1.9, 2.4, and 3.4 days longer than the control in F0 when exposed to 50, 100, and 500 μg/L As(V), respectively, and the toxicity increased with generations. Moreover, As(V) reduced the reproductive capacity of copepods, and this effect become more severe during generation succession. The 10-day fecundities were reduced from 80 to 85 eggs per female in the control to 42 eggs per female, the lowest level, in 500 μg/L As(V) exposure group in F3. Nevertheless, the fecundity was recovered to the control level in the offspring of the 50 and 100 μg/L As(V) exposed groups (F4), suggesting it was an acclimation effect of copepods during As(V) exposure. In addition, the survival rate, development time, and reproductive parameters were significantly correlated with the As accumulation in copepods. Overall, As(V) exposure caused As bioaccumulation which negatively affected copepods' survival, development, and reproductive traits, and this toxic effect was amplified with generations and concentrations. Therefore, the multigenerational toxicity of As should be considered in the environmental risk assessments.

Transcriptomic analysis reveals responses to a polluted sediment in the Mediterranean copepod Acartia clausi

Marine sediments are regarded as sinks for several classes of contaminants. Characterization and effects of sediments on marine biota now require a multidisciplinary approach, which includes chemical and ecotoxicological analyses and molecular biomarkers. Here, a gene expression study was performed to measure the response of adult females of the Mediterranean copepod Acartia clausi to elutriates of polluted sediments (containing high concentrations of polycyclic aromatic hydrocarbons, PAHs, and heavy metals) from an industrial area in the Southern Tyrrhenian Sea (Bagnoli-Coroglio). Functional annotation of the A. clausi transcriptome generated as reference here, showed a good quality of the assembly and great homology with other copepod and crustacean sequences in public databases. This is one of the few available transcriptomic resources for this widespread copepod species of great ecological relevance in temperate coastal areas. Differential expression analysis between females exposed to the elutriate and those in control seawater identified 1000 differentially expressed genes, of which 743 up- and 257 down-regulated. Within the up-regulated genes, the most represented functions were related to proteolysis (lysosomal protease, peptidase, cathepsin), response to stress and detoxification (heat-shock protein, superoxide dismutase, glutathione-S-transferase, cytochrome P450), and cytoskeleton structure (α- and β-tubulin). Down-regulated genes were mostly involved with ribosome structure (ribosomal proteins) and DNA binding (histone proteins, transcription factors). Overall, these results suggest that processes such as transcription, translation, protein degradation, metabolism of biomolecules, reproduction, and xenobiotic detoxification were altered in the copepod in response to polluted elutriates. In conclusion, our results contribute to gaining information on the transcriptomic responses of copepods to polluted sediments. They will also prompt the selection of genes of interest to be used as biomarkers of exposure to PAHs and heavy metals in molecular toxicology studies on copepods, and in general, in comparative functional genomic studies on marine zooplankton.

High and diurnally fluctuating carbon dioxide exposure produces lower mercury toxicity in a marine copepod

Coastal waters have experienced fluctuations in partial pressure of carbon dioxide (pCO₂) and mercury (Hg) pollution, yet little is known concerning how natural pCO₂ fluctuations affect Hg biotoxicity. Here, a marine copepod Tigriopus japonicus was interactively exposed to different seawater pCO₂ (ambient 400, steady elevated 1000, and fluctuating elevated 1000 ± 600 μatm) scenarios and Hg (control, 2 μg/L) treatments for 7 d. The results showed that elevated pCO₂ decreased Hg bioaccumulation, and it was even more under fluctuating elevated pCO₂ condition. We found energy depletion and oxidative stress under Hg-treated copepods, while combined exposure initiated compensatory response to alleviate Hg toxicity. Intriguingly, fluctuating acidification presented more immune defense related genes/processes in Hg-treated copepods when compared to steady acidification, probably linking with the greater decrease in Hg bioaccumulation. Collectively, understanding how fluctuating acidification interacts with Hg contaminant will become more crucial in predicting their risks to coastal biota and ecosystems.

Gene expression study alerted to possible impairment in Daphnia magna individuals as a consequence of exposure to sublethal concentrations of prochloraz

In the present study, Daphnia magna individuals were exposed for 21 days to 87, 130, 170, 230 and 380 μg/L of prochloraz. The expression of genes related to lipid metabolism (fabd), oxidative stress (cat and gst), heat shock proteins synthesis (hsp70 and hsp90), haemoglobin synthesis (hgb1 and hgb2), metallothioneins synthesis (mt-a, mt-b and mt-c), and vitellogenin synthesis (vgt1 y vgt2) were analyzed. Results showed that some gene expression in D. magna was altered as a consequence of the individual exposure to the fungicide. The genes fabd, vtg1 and vtg2, cat and gst resulted unaltered by the exposure of the daphnids to different fungicide concentrations. However, daphnid exposure to 380 μg/L of prochloraz resulted in an overexpression (p < 0.05) of hsp70 gene which indicated an alteration of the normal protein synthesis and its integrity maintenance. On the other hand, mt-b gene resulted significantly underexpressed (p < 0.05) in daphnids exposed to the lowest fungicide concentrations (87, 130 and 170 μg/L, respectively). In addition, hgb1 and hgb2 genes which are related with the haemoglobin synthesis weresignificantly overexpressed (p < 0.05). Results showed that hgb1 gene was overexpressed almost 100 times more in daphnids exposed for 21 days to 170, 230 and 380 μg/L than control values. However, many authors advocate for an association of these gene expression changes with the presence of contaminants in the medium, in fact they could be used as a good indicator of early contamination at low concentrations of toxicants.

Mercury can be transported into marine copepod by polystyrene nanoplastics but is not bioaccumulated: An increased risk?

Plastic pollution is a serious problem in the global marine environment because it can produce negative effects at the biological and ecological levels. Due to large surface-area-to-volume ratio and inherent hydrophobicity, nanoplastics can serve as carriers of contaminants, and may affect their fate and toxicity in marine environments. However, the combined effects of nanoplastics and mercury (Hg) in marine organisms have not been well characterized. In this study, after verifying the ingestion of polystyrene nano-size plastics (PS NPs, 50 nm) by the copepod Tigriopus japonicus and adsorption of Hg to PS NPs, we investigated the effects of PS NPs and Hg exposure (alone or in combination) for 48 h on the copepods. Specifically, a 72-h depuration was performed after 48 h exposure. The results showed that after 48 h exposure, the copepod's Hg concentration was significantly increased in the combined exposure group compared to that in the Hg treatment group, but these differences did not persist following 24 h of depuration. Therefore, PS NPs transported Hg into the copepods but did not promote Hg bioaccumulation. Treatment with PS NPs alone did not induce toxicity in T. japonicus, but co-exposure to PS NPs and Hg resulted in elevated transcription of genes related to energy production, antioxidant response, and detoxification/stress defense when compared with Hg treatment alone, demonstrating the synergistic interaction between PS NPs and Hg. Our findings contribute to a comprehensive understanding about the combined toxicity of nanoplastics and metals and the potential ecological risks of associated with these effects in marine environments.

First transcriptome of the copepod Gladioferens pectinatus subjected to chronic contaminant exposures

Contaminants are often at low concentrations in ecosystems and their effects on exposed organisms can occur over long periods of time and across multiple generations. Alterations to subcellular mechanistic pathways in response to exposure to contaminants can provide insights into mechanisms of toxicity that methods measuring higher levels of biological may miss. Analysis of the whole transcriptome can identify novel mechanisms of action leading to impacts in exposed biota. The aim of this study was to characterise how exposures to copper, benzophenone and diclofenac across multiple generations altered molecular expression pathways in the marine copepod Gladioferens pectinatus. Results of the study demonstrated differential gene expression was observed in cultures exposure to diclofenac (569), copper (449) and benzophenone (59). Pathways linked to stress, growth, cellular and metabolic processes were altered by exposure to all three contaminants with genes associated with oxidative stress and xenobiotic regulation also impacted. Protein kinase functioning, cytochrome P450, transcription, skeletal muscle contraction/relaxation, mitochondrial phosphate translocator, protein synthesis and mitochondrial methylation were all differentially expressed with all three chemicals. The results of the study also suggested that using dimethyl sulfoxide as a dispersant influenced the transcriptome and future research may want to investigate it's use in molecular studies. Data generated in this study provides a first look at transcriptomic response of G. pectinatus exposed to contaminants across multiple generations, future research is needed to validate the identified biomarkers and link these results to apical responses such as population growth to demonstrate the predictive capacity of molecular tools.

Multigenerational study of life history traits, bioaccumulation, and molecular responses of Pseudodiaptomus annandalei to cadmium

Metal pollution provide a substantial challenge for environmental health. This study investigated the multigeneration effects of cadmium on populations of the copepod species Pseudodiaptomus annandalei, exposed to a sublethal concentration, 40 µg/L of cadmium (Cd), over 10 generations. At the end of each generation, copepod individuals were collected to estimate fecundity, bioaccumulation, and real time qPCR quantification of selected differentially expressed genes to evaluate Cd effects and sex-specific responses of copepods across multiple generations. Our results revealed a sex-specific accumulation of Cd integrating 10 successive generations. The concentration of Cd was significantly higher (p < 0.05) in males than in females. We also observed a generational increase in Cd accumulation. Fecundity increased, with the exception of the first generation, possibly as a compensation for a decrease of copepod population size under Cd exposure. Protein expression of copepods exposed to Cd occurred in a sex-specific manner. Hemerythrin was mostly up-regulated in both copepod sexes exposed to Cd with males having the highest expression levels, while heat shock protein 70 was mostly up-regulated in males and down-regulated in female copepods, both exposed to Cd. Although copepods are known to develop adaptive mechanisms to tolerate toxic chemicals, continuous exposure to metals could lead to the bioaccumulation of metals in their offspring through maternal transfer and direct uptake from the medium over several generations. As a consequence, increased metal concentrations in copepods could result in physiological damage, reducing their fitness, and possibly compromise copepod population structures. This study showed that mortality, life history traits and molecular responses of a copepod species provided important toxicological endpoints and bio-markers for environmental risk assessments. Environmental pressure resulting from continuous exposure to persistent pollutants like Cd, could have evolutionary significance. The tendency for copepods to selectively adapt to a toxic environment through modifications, could increase their chance of survival over a long term.

Projected near-future ocean acidification decreases mercury toxicity in marine copepods

Here, we examined the combinational effect of ocean acidification (OA) and mercury (Hg) in the planktonic copepod Pseudodiaptomus annandalei in cross-factored response to different pCO₂ (400, 800 μatm) and Hg (control, 1.0 and 2.5 μg/L) exposures for three generations (F0-F2), followed by single-generation recovery (F3) under clean condition. Several phenotypic traits and Hg accumulation were analyzed for F0-F3. Furthermore, shotgun-based quantitative proteomics was performed for F0 and F2. Our results showed that OA insignificantly influenced the traits. During F0-F2, combined exposure reduced Hg accumulation as compared with the counterpart Hg treatment, supporting the mitigating effect of OA on Hg toxicity in copepods. Proteomics analysis indicated that the copepods probably increased energy production/storage and stress response to ensure physiological resilience against OA. However, Hg induced many toxic events (e.g., energy depletion and degenerated organomorphogenesis/embryogenesis for F0; cell cycle arrest and detrimental stress-defense for F2), which were translated to the population-level adverse outcome, i.e., compromised growth/reproduction. Particularly, compensatory proteome response was identified (e.g., increased immune defense for F0; energetic compensation and enhanced embryogenesis for F2), accounting for a negative interaction between OA and Hg. Together, this study provides the molecular mechanisms behind the effects of OA and Hg pollution in marine copepods.

Differential gene expression profile of male and female copepods in response to cadmium exposure

In this study, the whole transcriptome and sex-specific differential gene expression of the copepod Pseudodiaptomus annandalei exposed to cadmium (Cd) were investigated. P. annandalei were exposed to 40 μg/L Cd from the naupliar stage to male and female adults. High-throughput transcriptome sequencing (RNA-seq) was performed with copepod samples using an Illumina Hiseq™ 2000 platform. TransDecoder analysis found 32,625 putative open reading frame contigs. At p-values of <0.001, a total of 4756 differentially expressed genes (DEGs) (2216 up-regulated and 2540 down-regulated genes) were found in male copepods. Whereas a total of 2879 DEGs (2007 up-regulated and 872 down-regulated genes) were found in female copepods. A few selected cellular stress response genes, involved in xenobiotic metabolism, energy metabolism, growth, and development as a result of Cd exposure in the copepods were discussed. The study showed that most of these processes were changed in a sex-specific manner, accounting for the different sensitivities of male and female copepods. Results suggest and reinforce that sex is an important factor to be considered in ecotoxicogenomics.

Warmer temperature increases mercury toxicity in a marine copepod

Marine biota have been co-challenged with ocean warming and mercury (Hg) pollution over many generations because of human activities; however, the molecular mechanisms to explain their combined effects are not well understood. In this study, a marine planktonic copepod Pseudodiaptomus annandalei was acutely exposed to different temperature (22 and 25 °C) and Hg (0 and 118 μg/L) treatments in a 24-h cross-factored experiment. Hg accumulation and its subcellular fractions were determined in the copepods after exposure. The expression of the genes of superoxide dismutase (SOD), glutathione peroxidase (GPx), metallothionein1 (mt1), heat shock protein 70 (hsp70), hsp90, hexokinase (hk), and pyruvate kinase (pk) was also analyzed. Both the Hg treatment alone and the combined exposure of warmer temperature plus Hg pollution remarkably facilitated Hg bioaccumulation in the exposed copepods. Compared with the Hg treatment alone, the combined exposure increased total Hg accumulation and also the amount of Hg stored in the metal-sensitive fractions (MSF), suggesting elevated Hg toxicity in P. annandalei under a warmer environment, given that the MSF is directly related to metal toxicity. The warmer temperature significantly up-regulated the mRNA levels of mt1, hsp70, hsp90, and hk, indicating the copepods suffered from thermal stress. With exposure to Hg, the mRNA level of SOD increased strikingly but the transcript levels of hsp90, hk, and pk decreased significantly, indicating that Hg induced toxic events (e.g., oxidative damage and energy depletion). Particularly, in contrast to the Hg treatment alone, the combined exposure significantly down-regulated the mRNA levels of SOD and GPx but up-regulated the mRNA levels of mt1, hsp70, hsp90, hk, and pk. Collectively, the results of this study indicate that ocean warming will potentially boost Hg toxicity in the marine copepod P. annandalei, which is information that will increase the accuracy of the projections of marine ecosystem responses to the joint effects of climate change stressors and metal pollution on the future ocean.

Toxicity of mercury: Molecular evidence

Minamata disease in Japan and the large-scale poisoning by methylmercury (MeHg) in Iraq caused wide public concerns about the risk emanating from mercury for human health. Nowadays, it is widely known that all forms of mercury induce toxic effects in mammals, and increasing evidence supports the concern that environmentally relevant levels of MeHg could impact normal biological functions in wildlife. The information of mechanism involved in mercurial toxicity is growing but knowledge gaps still exist between the adverse effects and mechanisms of action, especially at the molecular level. A body of data obtained from experimental studies on mechanisms of mercurial toxicity in vivo and in vitro points to that disruption of the antioxidant system may play an important role in the mercurial toxic effects. Moreover, the accumulating evidence indicates that signaling transduction, protein or/and enzyme activity, and gene regulation are involving in mediating toxic and adaptive response to mercury exposure. We conducted here a comprehensive review of mercurial toxic effects on wildlife and human, in particular synthesized key findings of molecular pathways involved in mercurial toxicity from the cells to human. We discuss the molecular evidence related mercurial toxicity to the adverse effects, with particular emphasis on the gene regulation. The further studies relying on Omic analysis connected to adverse effects and modes of action of mercury will aid in the evaluation and validation of causative relationship between health outcomes and gene expression.

Invertebrates facing environmental contamination by endocrine disruptors: Novel evidences and recent insights

The crisis of biodiversity we currently experience raises the question of the impact of anthropogenic chemicals on wild life health. Endocrine disruptors are notably incriminated because of their possible effects on development and reproduction, including at very low doses. As commonly recorded in the field, the burden they impose on wild species also concerns invertebrates, with possible specificities linked with the specific physiology of these animals. A better understanding of chemically-mediated endocrine disruption in these species has clearly gained from knowledge accumulated on vertebrate models. But the molecular pathways specific to invertebrates also need to be reckoned, which implies dedicated research efforts to decipher their basic functioning in order to be able to assess its possible disruption. The recent rising of omics technologies opens the way to an intensification of these efforts on both aspects, even in species almost uninvestigated so far.

Diverse expression regulation of Hsp70 genes in scallops after exposure to toxic Alexandrium dinoflagellates

Heat shock proteins 70KD (Hsp70s) are highly conserved molecular chaperones with essential roles against biotic and abiotic stressors. Marine bivalves inhabit highly complex environments and could accumulate paralytic shellfish toxins (PSTs), the well-noted neurotoxins generated during harmful algal blooms. Here, we systematically analyzed Hsp70 genes (CfHsp70s) in Zhikong scallop (Chlamys farreri), an important aquaculture mollusk in China. Sixty-five CfHsp70s from eight sub-families were identified, and 47 of these genes showed expansion in the Hspa12 sub-family. After exposure to different PST-producing dinoflagellates, Alexandrium minutum and Alexandrium catenella, diverse CfHsp70s regulation presented in scallop hepatopancreas, mainly accumulating incoming PSTs, and kidneys, transforming PSTs into higher toxic analogs. All the up-regulated CfHsp70s were from CfHsp70B2, CfHspa12, and CfHspa5 sub-families. CfHsp70B2 sub-family was mainly induced in the hepatopancreas, and CfHspa12 sub-family was highly induced in the kidneys. CfHsp70s up-regulation under two dinoflagellates exposure was stronger in the kidneys (log₂FC: 19.5 and 18.6) than that in hepatopancreas (log₂FC: 4.3 and 6.1). Exposure to different Alexandrium species had varying effects, that in hepatopancreas, CfHsp70B2s were chronically induced only after A. catenella exposure, whereas in kidney, CfHspa12s were more acutely induced after exposure of A. minutum than A. caenella. Moreover, in Yesso scallops (Patinopecten yessoensis), only Hspa12s were up-regulated in hepatopancreas after A. catenella exposure, and all the Hsp70B2s were down-regulated. These organ-, toxin-, and species-dependent Hsp70 regulation suggested the functional diversity of duplicated Hsp70s in response to the stress by PST-producing algae. Our findings provide insights into the evolution and functional characteristics of Hsp70s in scallops.

Transgenerational Proteome Plasticity in Resilience of a Marine Copepod in Response to Environmentally Relevant Concentrations of Microplastics

Here, we examined the multigenerational effect of microplastics (6-μm polystyrene beads; with different environmentally relevant concentrations of 0.023 and 0.23 mg/L in seawater) on the marine copepod Tigriopus japonicus under two-generation exposure (F0-F1) followed by one-generation recovery (F2) in clean seawater. Also, the seven life-history traits (survival, sex ratio, developmental time of nauplius phase, developmental time to maturation, number of clutches, number of nauplii/clutch, and fecundity) were measured for each generation. Furthermore, to investigate within-generation proteomic response and transgenerational proteome plasticity, proteome profiling was conducted for the F1 and F2 copepods under the control and 0.23 mg/L microplastics treatment. The results showed successful ingestion of microplastics in F0-F1 under both exposure concentrations, while higher concentration (0.23 mg/L) of microplastics resulted in the significant reduction in survival rate, number of nauplii/clutch, and fecundity. However, the affected traits were totally restored in the recovery generation (F2). Proteomic analysis demonstrated that microplastics exposure increased several cellular biosynthesis processes and, in turn, reduced energy storage due to the trade-off, hence compromising survival and reproduction of the treated copepods in F1. Interestingly, the two-generational effect of microplastics in copepods had significant transgenerational proteome plasticity as demonstrated by increased energy metabolism and stress-related defense pathway, which accounts for regaining of the compromised phenotypic traits during recovery (i.e., F2). Overall, this study provides a molecular understanding on the effect of microplastics at a translational level under long-term multigenerational exposure in marine copepods, and also the transgenerational proteome plasticity is likely rendering the robustness of copepods in response to microplastics pollution.

Transcriptome analysis on chlorpyrifos detoxification in Uronema marinum (Ciliophora, Oligohymenophorea)

Chlorpyrifos (CPF) pollution has drawn widespread concerns in aquatic environments due to its risks to ecologic system, however, the response mechanisms of ciliates to CPF pollution were poorly studied. In our current work, the degradation of CPF by ciliates and the morphological changes of ciliates after CPF exposure were investigated. In addition, the transcriptomic profiles of the ciliate Uronema marinum, with and without exposure with CPF, were detected using digital gene expression technologies. De novo transcriptome assembly 166,829,634 reads produced from three groups (untreated, CPF treatment at 12 h and 24 h) by whole transcriptome sequencing (RNA-Seq). Gene ontology (GO) and Kyoto Encyclopaedia of Genes and Genomes (KEGG) pathways were analyzed in all unigenes and different expression genes to identify their biological functions and processes. Furthermore, the results indicated that genes related to the stress response, cytoskeleton and cell structure proteins, and antioxidant systems might play an important role in the resistance mechanism of ciliates. The enzyme activities of SOD and GST after CPF stress were also analyzed, and the result showed the good antioxidant capacity of SOD and GST in ciliates inferred from the increase of the activities of the two enzymes. The ciliate Uronema marinum showed a resistance response to chlorpyrifos stress at the transcriptomic level in the present work, which indicates that ciliates can be considered as a potential bioremediation agent.

De novo transcriptome assembly and differential gene expression analysis of the calanoid copepod Acartia tonsa exposed to nickel nanoparticles

The calanoid copepod Acartia tonsa is a reference species in standardized ecotoxicology bioassay. Despite this interest, there is a lack of knowledge on molecular responses of A. tonsa to contaminants. We generated a de novo assembled transcriptome of A. tonsa exposed 4 days to 8.5 and 17 mg/L nickel nanoparticles (NiNPs), which have been shown to reduce egg hatching success and larval survival but had no effects on the adults. Aims of our study were to 1) improve the knowledge on the molecular responses of A. tonsa copepod and 2) increase the genomic resources of this copepod for further identification of potential biomarkers of NP exposure. The de novo assembled transcriptome of A. tonsa consisted of 53,619 unigenes, which were further annotated to nr, GO, KOG and KEGG databases. In particular, most unigenes were assigned to Metabolic and Cellular processes (34-45%) GO terms, and to Human disease (28%) and Organismal systems (23%) KEGG categories. Comparison among treatments showed that 373 unigenes were differentially expressed in A. tonsa exposed to NiNPs at 8.5 and 17 mg/L, with respect to control. Most of these genes were downregulated and took part in ribosome biogenesis, translation and protein turnover, thus suggesting that NiNPs could affect the copepod ribosome synthesis machinery and functioning. Overall, our study highlights the potential of toxicogenomic approach in gaining more mechanistic and functional information about the mode of action of emerging compounds on marine organisms, for biomarker discovering in crustaceans.

Quantitative Shotgun Proteomics Associates Molecular-Level Cadmium Toxicity Responses with Compromised Growth and Reproduction in a Marine Copepod under Multigenerational Exposure

In this study, the copepod Tigriopus japonicus was exposed to different cadmium (Cd) treatments (0, 2.5, 5, 10, and 50 μg/L in seawater) for five generations (F0-F4), followed by a two-generation (F5-F6) recovery period in clean seawater. Six life-history traits (survival, developmental time of nauplius phase, developmental time to maturation, number of clutches, number of nauplii/clutch, and fecundity) were examined for each generation. Metal accumulation was also analyzed for generations F0-F6. Additionally, proteome profiling was performed for the control and 50 μg/L Cd-treated F4 copepods. In F0-F4 copepods, Cd accumulated in a concentration-dependent manner, prolonging the development of the nauplius phase and maturation and reducing the number of nauplii/clutch and fecundity. However, during F5-F6, Cd accumulation decreased rapidly, and significant but subtle effects on growth and reproduction were observed only for the highest metal treatment at F5. Proteomic analysis revealed that Cd treatment had several toxic effects including depressed nutrient absorption, dysfunction in cellular redox homeostasis and metabolism, and oxidative stress, resulting in growth retardation and reproduction limitation in this copepod species. Taken together, our results demonstrate the relationship between molecular toxicity responses and population-level adverse outcomes in T. japonicus under multigenerational Cd exposure.

Global Proteome Profiling of a Marine Copepod and the Mitigating Effect of Ocean Acidification on Mercury Toxicity after Multigenerational Exposure

Previously, we found that ocean acidification (OA) mitigates mercury (Hg) toxicity to marine copepod Tigriopus japonicus under multigenerational exposure (four generations, F0-F3). To determine the response mechanisms of T. japonicus against long-term exposure to OA and Hg pollution, we investigated the proteome of F3 copepods after multigenerational exposure to four conditions: pCO₂ 400 μatm + control; pCO₂ 1000 μatm + control; pCO₂ 400 μatm +1.0 μg/L Hg; and pCO₂ 1000 μatm +1.0 μg/L Hg. Functional enrichment analysis indicated that OA enhanced the copepod's energy production mainly by increasing protein assimilation and proteolysis as a compensatory strategy, which explained its physiological resilience to reduced pH. Conversely, Hg treatment decreased many critical processes, including ferric iron binding, antioxidant activity, cellular homeostasis, and glutathione metabolism, and these toxic events could translate into higher-level responses, i.e., restrained reproduction in copepods. Importantly, the mediation of Hg toxicity in T. japonicus by OA could be explained by the enhanced lysosome-autophagy pathway proteomes that are responsible for repairing and removing damaged proteins and enzymes under stress. Overall, this study provided molecular insights into the response of T. japonicus to long-term exposure of OA and Hg, with a particular emphasis on the mitigating impact of the CO₂-driven acidification on Hg toxicity.