An integrated multi-disciplinary approach for studying multiple stressors in freshwater ecosystems: Daphnia as a model organism

Could a residential wood ash recycling programme be part of the solution to calcium decline in lakes and forests in Muskoka (Ontario, Canada)?

One possible solution to the recent decline of calcium (Ca) concentrations in Canadian Shield forests and lakes in eastern North America is the addition of Ca-rich wood ash to watersheds. We investigated the feasibility of using small, mainly residential sources of non-industrial wood ash (NIWA) for this purpose by quantifying concentrations of its major nutrients and metals, its toxicity to Daphnia in aqueous extracts, and estimating the amount of NIWA available in the District of Muskoka in central Ontario. Locally collected NIWA averaged 30% Ca, and also contained smaller but significant amounts of K, Mg, Na, and P. Of these, K was so soluble that it was toxic to Daphnia over 48 h in the concentrate and 10-fold dilution; however, sedimented ash was not toxic over 15 d. Most metal levels in NIWA were below targets permitting unrestricted land application. However, Cu and Zn were just above these targets, but well below those for conditional use. Muskoka residents generate about 235 000 kg of NIWA annually, not enough to treat all central Ontario areas affected; however, a NIWA recycling programme implemented across southern Ontario could generate enough ash to solve the Ca decline problem in Muskoka’s forests and lakes.

Variation in transcriptional responses to copper exposure across Daphnia pulex lineages

Copper pollution is pervasive in aquatic habitats and is particularly harmful to invertebrates sensitive to environmental changes such as Daphnia pulex. Mechanisms of toxicity and tolerance to copper are not well understood. We used RNA-sequencing to investigate these mechanisms in three genetically distinct D. pulex clonal lineages with different histories of copper exposure. Upregulated genes after copper exposure were enriched with Gene Ontology (GO) categories involved in digestion, molting and growth, whereas downregulated genes after copper exposure were enriched in the metal-regulatory system, immune response and epigenetic modifications. The three D. pulex clones in our study show largely similar transcriptional patterns in response to copper, with only a total of twenty genes differentially expressed in a single clonal lineages. We also detected lower relative expression of some genes known to be important for copper tolerance, metallothionein and glutathione-S-transferase, in a sensitive lineage sampled from an uncontaminated habitat. Daphnia-specific genes (without orthologs outside the genus) and Daphnia-specific duplications (genes duplicated in the Daphnia lineage) were overrepresented in differentially expressed genes, highlighting an important role for newly emerged genes in tolerating environmental stressors. The results indicate that the D. pulex lineages tested in this study generally respond to copper stress using the same major pathways, but that the more resistant clone with previous copper exposure might be better able to regulate key genes. This finding highlights the important nuances in gene expression among clones, shaped by historical exposure and influencing copper tolerance.

The concentration of dissolved organic matter impacts the metabolic response in Daphnia magna exposed to 17α-ethynylestradiol and perfluorooctane sulfonate

The pharmaceutical 17α-ethynylestradiol (EE2) and the industrial chemical perfluorooctane sulfonate (PFOS) are organic contaminants frequently detected in freshwater environments. It is hypothesized that hydrophobic organic contaminants can sorb to dissolved organic matter (DOM) and this may reduce the toxicity of these contaminants by reducing the contaminants' bioavailability. To investigate this hypothesis, ¹H nuclear magnetic resonance (NMR)-based metabolomics was used to determine how the metabolome of Daphnia magna changes when a range of DOM concentrations are added during EE2 and PFOS exposure experiments. D. magna were exposed for 48 h to sub-lethal concentrations of 1 mg/L EE2 or 30 mg/L PFOS in the presence of 0, 1, 2, 3 and 4 mg dissolved organic carbon (DOC)/L. EE2 exposure resulted in increased amino acids and decreased glucose in D. magna. All DOM concentrations were able to lessen these metabolite disturbances from EE2 exposure, likely due to reductions in the bioavailability of EE2 through interactions with DOM. Exposure to PFOS resulted in decreased amino acids, and the presence of 1 mg DOC/L did not alter this metabolic response. However, PFOS exposure with the higher DOM concentrations resulted in a different pattern of metabolite changes which may be due to combined impacts of PFOS and DOM on the metabolome or due to an increase in PFOS bioavailability and uptake in D. magna. These results suggest that the concentration of DOM influences the sensitive biochemical changes in organisms that occur during acute sub-lethal exposure to organic contaminants.

Transgenerational response to early spring warming in Daphnia

Temperature and photoperiod regulate key fitness traits in plants and animals. However, with temperature increase due to global warming, temperature cue thresholds are experienced at shorter photoperiods, disrupting the optimal seasonal timing of physiological, developmental and reproductive events in many species. Understanding the mechanisms of adaptation to the asynchrony between temperature and photoperiod is key to inform our understanding of how species will respond to global warming. Here, we studied the transgenerational mechanisms of responses of the cyclical parthenogen Daphnia magna to different photoperiod lengths co-occurring with warm temperature thereby assessing the impact of earlier spring warming on its fitness. Daphnia uses temperature and photoperiod cues to time dormancy, and to switch between sexual and asexual reproduction. Daphnia life cycle offers the opportunity to measure the relative contribution of plastic and genetic responses to environmental change across generations and over evolutionary time. We use transgenerational common garden experiments on three populations 'resurrected' from a biological archive experiencing temperature increase over five decades. Our results suggest that response to early spring warming evolved underpinned by a complex interaction between plastic and genetic mechanisms while a positive maternal contribution at matching environments between parental and offspring generation was also observed.

Functional responses of Daphnia magna to zero-mean flow turbulence

Daphnia are important to understanding the biogeochemistry of aquatic ecosystems, mainly because of their ability to filter bacteria, algae and inorganic particles as well. Although there are many studies on the general effects that biotic and abiotic stressors, increased temperature and hypoxia, salinity, metals, pharmaceuticals, pesticides, etc., have on Daphnia populations, little is known about the impact elevated turbulence has. Here, we show that turbulence affects Daphnia magna survival, swimming behaviour and filtering capacity. Our data demonstrate that altering their habitat by induced mixing from turbulence, induces an increased filtering capacity of the Daphnia magna individuals, provided the level of background turbulence (defined by the dissipation of turbulent kinetic energy) is lower than ε = 0.04 cm² s⁻³. The filtering capacity reduced exponentially with increasing ε, and at ε > 1 cm² s⁻³ both mobility and filtration were suppressed and eventually led to the death of all the Daphnia magna individuals.

Temporal variation in zooplankton and phytoplankton community species composition and the affecting factors in Lake Taihu-a large freshwater lake in China

Monitoring diverse components of aquatic ecosystems is vital for elucidation of diversity dynamics and processes, which alter freshwater ecosystems, but such studies are seldom conducted. Phytoplankton and zooplankton are integral components which play indispensable parts in the structure and ecological service function of water bodies. However, few studies were made on how zooplankton and phytoplankton community may respond simultaneously to change of circumstance and their mutual relationship. Therefore, we researched synchronously the phytoplankton communities as well as zooplankton communities based on monthly monitoring data from September 2011 to August 2012 in heavily polluted areas and researched their responses to variation in environmental parameters and their mutual relationship. As indicated by Time-lag analysis (TLA), the long-term dynamics of phytoplankton and zooplankton were undergoing directional variations, what's more, there exists significant seasonal variations of phytoplankton and zooplankton communities as indicated by Non-Metric Multidimensional scaling (NMDS) methods. Also, Redundancy Analysis (RDA) demonstrated that environmental indicators together accounted for 25.6% and 50.1% variance of phytoplankton and zooplankton, respectively, indicating that environmental variations affected significantly on the temporal dynamics of phytoplankton as well as zooplankton communities. What's more, variance partioning suggested that the major environmental factors influencing variation structures of zooplankton communities were water temperature, concentration of nitrogen, revealing the dominating driving mechanism which shaped the communities of zooplankton. It was also found that there was significant synchronization between zooplankton biomass and phytoplankton biomass (expressed as Chl-a concentration), which suggested that zooplankton respond to changes in dynamic structure of phytoplankton community and can initiate a decrease in phytoplankton biomass through grazing in a few months.

The Chronic Effects of Copper and Cadmium on Life History Traits Across Cladocera Species: A Meta-analysis

The effect of sublethal concentrations of heavy metals on cladoceran growth and reproduction is a cornerstone of modern ecotoxicology. However, the literature contains assays across numerous concentrations, on numerous species and genotypes, and conditions are far from consistent. We undertook a systematic review of the sublethal effects of copper and cadmium concentrations on Cladocera spp. life history (reproduction, maturation age, and somatic growth rate). Using meta-analysis, we tested the hypothesis that the effects of increasing Cu and Cd concentrations on traits may vary by species. We also evaluated where possible whether the effect of metal concentrations on traits vary by water hardness, exposure duration, or whether the metals were delivered in aqueous solution or via food. We surveyed > 200 papers, resulting in a set of 32 experimental studies representing 446 trials where the results were presented compared with Daphnia magna-the most commonly assayed species. We found qualitatively similar effects of Cu and Cd on life history traits that included reduction in reproduction and somatic growth rate and delay of maturation. Cladocera species showed marked variations in their susceptibility to metals, and D. magna was found to be the least sensitive species to sublethal changes in reproduction. The effects were largely consistent for aqueous vs. dietary food. Water hardness, where data were available, had no detectable effect. Available data indicate that exposure duration had no effect on the toxicity of Cu but did for D. magna reproductive response to Cd. Our study highlights the importance of including species identity when considering toxicological testing and regulation development.

Accelerated rates of large-scale mutations in the presence of copper and nickel

Mutation rate variation has been under intense investigation for decades. Despite these efforts, little is known about the extent to which environmental stressors accelerate mutation rates and influence the genetic load of populations. Moreover, most studies on stressors have focused on unicellular organisms and point mutations rather than large-scale deletions and duplications (copy number variations [CNVs]). We estimated mutation rates in Daphnia pulex exposed to low levels of environmental stressors as well as the effect of selection on de novo mutations. We conducted a mutation accumulation (MA) experiment in which selection was minimized, coupled with an experiment in which a population was propagated under competitive conditions in a benign environment. After an average of 103 generations of MA propagation, we sequenced 60 genomes and found significantly accelerated rates of deletions and duplications in MA lines exposed to ecologically relevant concentrations of metals. Whereas control lines had gene deletion and duplication rates comparable to other multicellular eukaryotes (1.8 × 10-6 per gene per generation), the presence of nickel and copper increased these rates fourfold. The realized mutation rate under selection was reduced to 0.4× that of control MA lines, providing evidence that CNVs contribute to mutational load. Our CNV breakpoint analysis revealed that nonhomologous recombination associated with regions of DNA fragility is the primary source of CNVs, plausibly linking metal-induced DNA strand breaks with higher CNV rates. Our findings suggest that environmental stress, in particular multiple stressors, can have profound effects on large-scale mutation rates and mutational load of multicellular organisms.

High stoichiometric food quality increases moulting organism vulnerability to pollutant impacts: An experimental test with Gammarus fossarum (Crustacea: Amphipoda)

Headwater organisms are most often simultaneously faced with multiple stressors such as low resource quality and pollutants. Higher food quality has been hypothesized to enhance the tolerance of organisms to pollutants, but the interactive effects of food quality and pollutants on species and ecosystems remain poorly studied. To better understand these interactive effects, we experimentally manipulated the phosphorus (P) content of two leaf litters with contrasted carbon quality (alder and maple). During four weeks, individuals of the detritivorous crustacean Gammarus fossarum were exposed to low levels of cadmium ([Cd] = 0, 0.35 or 0.70 μg L^-1) while being fed with one of the leaf P treatments. When organisms were not exposed to Cd, their high survival rate was more driven by the carbon quality of the resource (litter species) than by its stoichiometric quality. In contrast, their number of moults and growth rates were primarily increased by the P content of resources. When exposed to Cd, G. fossarum survival rate was reduced, but this effect was largely magnified by a higher P level in resources. Our results showed that despite positive effects of resource stoichiometric quality on organism life history traits (growth, survival), a resource of high stoichiometric quality might be detrimental for organisms exposed to low and environmentally realistic levels of pollutants. Two non-exclusive hypotheses are proposed to explain these results. First, organisms fed on the highest quality resource exhibited the highest moulting frequencies (moults being the most critical life cycle step of arthropods), which could have rendered them more sensitive to pollutants. Secondly, the metabolism of organisms fed on higher quality resources was potentially enhanced, increasing the uptake of dissolved Cd by gammarids. This study suggests that species sensitivity to pollutants might be underestimated in ecosystems facing both nutrient constraint and pollutants.

Predictability of the impact of multiple stressors on the keystone species Daphnia

Eutrophication and climate change are two of the most pressing environmental issues affecting up to 50% of aquatic ecosystems worldwide. Mitigation strategies to reduce the impact of environmental change are complicated by inherent difficulties of predicting the long-term impact of multiple stressors on natural populations. Here, we investigated the impact of temperature, food levels and carbamate insecticides, in isolation and in combination, on current and historical populations of the freshwater grazer Daphnia. We used common garden and competition experiments on historical and modern populations of D. magna ‘resurrected’ from a lake with known history of anthropogenic eutrophication and documented increase in ambient temperature over time. We found that these populations response dramatically differed between single and multiple stressors. Whereas warming alone induced similar responses among populations, warming combined with insecticides or food limitation resulted in significantly lower fitness in the population historically exposed to pesticides. These results suggest that the negative effect of historical pesticide exposure is magnified in the presence of warming, supporting the hypothesis of synergism between chemical pollution and other stressors.

Evaluation of Daphnia magna metabolic responses to organic contaminant exposure with and without dissolved organic matter using 1H nuclear magnetic resonance (NMR)-based metabolomics

Previous studies have shown that contaminant toxicity to target organisms is altered by the presence of dissolved organic matter (DOM). Contaminants can bind to DOM and this may alter the bioavailability and subsequent toxicity of the contaminants. However, molecular-level techniques are needed to more closely evaluate the impact of DOM on the sub-lethal biochemical responses to emerging contaminants. To investigate how DOM may alter the metabolic response to organic contaminant exposure, ¹H nuclear magnetic resonance (NMR)-based metabolomics was used to investigate how the metabolome of Daphnia magna changes when Suwannee River DOM (5 mg organic carbon/L) is included in the acute exposure of four contaminants with varying hydrophobicity. Sub-lethal concentrations of the hydrophobic contaminant 17α-ethynylestradiol (EE2), the relatively more polar compounds carbamazepine and imidacloprid, or the anionic contaminant perfluorooctane sulfonate (PFOS) were used. A 48-h exposure to DOM alone had a minor impact on the metabolome of D. magna. There were significant increases in amino acids from EE2 exposure which were reduced in the presence of DOM, suggesting that DOM may alleviate the sub-lethal metabolic response from EE2 exposure through sorption and a reduction in freely dissolved EE2. The metabolome was relatively unaltered with exposure to carbamazepine and imidacloprid in the presence of DOM which is likely because these contaminants are water soluble and did not strongly interact with DOM. PFOS exposure resulted in a more significant metabolic response with DOM suggesting that DOM enhanced the uptake and bioavailability of PFOS in D. magna. As such, the presence of DOM should be considered when determining sensitive molecular-level changes in organisms to sub-lethal organic contaminant exposure.

Intraspecific phenotypic variation in life history traits of Daphnia galeata populations in response to fish kairomones

Phenotypic plasticity is the ability of a genotype to produce different phenotypes depending on the environment. It has an influence on the adaptive potential to environmental change and the capability to adapt locally. Adaptation to environmental change happens at the population level, thereby contributing to genotypic and phenotypic variation within a species. Predation is an important ecological factor structuring communities and maintaining species diversity. Prey developed different strategies to reduce their vulnerability to predators by changing their behaviour, their morphology or their life history. Predator-induced life history responses in Daphnia have been investigated for decades, but intra-and inter-population variability was rarely addressed explicitly. We addressed this issue by conducting a common garden experiment with 24 clonal lines of European Daphnia galeata originating from four populations, each represented by six clonal lines. We recorded life history traits in the absence and presence of fish kairomones. Additionally, we looked at the shape of experimental individuals by conducting a geometric morphometric analysis, thus assessing predator-induced morphometric changes. Our data revealed high intraspecific phenotypic variation within and between four D. galeata populations, the potential to locally adapt to a vertebrate predator regime as well as an effect of the fish kairomones on morphology of D. galeata.

Investigation of Daphnia magna Sub-Lethal Exposure to Organophosphate Esters in the Presence of Dissolved Organic Matter Using ¹H NMR-Based Metabolomics

Organophosphate esters (OPEs) are frequently detected in aquatic environments. Hydrophobic OPEs with high octanol-water partition coefficients (Log K_OW) will likely sorb to dissolved organic matter (DOM) and consequently alter OPE bioavailability and sub-lethal toxicity. ¹H nuclear magnetic resonance (NMR)-based metabolomics was used to evaluate how DOM (5 mg organic carbon/L) alters the metabolic response of Daphnia magna exposed to sub-lethal concentrations of three individual OPEs with varying hydrophobicity. D. magna exposed to the hydrophilic contaminant (Log K_OW = 1.43) tris(2-chloroethyl) phosphate (TCEP) did not have substantial metabolic changes and DOM did not alter the metabolic response. There were significant increases in amino acids and a decrease in glucose from exposure to the hydrophobic contaminant (Log K_OW = 3.65) tris(2-butoxyethyl) phosphate (TBOEP) which DOM did not mitigate, likely due to the high sub-lethal toxicity of TBOEP. Exposure to DOM and the hydrophobic contaminant (Log K_OW = 4.76) triphenyl phosphate (TPhP) resulted in a unique metabolic response which was unlike TPhP only exposure, perhaps because DOM may be an additional stressor with TPhP exposure. Therefore, Log K_OW values may not always predict how sub-lethal contaminant toxicity will change with DOM and there should be more consideration to incorporate DOM in sub-lethal ecotoxicology testing.

Differential gene transcription across the life cycle in Daphnia magna using a new all genome custom-made microarray

Background

Unravelling the link between genes and environment across the life cycle is a challenging goal that requires model organisms with well-characterized life-cycles, ecological interactions in nature, tractability in the laboratory, and available genomic tools. Very few well-studied invertebrate model species meet these requirements, being the waterflea Daphnia magna one of them. Here we report a full genome transcription profiling of D. magna during its life-cycle. The study was performed using a new microarray platform designed from the complete set of gene models representing the whole transcribed genome of D. magna.

Results

Up to 93% of the existing 41,317 D. magna gene models showed differential transcription patterns across the developmental stages of D. magna, 59% of which were functionally annotated. Embryos showed the highest number of unique transcribed genes, mainly related to DNA, RNA, and ribosome biogenesis, likely related to cellular proliferation and morphogenesis of the several body organs. Adult females showed an enrichment of transcripts for genes involved in reproductive processes. These female-specific transcripts were essentially absent in males, whose transcriptome was enriched in specific genes of male sexual differentiation genes, like doublesex.

Conclusion

Our results define major characteristics of transcriptional programs involved in the life-cycle, differentiate males and females, and show that large scale gene-transcription data collected in whole animals can be used to identify genes involved in specific biological and biochemical processes.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4725-7) contains supplementary material, which is available to authorized users.

Metabolomic and oxidative effects of quantum dots-indolicidin on three generations of Daphnia magna

This study evaluated the effect of QDs functionalized with the antimicrobial peptide indolicidin on oxidative stress and metabolomics profiles of Daphnia magna across three generations (F0, F1, and F2). Exposing D. magna to sub-lethal concentrations of the complex QDs-indolicidin, a normal survival of daphnids was observed from F0 to F2, but a delay of first brood, fewer broods per female, a decrease of length of about 50% compared to control. In addition, QDs-indolicidin induced a significantly higher production of reactive oxygen species (ROS) gradually in each generation and an impairment of enzymes response to oxidative stress such as superoxide dismutase (SOD), catalase (CAT) and glutathione transferase (GST). Effects were confirmed by metabolomics profiles that pointed out a gradual decrease of metabolomics content over the three generations and a toxic effect of QDs-indolicidin likely related to the higher accumulation of ROS and decreased antioxidant capacity in F1 and F2 generations. Results highlighted the capability of metabolomics to reveal an early metabolic response to stress induced by environmental QDs-indolicidin complex.

Early ecotoxic effects of ZnO nanoparticle chronic exposure in Mytilus galloprovincialis revealed by transcription of apoptosis and antioxidant-related genes

Recently, China became one of the largest nanomaterial markets in the world. The wide use of ZnO nanoparticles in a number of products implies an increasing release in marine environment and consequently the evaluation of the potential effects upon marine organisms largely cultured in China for commercial purposes, such as invertebrate bivalves is a current need. To this aim, survival, bioaccumulation, and transcription pattern of key genes, p53, PDRP, SOD, CAT, and GST, involved in DNA damage/repair and antioxidation, in Mytilus galloprovincialis digestive gland, exposed to ZnO NPs (<100 nm) and ZnO bulk (150-200 nm) for 4 weeks, were evaluated. ZnSO₄ was also assessed to appraise the role of zinc ions. Starting from 72 h, increasing mortality values along the exposure time were observed for all ZnO compounds. The highest difference was evident after 28 d when NPs resulted three times more toxic than bulk, (LC₅₀) = 0.78 mg Zn/L (confidence limits: 0.64, 1.00) and 2.62 mg Zn/L (confidence limits: 1.00, 4.00), respectively. For ZnSO₄ the (LC₅₀) was always the lowest reaching the minimum value at 28 d 0.25 mg Zn/L (confidence limits: 0.10-0.40). Digestive gland showed higher uptake rate of ionic Zn respect to ZnO NPs and bulk during the first three days of exposure. In particular at the end of the exposure time (28 d) at 1 mg Zn/L the rank of Zn uptake rate was Zinc ion > ZnO NPs > ZnO bulk. The relative expression of investigated genes evidenced that distinct actions of apoptosis and antioxidation occurred in M. galloprovincialis exposed to ZnO NPs with a peculiar pattern dependent on exposure time and concentration. Application of the qRT-PCR technique revealed evidence of sensitivity to the nanomaterial since the first time of exposure.

Haemoglobin-mediated response to hyper-thermal stress in the keystone species Daphnia magna

Anthropogenic global warming has become a major geological and environmental force driving drastic changes in natural ecosystems. Due to the high thermal conductivity of water and the effects of temperature on metabolic processes, freshwater ecosystems are among the most impacted by these changes. The ability to tolerate changes in temperature may determine species long-term survival and fitness. Therefore, it is critical to identify coping mechanisms to thermal and hyper-thermal stress in aquatic organisms. A central regulatory element compensating for changes in oxygen supply and ambient temperature is the respiratory protein haemoglobin (Hb). Here, we quantify Hb plastic and evolutionary response in Daphnia magna subpopulations resurrected from the sedimentary archive of a lake with known history of increase in average temperature and recurrence of heat waves. By measuring constitutive changes in crude Hb protein content among subpopulations, we assessed evolution of the Hb gene family in response to temperature increase. To quantify the contribution of plasticity in the response of this gene family to hyper-thermal stress, we quantified changes in Hb content in all subpopulations under hyper-thermal stress as compared to nonstressful temperature. Further, we tested competitive abilities of genotypes as a function of their Hb content, constitutive and induced. We found that Hb-rich genotypes have superior competitive abilities as compared to Hb-poor genotypes under hyper-thermal stress after a period of acclimation. These findings suggest that whereas long-term adjustment to higher occurrence of heat waves may require a combination of plasticity and genetic adaptation, plasticity is most likely the coping mechanism to hyper-thermal stress in the short term. Our study suggests that with higher occurrence of heat waves, Hb-rich genotypes may be favoured with potential long-term impact on population genetic diversity.

Multi-trophic level response to extreme metal contamination from gold mining in a subarctic lake

Giant Mine, located in the city of Yellowknife (Northwest Territories, Canada), is a dramatic example of subarctic legacy contamination from mining activities, with remediation costs projected to exceed $1 billion. Operational between 1948 and 2004, gold extraction at Giant Mine released large quantities of arsenic and metals from the roasting of arsenopyrite ore. We examined the long-term ecological effects of roaster emissions on Pocket Lake, a small lake at the edge of the Giant Mine lease boundary, using a spectrum of palaeoenvironmental approaches. A dated sedimentary profile tracked striking increases (approx. 1700%) in arsenic concentrations coeval with the initiation of Giant Mine operations. Large increases in mercury, antimony and lead also occurred. Synchronous changes in biological indicator assemblages from multiple aquatic trophic levels, in both benthic and pelagic habitats, indicate dramatic ecological responses to extreme metal(loid) contamination. At the peak of contamination, all Cladocera, a keystone group of primary consumers, as well as all planktonic diatoms, were functionally lost from the sediment record. No biological recovery has been inferred, despite the fact that the bulk of metal(loid) emissions occurred more than 50 years ago, and the cessation of all ore-roasting activities in Yellowknife in 1999.

Paleo-ecotoxicology: What Can Lake Sediments Tell Us about Ecosystem Responses to Environmental Pollutants?

The development of effective risk reduction strategies for aquatic pollutants requires a comprehensive understanding of toxic impacts on ecosystems. Classical toxicological studies are effective for characterizing pollutant impacts on biota in a controlled, simplified environment. Nonetheless, it is well-acknowledged that predictions based on the results of these studies must be tested over the long-term in a natural ecosystem setting to account for increased complexity and multiple stressors. Paleolimnology (the study of lake sediment cores to reconstruct environmental change) can address many key knowledge gaps. When used as part of a weight-of-evidence framework with more traditional approaches in ecotoxicology, it can facilitate rapid advances in our understanding of the chronic effects of pollutants on ecosystems in an environmentally realistic, multistressor context. Paleolimnology played a central role in the Acid Rain debates, as it was instrumental in demonstrating industrial emissions caused acidification of lakes and associated ecosystem-wide impacts. "Resurrection Ecology" (hatching dormant resting eggs deposited in the past) records evolutionary responses of populations to chronic pollutant exposure. With recent technological advances (e.g., geochemistry, genomic approaches), combined with an emerging paleo-ecotoxicological framework that leverages strengths across multiple disciplines, paleolimnology will continue to provide valuable insights into the most pressing questions in ecotoxicology.

Endocrine disruption in aquatic systems: up-scaling research to address ecological consequences

Endocrine‐disrupting chemicals (EDCs) can alter biological function in organisms at environmentally relevant concentrations and are a significant threat to aquatic biodiversity, but there is little understanding of exposure consequences for populations, communities and ecosystems. The pervasive nature of EDCs within aquatic environments and their multiple sub‐lethal effects make assessments of their impact especially important but also highly challenging. Herein, we review the data on EDC effects in aquatic systems focusing on studies assessing populations and ecosystems, and including how biotic and abiotic processes may affect, and be affected by, responses to EDCs. Recent research indicates a significant influence of behavioural responses (e.g. enhancing feeding rates), transgenerational effects and trophic cascades in the ecological consequences of EDC exposure. In addition, interactions between EDCs and other chemical, physical and biological factors generate uncertainty in our understanding of the ecological effects of EDCs within aquatic ecosystems. We illustrate how effect thresholds for EDCs generated from individual‐based experimental bioassays of the types commonly applied using chemical test guidelines [e.g. Organisation for Economic Co‐operation and Development (OECD)] may not necessarily reflect the hazards associated with endocrine disruption. We argue that improved risk assessment for EDCs in aquatic ecosystems urgently requires more ecologically oriented research as well as field‐based assessments at population‐, community‐ and food‐web levels.

How experimental biology and ecology can support evidence-based decision-making in conservation: avoiding pitfalls and enabling application

Policy development and management decisions should be based upon the best available evidence. In recent years, approaches to evidence synthesis, originating in the medical realm (such as systematic reviews), have been applied to conservation to promote evidence-based conservation and environmental management. Systematic reviews involve a critical appraisal of evidence, but studies that lack the necessary rigour (e.g. experimental, technical and analytical aspects) to justify their conclusions are typically excluded from systematic reviews or down-weighted in terms of their influence. One of the strengths of conservation physiology is the reliance on experimental approaches that help to more clearly establish cause-and-effect relationships. Indeed, experimental biology and ecology have much to offer in terms of building the evidence base that is needed to inform policy and management options related to pressing issues such as enacting endangered species recovery plans or evaluating the effectiveness of conservation interventions. Here, we identify a number of pitfalls that can prevent experimental findings from being relevant to conservation or would lead to their exclusion or down-weighting during critical appraisal in a systematic review. We conclude that conservation physiology is well positioned to support evidence-based conservation, provided that experimental designs are robust and that conservation physiologists understand the nuances associated with informing decision-making processes so that they can be more relevant.

Complexities of gene expression patterns in natural populations of an extremophile fish (Poecilia mexicana, Poeciliidae)

Variation in gene expression can provide insights into organismal responses to environmental stress and physiological mechanisms mediating adaptation to habitats with contrasting environmental conditions. We performed an RNA-sequencing experiment to quantify gene expression patterns in fish adapted to habitats with different combinations of environmental stressors, including the presence of toxic hydrogen sulphide (H2 S) and the absence of light in caves. We specifically asked how gene expression varies among populations living in different habitats, whether population differences were consistent among organs, and whether there is evidence for shared expression responses in populations exposed to the same stressors. We analysed organ-specific transcriptome-wide data from four ecotypes of Poecilia mexicana (nonsulphidic surface, sulphidic surface, nonsulphidic cave and sulphidic cave). The majority of variation in gene expression was correlated with organ type, and the presence of specific environmental stressors elicited unique expression differences among organs. Shared patterns of gene expression between populations exposed to the same environmental stressors increased with levels of organismal organization (from transcript to gene to physiological pathway). In addition, shared patterns of gene expression were more common between populations from sulphidic than populations from cave habitats, potentially indicating that physiochemical stressors with clear biochemical consequences can constrain the diversity of adaptive solutions that mitigate their adverse effects. Overall, our analyses provided insights into transcriptional variation in a unique system, in which adaptation to H2 S and darkness coincide. Functional annotations of differentially expressed genes provide a springboard for investigating physiological mechanisms putatively underlying adaptation to extreme environments.

Armour GE, Thakkar E, Mackay TFC, Anholt RRH. A Drosophila model for toxicogenomics: Genetic variation in susceptibility to heavy metal exposure

The genetic factors that give rise to variation in susceptibility to environmental toxins remain largely unexplored. Studies on genetic variation in susceptibility to environmental toxins are challenging in human populations, due to the variety of clinical symptoms and difficulty in determining which symptoms causally result from toxic exposure; uncontrolled environments, often with exposure to multiple toxicants; and difficulty in relating phenotypic effect size to toxic dose, especially when symptoms become manifest with a substantial time lag. Drosophila melanogaster is a powerful model that enables genome-wide studies for the identification of allelic variants that contribute to variation in susceptibility to environmental toxins, since the genetic background, environmental rearing conditions and toxic exposure can be precisely controlled. Here, we used extreme QTL mapping in an outbred population derived from the D. melanogaster Genetic Reference Panel to identify alleles associated with resistance to lead and/or cadmium, two ubiquitous environmental toxins that present serious health risks. We identified single nucleotide polymorphisms (SNPs) associated with variation in resistance to both heavy metals as well as SNPs associated with resistance specific to each of them. The effects of these SNPs were largely sex-specific. We applied mutational and RNAi analyses to 33 candidate genes and functionally validated 28 of them. We constructed networks of candidate genes as blueprints for orthologous networks of human genes. The latter not only provided functional contexts for known human targets of heavy metal toxicity, but also implicated novel candidate susceptibility genes. These studies validate Drosophila as a translational toxicogenomics gene discovery system.

Although physiological effects of environmental toxins are well documented, we know little about the genetic factors that determine individual variation in susceptibility to toxins. Such information is difficult to obtain in human populations due to heterogeneity in genetic background and environmental exposure, and the diversity of symptoms and time lag with which they appear after toxic exposure. Here, we show that the fruit fly, Drosophila, can serve as a powerful genetic model system to elucidate the genetic underpinnings that contribute to individual variation in resistance to toxicity, using lead and cadmium exposure as an experimental paradigm. We identified genes that harbor genetic variants that contribute to individual variation in resistance to heavy metal exposure. Furthermore, we constructed genetic networks on which we could superimpose human counterparts of Drosophila genes. We were able to place human genes previously implicated in heavy metal toxicity in biological context and identify novel targets for heavy metal toxicity. Thus, we demonstrate that based on evolutionary conservation of fundamental biological processes, we can use Drosophila as a powerful translational model for toxicogenomics studies.

Evolutionary consequences of historical metal contamination for natural populations of Chironomus riparius (Diptera: Chironomidae)

Populations inhabiting metal-impacted freshwater systems located nearby industrial and urban areas may be under intense selection. The present study aims to address two fundamental microevolutionary aspects of metal contamination in the midge Chironomus riparius (Meigen): Are populations inhabiting historically metal contaminated sites genetically adapted to metals? And, are populations from these sites genetically eroded? To answer these questions, C. riparius populations were sampled from three sites with well-known histories of metal contamination and three nearby-located references. Genetic adaptation to metals was investigated through acute and chronic exposures to cadmium (Cd), after rearing all populations for at least six generations under laboratory clean conditions. Genetic diversity was estimated based on the allelic variation of seven microsatellite markers. Results showed higher acute tolerance to Cd in populations originating from metal contaminated sites compared to their respective references and significant differences in two out of three pairwise comparisons. However, there was a mismatch between acute and chronic tolerance to Cd with results of the partial life-cycle tests suggesting fitness costs under control clean conditions in two metal-adapted populations. Despite no evidences of genetic erosion in populations sampled from metal contaminated sites, our results suggest genetically inherited tolerance to Cd in populations inhabiting historically contaminated sites. These findings lend support to the use of C. riparius as a model organism in evolutionary toxicology and highlight the importance of coupling measures of neutral genetic diversity with assessments of chemical tolerance of populations for a better understanding of contaminant-induced adaptation and evolutionary processes.

Temporal genetic stability in natural populations of the waterflea Daphnia magna in response to strong selection pressure

Studies monitoring changes in genetic diversity and composition through time allow a unique understanding of evolutionary dynamics and persistence of natural populations. However, such studies are often limited to species with short generation times that can be propagated in the laboratory or few exceptional cases in the wild. Species that produce dormant stages provide powerful models for the reconstruction of evolutionary dynamics in the natural environment. A remaining open question is to what extent dormant egg banks are an unbiased representation of populations and hence of the species' evolutionary potential, especially in the presence of strong environmental selection. We address this key question using the water flea Daphnia magna, which produces dormant stages that accumulate in biological archives over time. We assess temporal genetic stability in three biological archives, previously used in resurrection ecology studies showing adaptive evolutionary responses to rapid environmental change. We show that neutral genetic diversity does not decline with the age of the population and it is maintained in the presence of strong selection. In addition, by comparing temporal genetic stability in hatched and unhatched populations from the same biological archive, we show that dormant egg banks can be consulted to obtain a reliable measure of genetic diversity over time, at least in the multidecadal time frame studied here. The stability of neutral genetic diversity through time is likely mediated by the buffering effect of the resting egg bank.

An integrated study on antimicrobial activity and ecotoxicity of quantum dots and quantum dots coated with the antimicrobial peptide indolicidin

This study attempts to evaluate the antimicrobial activity and the ecotoxicity of quantum dots (QDs) alone and coated with indolicidin. To meet this objective, we tested the level of antimicrobial activity on Gram-positive and Gram-negative bacteria, and we designed an ecotoxicological battery of test systems and indicators able to detect different effects using a variety of end points. The antibacterial activity was analyzed against Staphylococcus aureus (ATCC 6538), Pseudomonas aeruginosa (ATCC 1025), Escherichia coli (ATCC 11229), and Klebsiella pneumoniae (ATCC 10031), and the results showed an improved germicidal action of QDs-Ind. Toxicity studies on Daphnia magna indicated a decrease in toxicity for QDs-Ind compared to QDs alone, lack of bioluminescence inhibition on Vibrio fisheri, and no mutations in Salmonella typhimurium TA 100. The comet assay and oxidative stress experiments performed on D. magna showed a genotoxic and an oxidative damage with a dose-response trend. Indolicidin retained its activity when bound to QDs. We observed an enhanced activity for QDs-Ind. The presence of indolicidin on the surface of QDs was able to decrease its QDs toxicity.

Adaptation to climate change: trade-offs among responses to multiple stressors in an intertidal crustacean

Trade-offs may influence both physiological and evolutionary responses to co-occurring stressors, but their effects on both plastic and adaptive responses to climate change are poorly understood. To test for genetic and physiological trade-offs incurred in tolerating multiple stressors, we hybridized two populations of the intertidal copepod Tigriopus californicus that were divergent for both heat and salinity tolerance. Starting in the F₂ generation, we selected for increased tolerance of heat, low salinity, and high salinity in replicate lines. After five generations of selection, heat-selected lines had greater heat tolerance but lower fecundity, indicating an energetic cost to tolerance. Lines selected for increased salinity tolerance did not show evidence of adaptation to their respective environments; however, hypo-osmotic selection lines showed substantial loss of tolerance to hyperosmotic stress. Neither of the salinity selection regimes resulted in diminished heat tolerance at ambient salinity; however, simultaneous exposure to heat and hypo-osmotic stress led to decreased heat tolerance, implying a physiological trade-off in tolerance to the two stressors. When we quantified the transcriptomic response to heat and salinity stress via RNA sequencing, we observed little overlap in the stress responses, suggesting the observed synergistic effects of heat and salinity stress were driven by competing energetic demands, rather than shared stress response pathways.

Daphnia magna transcriptome by RNA-Seq across 12 environmental stressors

The full exploration of gene-environment interactions requires model organisms with well-characterized ecological interactions in their natural environment, manipulability in the laboratory and genomic tools. The waterflea Daphnia magna is an established ecological and toxicological model species, central to the food webs of freshwater lentic habitats and sentinel for water quality. Its tractability and cyclic parthenogenetic life-cycle are ideal to investigate links between genes and the environment. Capitalizing on this unique model system, the STRESSFLEA consortium generated a comprehensive RNA-Seq data set by exposing two inbred genotypes of D. magna and a recombinant cross of these genotypes to a range of environmental perturbations. Gene models were constructed from the transcriptome data and mapped onto the draft genome of D. magna using EvidentialGene. The transcriptome data generated here, together with the available draft genome sequence of D. magna and a high-density genetic map will be a key asset for future investigations in environmental genomics.

(1)H NMR-based metabolomics of Daphnia magna responses after sub-lethal exposure to triclosan, carbamazepine and ibuprofen

Pharmaceuticals and personal care products are a class of emerging contaminants that are present in wastewater effluents, surface water, and groundwater around the world. There is a need to determine rapid and reliable bioindicators of exposure and the toxic mode of action of these contaminants to aquatic organisms. (1)H nuclear magnetic resonance (NMR)-based metabolomics in combination with multivariate statistical analysis was used to determine the metabolic profile of Daphnia magna after exposure to a range of sub-lethal concentrations of triclosan (6.25-100μg/L), carbamazepine (1.75-14mg/L) and ibuprofen (1.75-14mg/L) for 48h. Sub-lethal triclosan exposure suggested a general oxidative stress condition and the branched-chain amino acids, glutamine, glutamate, and methionine emerged as potential bioindicators. The aromatic amino acids, serine, glycine and alanine are potential bioindicators for sub-lethal carbamazepine exposure that may have altered energy metabolism. The potential bioindicators for sub-lethal ibuprofen exposure are serine, methionine, lysine, arginine and leucine, which showed a concentration-dependent response. The differences in the metabolic changes were related to the dissimilar modes of toxicity of triclosan, carbamazepine and ibuprofen. (1)H NMR-based metabolomics gave an improved understanding of how these emerging contaminants impact the keystone species D. magna.

Metabolomics reveals energetic impairments in Daphnia magna exposed to diazinon, malathion and bisphenol-A

(1)H nuclear magnetic resonance (NMR)-based metabolomics was used to study the response of Daphnia magna to increasing sub-lethal concentrations of either an organophosphate (diazinon or malathion) or bisphenol-A (BPA). Principal component analysis (PCA) of (1)H NMR spectra were used to screen metabolome changes after 48h of contaminant exposure. The PCA scores plots showed that diazinon exposures resulted in aberrant metabolomic profiles at all exposure concentrations tested (0.009-0.135 μg/L), while for malathion the second lowest (0.08μg/L) and two highest exposure concentrations (0.32μg/L and 0.47μg/L) caused significant shifts from the control. Individual metabolite changes for both organophosphates indicated that the response to increasing exposure was non-linear and described perturbations in the metabolome that were characteristic of the severity of exposure. For example, intermediate concentrations of diazinon (0.045μg/L and 0.09μg/L) and malathion (0.08μg/L) elicited a decrease in amino acids such as leucine, valine, arginine, glycine, lysine, glutamate, glutamine, phenylalanine and tyrosine, with concurrent increases in glucose and lactate, suggesting a mobilization of energy resources to combat stress. At the highest exposure concentrations for both organophosphates there was evidence of a cessation in metabolic activity, where the same amino acids increased and glucose and lactate decreased, suggesting a slowdown in protein synthesis and depletion of energy stocks. This demonstrated a similar response in the metabolome between two organophosphates but also that intermediate and severe stress levels could be differentiated by changes in the metabolome. For BPA exposures, the PCA scores plot showed a significant change in metabolome at 0.1mg/L, 1.4mg/L and 2.1mg/L of exposure. Individual metabolite changes from 0.7 to 2.1mg/L of BPA exposure showed increases in amino acids such as alanine, valine, isoleucine, leucine, arginine, phenylalanine and tyrosine. These metabolite changes were correlated with decreases in glucose and lactate. This pattern of response was also seen in the highest organophosphate exposures and suggested a generalized stress response that could be related to altered energy dynamics in D. magna. Through studying increasing exposure responses, we have demonstrated the ability of metabolomics to identify discrete differences between intermediate and severe stress, and also to characterize how systemic stress is manifested in the metabolome.

Quantified biotic and abiotic responses to multiple stress in freshwater, marine and ground waters

We reviewed 219 papers and built an inventory of 532 items of ecological evidence on multiple stressor impacts in rivers, lakes, transitional and coastal waters, as well as groundwaters. Our review revealed that, despite the existence of a huge conceptual knowledge base in aquatic ecology, few studies actually provide quantitative evidence on multi-stress effects. Nutrient stress was involved in 71% to 98% of multi-stress situations in the three types of surface water environments, and in 42% of those in groundwaters. However, their impact manifested differently along the groundwater-river-lake-transitional-coastal continuum, mainly determined by the different hydro-morphological features of these ecosystems. The reviewed papers addressed two-stressor combinations most frequently (42%), corresponding with the actual status-quo of pressures acting on European surface waters as reported by the Member States in the WISE WFD Database (EEA, 2015). Across all biological groups analysed, higher explanatory power of the stress-effect models was discernible for lakes under multi-stressor compared to single stressor conditions, but generally lower for coastal and transitional waters. Across all aquatic environments, the explanatory power of stress-effect models for fish increased when multi-stressor conditions were taken into account in the analysis, qualifying this organism group as a useful indicator of multi-stress effects. In contrast, the explanatory power of models using benthic flora decreased under conditions of multiple stress.

Impact of Predator Cues on Responses to Silver Nanoparticles in Daphnia carinata

The past decades have witnessed a boom in nanotechnology that has led to increasing production and application of silver nanoparticles (AgNPs) in the textile industry due to their antimicrobial properties. Increase in the manufacture and use of NPs inevitably has resulted in their increased release into aquatic environments resulting in the exposure of organisms living in these environments. Recently, the risk of exposure to NPs and the potential interaction with biological systems has received increasing attention. The present study investigated the potential effects of predator cues on the toxicity of environmentally relevant concentrations of AgNPs in Daphnia carinata at organismal and biochemical levels. The results of this study show that exposure to environmentally relevant concentrations of AgNPs can result in adverse effects on daphnids with 24- and 48-h LC50 values of 3.56 and 1.75 μg/L, respectively. Furthermore, significant inhibition of reproduction was observed at concentrations as low as 0.5 μg/L. Exposure to predator cues alone resulted in an increase in reproduction and inhibition of superoxide dismutase activity in daphnids. However, coexposure to predator cues interacted in an antagonistic manner with AgNPs with a 24-h LC50 value of 10.81 μg/L compared with 3.56 μg/L for AgNPs alone. In summary, AgNPs could pose risks to aquatic invertebrates at environmentally relevant concentrations. Interestingly, the presence of other factors, such as predator cues, moderated the effects of exposure to AgNPs. Therefore, there is a need to further investigate the potential interactions between NPs and biological factors that can modulate toxicity of NPs for application to the risk assessment of aquatic invertebrates.

Development of an NMR microprobe procedure for high-throughput environmental metabolomics of Daphnia magna

Nuclear magnetic resonance (NMR) is the primary platform used in high-throughput environmental metabolomics studies because its non-selectivity is well suited for non-targeted approaches. However, standard NMR probes may limit the use of NMR-based metabolomics for tiny organisms because of the sample volumes required for routine metabolic profiling. Because of this, keystone ecological species, such as the water flea Daphnia magna, are not commonly studied because of the analytical challenges associated with NMR-based approaches. Here, the use of a 1.7-mm NMR microprobe in analyzing tissue extracts from D. magna is tested. Three different extraction procedures (D2O-based buffer, Bligh and Dyer, and acetonitrile : methanol : water) were compared in terms of the yields and breadth of polar metabolites. The D2O buffer extraction yielded the most metabolites and resulted in the best reproducibility. Varying amounts of D. magna dry mass were extracted to optimize metabolite isolation from D. magna tissues. A ratio of 1-1.5-mg dry mass to 40 µl of extraction solvent provided excellent signal-to-noise and spectral resolution using (1)H NMR. The metabolite profile of a single daphnid was also investigated (approximately 0.2 mg). However, the signal-to-noise of the (1)H NMR was considerably lower, and while feasible for select applications would likely not be appropriate for high-throughput NMR-based metabolomics. Two-dimensional NMR experiments on D. magna extracts were also performed using the 1.7-mm NMR probe to confirm (1)H NMR metabolite assignments. This study provides an NMR-based analytical framework for future metabolomics studies that use D. magna in ecological and ecotoxicity studies.

Linking Insects with Crustacea: Physiology of the Pancrustacea: An Introduction to the Symposium

Insects and crustaceans represent critical, dominant animal groups (by biomass and species number) in terrestrial and aquatic systems, respectively. Insects (hexapods) and crustaceans are historically grouped under separate taxonomic classes within the Phylum Arthropoda, and the research communities studying hexapods and crustaceans are quite distinct. More recently, the hexapods have been shown to be evolutionarily derived from basal crustaceans, and the clade Pancrustacea recognizes this relationship. This recent evolutionary perspective, and the fact that the Society for Integrative and Comparative Biology has strong communities in both invertebrate biology and insect physiology, provides the motivation for this symposium. Speakers in this symposium were selected because of their expertise in a particular field of insect or crustacean physiology, and paired in such a way as to provide a comparative view of the state of the current research in their respective fields. Presenters discussed what aspects of the physiological system are clearly conserved across insects and crustaceans and how cross-talk between researchers utilizing insects and crustaceans can fertilize understanding of such conserved systems. Speakers were also asked to identify strategies that would enable improved understanding of the evolution of physiological systems of the terrestrial insects from the aquatic crustaceans. The following collection of articles describes multiple recent advances in our understanding of Pancrustacean physiology.

Transcriptional profiling of predator-induced phenotypic plasticity in Daphnia pulex

Background

Predator-induced defences are a prominent example of phenotypic plasticity found from single-celled organisms to vertebrates. The water flea Daphnia pulex is a very convenient ecological genomic model for studying predator-induced defences as it exhibits substantial morphological changes under predation risk. Most importantly, however, genetically identical clones can be transcriptionally profiled under both control and predation risk conditions and be compared due to the availability of the sequenced reference genome. Earlier gene expression analyses of candidate genes as well as a tiled genomic microarray expression experiment have provided insights into some genes involved in predator-induced phenotypic plasticity. Here we performed the first RNA-Seq analysis to identify genes that were differentially expressed in defended vs. undefended D. pulex specimens in order to explore the genetic mechanisms underlying predator-induced defences at a qualitatively novel level.

Results

We report 230 differentially expressed genes (158 up- and 72 down-regulated) identified in at least two of three different assembly approaches. Several of the differentially regulated genes belong to families of paralogous genes. The most prominent classes amongst the up-regulated genes include cuticle genes, zinc-metalloproteinases and vitellogenin genes. Furthermore, several genes from this group code for proteins recruited in chromatin-reorganization or regulation of the cell cycle (cyclins). Down-regulated gene classes include C-type lectins, proteins involved in lipogenesis, and other families, some of which encode proteins with no known molecular function.

Conclusions

The RNA-Seq transcriptome data presented in this study provide important insights into gene regulatory patterns underlying predator-induced defences. In particular, we characterized different effector genes and gene families found to be regulated in Daphnia in response to the presence of an invertebrate predator. These effector genes are mostly in agreement with expectations based on observed phenotypic changes including morphological alterations, i.e., expression of proteins involved in formation of protective structures and in cuticle strengthening, as well as proteins required for resource re-allocation. Our findings identify key genetic pathways associated with anti-predator defences.

Electronic supplementary material

The online version of this article (doi:10.1186/s12983-015-0109-x) contains supplementary material, which is available to authorized users.

Ecotoxicogenomic approaches for understanding molecular mechanisms of environmental chemical toxicity using aquatic invertebrate, Daphnia model organism

Due to the rapid advent in genomics technologies and attention to ecological risk assessment, the term “ecotoxicogenomics” has recently emerged to describe integration of omics studies (i.e., transcriptomics, proteomics, metabolomics, and epigenomics) into ecotoxicological fields. Ecotoxicogenomics is defined as study of an entire set of genes or proteins expression in ecological organisms to provide insight on environmental toxicity, offering benefit in ecological risk assessment. Indeed, Daphnia is a model species to study aquatic environmental toxicity designated in the Organization for Economic Co-operation and Development’s toxicity test guideline and to investigate expression patterns using ecotoxicology-oriented genomics tools. Our main purpose is to demonstrate the potential utility of gene expression profiling in ecotoxicology by identifying novel biomarkers and relevant modes of toxicity in Daphnia magna. These approaches enable us to address adverse phenotypic outcomes linked to particular gene function(s) and mechanistic understanding of aquatic ecotoxicology as well as exploration of useful biomarkers. Furthermore, key challenges that currently face aquatic ecotoxicology (e.g., predicting toxicant responses among a broad spectrum of phytogenetic groups, predicting impact of temporal exposure on toxicant responses) necessitate the parallel use of other model organisms, both aquatic and terrestrial. By investigating gene expression profiling in an environmentally important organism, this provides viable support for the utility of ecotoxicogenomics.

Synergistic interactions of biotic and abiotic environmental stressors on gene expression

Understanding the response of organisms to multiple stressors is critical for predicting if populations can adapt to rapid environmental change. Natural and anthropogenic stressors often interact, complicating general predictions. In this study, we examined the interactive and cumulative effects of two common environmental stressors, lowered calcium concentration, an anthropogenic stressor, and predator presence, a natural stressor, on the water flea Daphnia pulex. We analyzed expression changes of five genes involved in calcium homeostasis — cuticle proteins (Cutie, Icp2), calbindin (Calb), and calcium pump and channel (Serca and Ip3R) — using real-time quantitative PCR (RT-qPCR) in a full factorial experiment. We observed strong synergistic interactions between low calcium concentration and predator presence. While the Ip3R gene was not affected by the stressors, the other four genes were affected in their transcriptional levels by the combination of the stressors. Transcriptional patterns of genes that code for cuticle proteins (Cutie and Icp2) and a sarcoplasmic calcium pump (Serca) only responded to the combination of stressors, changing their relative expression levels in a synergistic response, while a calcium-binding protein (Calb) responded to low calcium stress and the combination of both stressors. The expression pattern of these genes (Cutie, Icp2, and Serca) were nonlinear, yet they were dose dependent across the calcium gradient. Multiple stressors can have complex, often unexpected effects on ecosystems. This study demonstrates that the dominant interaction for the set of tested genes appears to be synergism. We argue that gene expression patterns can be used to understand and predict the type of interaction expected when organisms are exposed simultaneously to natural and anthropogenic stressors.

Distinct expression profiles of stress defense and DNA repair genes in Daphnia pulex exposed to cadmium, zinc, and quantum dots

The ever-increasing production and use of nanocrystaline semiconductors (Quantum dots; QDs) will inevitably result in increased appearance of these nanomaterials in the aquatic environment. However, the behavior and potential toxicity of heavy metal constituted nanoparticulates in aquatic invertebrates is largely unknown, especially with regard to molecular responses. The freshwater crustacean Daphnia pulex is a well-suited toxicological and ecological model to study molecular responses to environmental stressors. In this study, D. pulex were exposed for 48 h to sublethal doses of QDs (25% and 50% of LC50) with differing spectral properties (CdTe and CdSe/ZnS QDs) and Cd and Zn salts. Our data suggest that acute exposure to both CdSO4 and Cd-based QDs leads to Cd uptake in vivo, which was biologically supported by the observation of increased expression of metallothionein (MT-1). Furthermore, Cd, Zn, and CdSe/ZnS QDs induced different patterns of gene expression regarding stress defense and DNA repair, which furthers our knowledge regarding which response pathways are affected by nanoparticulate forms of metals versus ionic forms in aquatic crustaceans.

Effects of cadmium exposure on the gill proteome of Cottus gobio: modulatory effects of prior thermal acclimation

Temperature and trace metals are common environmental stressors, and their importance is increasing due to global climate change and anthropogenic pollution. The aim of the present study was to investigate whether acclimation to elevated temperature affects the response of the European bullhead (Cottus gobio) to subsequent cadmium (Cd) exposure by using enzymatic and proteomic approaches. Fish acclimated to 15 (standard temperature), 18 or 21 °C for 28 days were exposed to 1mg Cd/L for 4 days at the respective acclimation temperature. First, exposure to Cd significantly decreased the activity of the lactate dehydrogenase (LDH) in gills of fish acclimated to 15 or 18 °C. However, an acclimation to 21 °C suppressed the inhibitory effect of Cd. Second, using a proteomic analysis by 2D-DIGE, we observed that thermal acclimation was the first parameter affecting the protein expression profile in gills of C. gobio, while subsequent Cd exposure seemed to attenuate this temperature effect. Moreover, our results showed opposite effects of these two environmental stressors at protein expression level. From the 52 protein spots displaying significant interaction effects of temperature and Cd exposure, a total of 28 different proteins were identified using nano LC-MS/MS and the Peptide and Protein Prophet algorithms of Scaffold software. The identified differentially expressed proteins can be categorized into diverse functional classes, related to protein turnover, folding and chaperoning, metabolic process, ion transport, cell signaling and cytoskeleton. Within a same functional class, we further reported that several proteins displayed reverse responses following sequential exposure to heat and Cd. This work provides insights into the molecular pathways potentially involved in heat acclimation process and the interactive effects of temperature and Cd stress in ectothermic vertebrates.

Sucralose induces biochemical responses in Daphnia magna

The intense artificial sweetener sucralose has no bioconcentration properties, and no adverse acute toxic effects have been observed in standard ecotoxicity tests, suggesting negligible environmental risk. However, significant feeding and behavioural alterations have been reported in non-standard tests using aquatic crustaceans, indicating possible sublethal effects. We hypothesized that these effects are related to alterations in acetylcholinesterase (AChE) and oxidative status in the exposed animals and investigated changes in AChE and oxidative biomarkers (oxygen radical absorbing capacity, ORAC, and lipid peroxidation, TBARS) in the crustacean Daphnia magna exposed to sucralose (0.0001-5 mg L(-1)). The sucralose concentration was a significant positive predictor for ORAC, TBARS and AChE in the daphnids. Moreover, the AChE response was linked to both oxidative biomarkers, with positive and negative relationships for TBARS and ORAC, respectively. These joint responses support our hypothesis and suggest that exposure to sucralose may induce neurological and oxidative mechanisms with potentially important consequences for animal behaviour and physiology.

Copper alters the effect of temperature on mitochondrial bioenergetics in rainbow trout, Oncorhynchus mykiss

We investigated the interaction of temperature and copper (Cu) on mitochondrial bioenergetics to gain insight into how temperature fluctuations imposed by natural phenomena or anthropogenic activities would modulate the effects of Cu on cellular energy homeostasis. Mitochondria were isolated from rainbow trout livers and, in the first set of experiments, exposed to Cu (0-2.5 mM) at 5, 11, and 25 °C with measurement of mitochondrial complex II (mtCII)-driven respiration. In the second set of experiments, unenergized mitochondria were incubated for 30 or 60 min with lower concentrations (0-160 μM) of Cu to measure the effects on mtCII enzyme activity. Whereas maximal (state 3) respiration was inhibited by high Cu exposure, low Cu doses stimulated and high Cu doses inhibited resting (state 4) and 4ol (proton leak) respirations. High temperature alone increased mitochondrial respiration in all states. The Q10 values for state 3, state 4, and proton leak respirations suggested active processes with state 4 respiration and proton leak exhibiting greater thermal sensitivity than state 3 respiration. The differential thermal sensitivity of resting relative to phosphorylating mitochondrial state led to uncoupling and limitation of mitochondrial oxidative capacity at both high temperature and doses of Cu. Moreover, exposure to high Cu caused loss of thermal dependence of the mitochondrial bioenergetics culminating in Q10 values well below unity and decreased activation energies (E a) for both maximal and resting respiration rates. In addition, mtCII activity was increased by low and decreased by high doses of Cu indicating that direct effects on this enzyme contribute to Cu-induced mitochondrial dysfunction. Taken together, it appears that the substrate oxidation (electron transport chain and tricarboxylic acid cycle) and proton leak subsystems are targets of the deleterious effects of Cu and increased temperature on mitochondrial bioenergetics. However, mitochondrial effects of Cu and temperature may not be easily distinguished because they are generally qualitatively similar.

The interactive effects of multiple stressors on physiological stress responses and club cell investment in fathead minnows

Anthropogenic activities have dramatically increased over the past decades, with the consequence that many organisms are simultaneously exposed to multiple stressors. Understanding how organisms respond to these stressors is a key focus for scientists from many disciplines. Here we investigated the interactive effects of two stressors, UV radiation (UVR) and cadmium (Cd) exposure on a common freshwater fish, fathead minnow (Pimephales promelas). UVR is known to influence the density of epidermal club cells (ECCs), which are not only a key component of the innate immune system of fishes, but are also the source of chemical alarm cues that serve to warn other fishes of nearby predators. In contrast, Cd impairs the physiological stress response and ability of fish to respond to alarm cues. We used an integrative approach to examine physiological stress response as well as investment in ECCs. Fish exposed to UVR had higher levels of cortisol than non-exposed controls, but Cd reduced cortisol levels substantially for fish exposed to UVR. Fish exposed to UVR, either in the presence or absence of Cd, showed consistent decreases in ECC investment compared to non-exposed controls. Despite differences in ECC number, there was no difference in the potency of alarm cues prepared from the skin of UVR and Cd exposed or non-exposed fish indicating that UVR and Cd exposure combined may have little influence on chemically-mediated predator-prey interactions.

An investigation of the inter-clonal variation of the interactive effects of cadmium and Microcystis aeruginosa on the reproductive performance of Daphnia magna

Interactive effects between chemical and natural stressors as well as genetically determined variation in stress tolerance among individuals may complicate risk assessment and management of chemical pollutants in natural ecosystems. Although genetic variation in tolerance to single stressors has been described extensively, genetic variation in interactive effects between two stressors has only rarely been investigated. Here, we examined the interactive effects between a chemical stressor (Cd) and a natural stressor (the cyanobacteria Microcystis aeruginosa) on the reproduction of Daphnia magna in 20 genetically different clones using a full-factorial experimental design and with the independent action model of joint stressor action as the reference theoretical framework. Across all clones, the reduction of 21-day reproduction compared to the control treatment (no Cd, no M. aeruginosa) ranged from -10% to 98% following Cd exposure alone, from 44% to 89% for Microcystis exposure alone, and from 61% to 98% after exposure to Cd+Microcystis combined. Three-way ANOVA on log-transformed reproduction data of all clones together did not detect a statistically significant Cd×Microcystis interaction term (F-test, p=0.11), meaning that on average both stressors do not interact in inhibiting reproductive performance of D. magna. This finding contrasted expectations based on some known shared mechanisms of toxicity of Cd and Microcystis and therefore cautions against making predictions of interactive chemical+natural stressor effects from incomplete knowledge on affected biological processes and pathways. Further, still based on three-way ANOVA, we did not find statistically significant clone×Cd×Microcystis interaction when data for all clones were analyzed together (F-test, p=0.07), suggesting no inter-clonal variation of the interactive effect between Cd and Microcystis. However, when the same data were quantitatively analyzed on a clone-by-clone scale, we found a relatively wide range of deviations between observed and IA-model-predicted reproduction in combined Cd+Microcystis treatments (both in direction and magnitude), suggesting some biological significance of inter-clonal variation of interactive effects. In one of the twenty clones this deviation was statistically significant (two-way ANOVA, F-test, p=0.005), indicating an interactive Cd×Microcystis effect in this clone. Together, these two observations caution against the extrapolation of conclusions about mixed stressor data obtained with single clones to the level of the entire species and to the level of natural, genetically diverse populations.

(1)H NMR-based metabolomics investigation of Daphnia magna responses to sub-lethal exposure to arsenic, copper and lithium

Metal and metalloid contamination constitutes a major concern in aquatic ecosystems. Thus it is important to find rapid and reliable indicators of metal stress to aquatic organisms. In this study, we tested the use of (1)H nuclear magnetic resonance (NMR) - based metabolomics to examine the response of Daphnia magna neonates after a 48h exposure to sub-lethal concentrations of arsenic (49μgL(-1)), copper (12.4μgL(-1)) or lithium (1150μgL(-1)). Metabolomic responses for all conditions were compared to a control using principal component analysis (PCA) and metabolites that contributed to the variation between the exposures and the control condition were identified and quantified. The PCA showed that copper and lithium exposures result in statistically significant metabolite variations from the control. Contributing to this variation was a number of amino acids such as: phenylalanine, leucine, lysine, glutamine, glycine, alanine, methionine and glutamine as well as the nucleobase uracil and osmolyte glycerophosphocholine. The similarities in metabolome changes suggest that lithium has an analogous mode of toxicity to that of copper, and may be impairing energy production and ionoregulation. The PCA also showed that arsenic exposure resulted in a metabolic shift in comparison to the control population but this change was not statistically significant. However, significant changes in specific metabolites such as alanine and lysine were observed, suggesting that energy metabolism is indeed disrupted. This research demonstrates that (1)H NMR-based metabolomics is a viable platform for discerning metabolomic changes and mode of toxicity of D. magna in response to metal stressors in the environment.

Role of molting on the biodistribution of CeO2 nanoparticles within Daphnia pulex

As all arthropods, microcrustaceans shed their chitinous exoskeleton (cuticule, peritrophic membrane) to develop and grow. While the molting is the most crucial stage in their life cycle, it remains poorly investigated in term of pollutant biodistribution within the organisms. In this paper, we used optical, electronic, and X ray-based microscopies to study the uptake and release of CeO2 nanoparticles by/from Daphnia pulex over a molting stage. We measured that D. pulex molts every 59 ± 21 h (confidence interval) with growth rates about 1.1 or 1.8 μm per stage as a function of the pieces measured. Ingestion via food chain was the main route of CeO2 nanoparticles uptake by D. pulex. The presence of algae during the exposure to nanoparticles (sub-lethal doses) enhanced by a factor of 3 the dry weight concentration of Ce on the whole D. pulex. Nanoparticles were localized in the gut content, in direct contact with the peritrophic membrane, and on the cuticle. Interestingly, the depuration (24 h with Chlorella pseudomonas) was not efficient to remove the nanoparticles from the organisms. From 40% to 100% (depending on the feeding regime during exposure) of the CeO2 taken up by D. pulex is not release after the depuration process. However, we demonstrated for the first time that the shedding of the chitinous exoskeleton was the crucial mechanism governing the released of CeO2 nanoparticles regardless of the feeding regime during exposure.