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Nearly all animal species have utilized photoperiod to cue seasonal behaviours and life history traits. We investigated photoperiod responses in keystone species, Daphnia magna, to identify molecular processes underlying ecologically important behaviours and traits using functional transcriptomic analyses. Daphnia magna were photoperiod‐entrained immediately posthatch to a standard control photoperiod of 16 light/ 8 dark hours (16L:8D) relative to shorter (4L:20D, 8L:16D, 12L:12L) and longer (20L:4D) day length photoperiods. Short‐day photoperiods induced significantly increased light‐avoidance behaviours relative to controls. Correspondingly, significant differential transcript expression for genes involved in glutamate signalling was observed, a critical signalling pathway in arthropod light‐avoidance behaviour. Additionally, period circadian protein and proteins coding F‐box/LRR‐repeat domains were differentially expressed which are recognized to establish circadian rhythms in arthropods. Indicators of metabolic rate increased in short‐day photoperiods which corresponded with broadscale changes in transcriptional expression across system‐level energy metabolism pathways. The most striking observations included significantly decreased neonate production at the shortest day length photoperiod (4L:20D) and significantly increased male production across short‐day and equinox photoperiods (4L:20D, 8L:16D and 12L:12D). Transcriptional expression consistent with putative mechanisms of male production was observed including photoperiod‐dependent expression of transformer‐2 sex‐determining protein and small nuclear ribonucleoprotein particles (snRNPs) which control splice variant expression for genes like transformer. Finally, increased transcriptional expression of glutamate has also been shown to induce male production in Daphnia pulex via photoperiod‐sensitive mechanisms. Overall, photoperiod entrainment affected molecular pathways that underpin critical behavioural and life history traits in D. magna providing fundamental insights into biological responses to this primary environmental cue.

Crude oil released from the Deepwater Horizon disaster into the Gulf of Mexico posed potential impacts to infaunal invertebrates inhabiting near shore habitats. The effects of sediment-associated weathered slick oil on the amphipod Leptocheirus plumulosus was assessed using 28-d exposures to total PAH sediment concentrations ranging from 0.3 to 24 mg/kg (sum of 50 PAHs or tPAH50). Survival and growth rate were significantly decreased in the 2.6, 11.4 and 24.2 mg/kg treatments, but only growth in 5.5 mg/kg. Offspring production was dramatically decreased but was variable and significantly different only for 24.2 mg/kg. The concentrations associated with 20% decreases relative to reference were 1.05 (95% CI = 0–2.89) mg/kg tPAH50 for growth rate and 0.632 (95% CI = 0.11–2.15) mg/kg tPAH50 for offspring production. The concentrations of PAHs affecting amphipods are within the range of concentrations measured in marsh areas reportedly impacted by DWH oil after its release.

This work investigates whether the scale-up to multi-animal exposures that is commonly applied in genomics studies provides equivalent toxicity outcomes to single-animal experiments of standard Daphnia magna toxicity assays. Specifically, we tested the null hypothesis that intraspecific interactions (ISI) among D. magna have neither effect on the life history strategies of this species, nor impact toxicological outcomes in exposure experiments with Cu and Pb. The results show that ISI significantly increased mortality of D. magna in both Cu and Pb exposure experiments, decreasing 14 day LC50 s and 95 % confidence intervals from 14.5 (10.9–148.3) to 8.4 (8.2–8.7) µg Cu/L and from 232 (156–4810) to 68 (63–73) µg Pb/L. Additionally, ISI potentiated Pb impacts on reproduction eliciting a nearly 10-fold decrease in the no-observed effect concentration (from 236 to 25 µg/L). As an indication of environmental relevance, the effects of ISI on both mortality and reproduction in Pb exposures were sustained at both high and low food rations. Furthermore, even with a single pair of Daphnia, ISI significantly increased (p

As integration of nanoparticles (NPs) into products becomes more common, the need to address the paucity of chronic hazard information for aquatic environments required to determine risk potential increases. This study generated acute and chronic toxicity reference values for Ceriodaphnia dubia exposed to 20 and 100 nm silver nanoparticles (AgNPs) and gold nanoparticles (AuNPs) to generate and evaluate potential differences in acute-to-chronic ratios (ACR) using two different feeding methods. A modified feeding procedure was employed alongside the standard procedures to investigate the influence of food on organism exposure. An 8-h period before food was added allowed direct organism exposure to NP dispersions (and associated ions) without food-to-NP interactions. The AgNPs [chronic lethal median concentrations (LC50) between 18.7 and 31.9 µg/L] were substantially more toxic than AuNPs (LC50 = 21 507 to >26 384 µg/L). The modified chronic testing method resulted in greater sensitivity in AgNPs exposures. However, the modified feeding ration had less of an effect in exposures to the larger (100 nm) AgNPs compared to smaller particles (20 nm). The ACRs for AgNPs using the standard feeding ration were 1.6 and 3.5 for 20 nm and 100 nm, respectively. The ACRs for AgNPs using the modified feeding ration were 3.4 and 7.6 for 20 nm and 100 nm NPs, respectively. This supports that the addition of the standard feeding ration decreases C. dubia chronic sensitivity to AgNPs, although it must also be recognized organisms may be sensitized due to less access to food. The ACRs for 20 nm and 100 nm AuNPs (standard ration only) were 4.0 and 3.0, respectively. It is important to also consider that dissolved Ag+ ions are more toxic than AgNPs, based on both acute toxicity values in the cited literature and chronic toxicity thresholds generated in this study that support existing thresholds that Ag+ are likely protective of AgNPs effects.

Nanoparticles are of concern because of widespread use, but it is unclear if metal nanoparticles cause effects directly or indirectly. We explored whether polyvinylpyrrolidone-coated silver nanoparticles (PVP-AgNPs) cause effects through intact nanoparticles or dissolved silver. Females of the model species fathead minnow (Pimephales promelas) were exposed to either 4.8 μg/L of AgNO3 or 61.4 μg/L of PVP-AgNPs for 96h. Microarray analyses were used to identify impacted receptors and toxicity pathways in liver and brain tissues that were confirmed using in vitro mammalian assays. AgNO3 and PVP-AgNP exposed fish had common and distinct effects consistent with both intact nanoparticles and dissolved silver causing effects. PVP-AgNPs and AgNO3 both affected pathways involved in Na(+), K(+), and H(+) homeostasis and oxidative stress but different neurotoxicity pathways. In vivo effects were supported by PVP-AgNP activation of five in vitro nuclear receptor assays and inhibition of ligand binding to the dopamine receptor. AgNO3 inhibited ligand binding to adrenergic receptors α1 and α2 and cannabinoid receptor CB1, but had no effect in nuclear receptor assays. PVP-AgNPs have the potential to cause effects both through intact nanoparticles and metal ions, each interacting with different initiating events. Since the in vitro and in vivo assays examined here are commonly used in human and ecological hazard screening, this work suggests that environmental health assessments should consider effects of intact nanoparticles in addition to dissolved metals.

Studies investigating the impact of particle size and capping agents on nanosilver toxicity in pristine laboratory conditions are becoming available. However, the relative importance of known environmental mitigating factors for dissolved silver remains poorly characterized for nanosilver in context with existing predictive toxicity models. This study investigated the implications of freshly prepared versus stored 20 and 100 nm nanosilver stocks to freshwater zooplankton (Ceriodaphnia dubia) in presence and absence of dissolved organic carbon (DOC). Results indicated that while the acute toxicity of nanosilver decreased significantly with larger size and higher DOC, storage resulted in significant increases in toxicity and ion release. The most dramatic decrease in toxicity due to DOC was observed for the 20 nm particle (2.5-6.7 fold decrease), with more modest toxicity reductions observed for the 100 nm particle (2.0-2.4 fold) and dissolved silver (2.7-3.1 fold). While a surface area dosimetry presented an improvement over mass when DOC was absent, the presence of DOC confounded its efficacy. The fraction of dissolved silver in the nanosilver suspensions was most predictive of acute toxicity regardless of system complexity. Biotic Ligand Model (BLM) predictions based on the dissolved fraction in nanosilver suspensions were comparable to observed toxicity.

Mass concentration is the standard convention to express exposure in ecotoxicology for dissolved substances. However, nanotoxicology has challenged the suitability of the mass concentration dose metric. Alternative metrics often discussed in the literature include particle number, surface area, and ion release (kinetics, equilibrium). It is unlikely that any single metric is universally applicable to all types of nanoparticles. However, determining the optimal metric for a specific type of nanoparticle requires novel studies to generate supportive data and employ methods to compensate for current analytical capability gaps. This investigation generated acute toxicity data for two standard species (Ceriodaphnia dubia, Pimephales promelas) exposed to five sizes (10, 20, 30, 60, 100 nm) of monodispersed citrate- and polyvinylpyrrolidone-coated silver nanoparticles. Particles were sized by various techniques to populate available models for expressing the particle number, surface area, and dissolved fraction. Results indicate that the acute toxicity of the tested silver nanoparticles is best expressed by ion release, and is relatable to total exposed surface area. Particle number was not relatable to the observed acute silver nanoparticle effects.

Carbon nanotubes were previously demonstrated to accumulate on the carapace and in the gut of daphnids in aquatic exposures. The purpose of the present study was to assess the effects of multiwalled carbon nanotube (MWCNT) exposure on the sublethal Daphnia magna endpoints swimming behavior, algal feeding, growth, and reproduction and to determine the relative magnitude of difference between lethal and sublethal toxicity thresholds in 48-h and 14-d exposures. A stable dispersion of MWCNTs was prepared using 100 mg/L natural organic matter (NOM), and all treatments were compared statistically to a NOM control. The swimming behavior endpoints of mean velocity and total distance moved were determined using digital tracking software. For the acute (48-h) exposure, a 50% lethal concentration (LC50) of 29.3 (23.6-36.3) mg/L and a 50% effective concentration (EC50) of 6.7 mg/L in the swimming velocity endpoint were determined. When swimming response was nonmonotonic below 2 mg/L, consistent reductions in velocity were observed at 6.9 mg/L and above. Median effect concentrations were lower in the chronic (14-d) bioassay. The 14-d LC50 was 4.3 mg/L (3.3-5.6 mg/L), and the reproduction EC50 was 5.0 mg/L. Lowest-observed-effect concentrations for survival and reproduction were 5.4 mg/L and 1.7 mg/L, respectively. Significantly fewer (23.1%) algal cells were consumed in the 3.9-mg/L treatment relative to the control. No significant effects on swimming behavior were observed for the 14-d bioassay. Less traditional sublethal endpoints such as swimming behavior and feeding rate may be especially important to assess for MWCNTs and other materials expected to be more physically than chemically toxic through mechanisms such as gut clogging.

Insensitive munitions offer increased safety because of their "insensitivity" to unintended detonation relative to historically used formulations such as 2,4,6-trinitrotoluene (TNT). Dinitroanisole (DNAN) is an insensitive munition constituent, and its solubility and stability warrant investigations of potential toxicological hazard related to manufacturing discharges and training ranges. Although ecotoxicology data are available for other insensitive munition constituents, few data are available for DNAN. In the present study, acute and chronic exposures of a fish (Pimephales promelas) and 2 cladocerans (Ceriodaphnia dubia, Daphnia pulex) were conducted. The 50% lethal concentration (LC50) values of DNAN ranged from 14.2 mg/L to 42.0 mg/L, depending on species. In chronic exposures, fish survival (LC50 = 10.0 mg/L) was more sensitive than cladoceran survival (LC50 = 13.7 to24.2 mg/L). However, cladoceran reproduction was equally or more sensitive to DNAN (50% inhibition values 2.7-10.6 mg/L, depending on species) than fish endpoints. Daphnia pulex was the most sensitive species, with only slight differences between the 3 populations tested. Although the aquatic toxicity of DNAN was lower than previously reported in the literature for TNT, future research is needed to determine the potential synergistic toxicity of all the constituents in insensitive munition mixtures and the implications of photo-oxidation.

As the production and applications of silver nanoparticles (AgNPs) increase, it is essential to characterize fate and effects in environmental systems. Nanosilver materials may settle from suspension; therefore, the authors' objective was to utilize environmentally relevant bioassays and study the impact, bioaccumulation, tissue distribution, uptake, and depuration of AgNPs on a sediment-dwelling invertebrate, Lumbriculus variegatus. Hydrodynamic diameters of uncoated 30-nm, 80-nm, and 1500-nm AgNP powders and a polyvinyl pyrrolidone (PVP) AgNP suspension were measured utilizing dynamic light scattering in freshwater media (0-280 µS/cm). Aggregation for 30 nm, 80 nm, and 1500 nm silver increased with conductivity but was minimal for PVP silver. Lumbriculus variegatus were exposed to AgNPs or silver nitrate (AgNO3 ) spiked into sediment (nominally 100 mg/kg) and water (PVP 30 nm and 70 nm Ag, nominally 5 mg/L). Uptake was assessed through inductively coupled plasma mass spectroscopy (ICP-MS) and hyperspectral imaging. Particle sizes were examined through field flow fractionation-ICP-MS (FFF-ICP-MS) and ICP-MS in single particle mode (SP-ICP-MS). Lumbriculus variegatus were also depurated for 6 h, 8 h, 24 h, and 48 h to determine gut clearance. Bioaccumulation factors of sediment-exposed L. variegatus were similar regardless of particle size or coatings. The FFF-ICP-MS and SP-ICP-MS detected AgNPs for up to 48 h post depuration. The present study provides information on bioaccumulation and interactions of AgNPs within biological systems.

Small organisms can be used as biomonitoring tools to assess chemicals in the environment. Chemical stressors are especially hard to assess and monitor when present as complex mixtures. Here, fifteen polymerase chain reaction assays targeting Daphnia magna genes were calibrated to responses elicited in D. magna exposed for 24 h to five different doses each of the munitions constituents 2,4,6-trinitrotoluene, 2,4-dinitrotoluene, 2,6-dinitrotoluene, trinitrobenzene, dinitrobenzene, or 1,3,5-trinitro-1,3,5-triazacyclohexane. A piecewise-linear model for log-fold expression changes in gene assays was used to predict response to munitions mixtures and contaminated groundwater under the assumption that chemical effects were additive. The correlations of model predictions with actual expression changes ranged from 0.12 to 0.78 with an average of 0.5. To better understand possible mixture effects, gene expression changes from all treatments were compared using high-density microarrays. Whereas mixtures and groundwater exposures had genes and gene functions in common with single chemical exposures, unique functions were also affected, which was consistent with the nonadditivity of chemical effects in these mixtures. These results suggest that, while gene behavior in response to chemical exposure can be partially predicted based on chemical exposure, estimation of the composition of mixtures from chemical responses is difficult without further understanding of gene behavior in mixtures. Future work will need to examine additive and nonadditive mixture effects using a much greater range of different chemical classes in order to clarify the behavior and predictability of complex mixtures.