Correlating Quantitative Measurements of Radical Production by Photocatalytic TiO2 with Daphnia magna Toxicity.

Increased use of titanium dioxide (TiO2 ) nanoparticles (NPs) in domestic and industrial applications has increased the risk for adverse environmental outcomes based on an elevated likelihood of organism exposure. Anatase TiO2 NP exposure to ultraviolet A (UV-A) radiation in aquatic environments generates radical oxygen species (ROS), which may ultimately be responsible for increased organism toxicity. We have identified and measured the 2 most relevant ROS species, hydroxyl and superoxide radicals, and described that ROS can be modeled using the highly reactive hydroxyl radical to provide an upper bound for toxicity. The TiO2 NPs were co-exposed to increasing natural organic matter (NOM) amounts (measured as concentration of dissolved organic carbon [DOC]) and simulated-sunlight UV-A intensities. Radical production rate was determined using fluorescence spectroscopy and was positively correlated with increases in TiO2 concentration and UV-A intensity, and negatively correlated with increased DOC concentration. Daphnia magna toxicity was also found to decrease with NOM addition, which is attributed to the decreased radical production rate with increased DOC concentrations. We demonstrate that the rate of ROS production from simulated-sunlight-irradiated TiO2 NPs can be quantified using relatively simple fluorescent techniques. We show that toxicity to TiO2 NP varies greatly with conditions, and that concentration alone is a poor predictor of toxicity. Describing toxicity/hydroxyl radical measurement may be a more accurate way to describe overall risk. We provide a framework for a simple model to describe toxicity/hydroxyl radical. These conclusions demonstrate the importance of considering exposure conditions as a means of risk management during TiO2 NP toxicity testing, waste management, and regulatory decisions. Environ Toxicol Chem 2021;40:1322-1334. © 2021 SETAC.

Toward Sustainable Environmental Quality: Priority Research Questions for North America.

Anticipating, identifying, and prioritizing strategic needs represent essential activities by research organizations. Decided benefits emerge when these pursuits engage globally important environment and health goals, including the United Nations Sustainable Development Goals. To this end, horizon scanning efforts can facilitate identification of specific research needs to address grand challenges. We report and discuss 40 priority research questions following engagement of scientists and engineers in North America. These timely questions identify the importance of stimulating innovation and developing new methods, tools, and concepts in environmental chemistry and toxicology to improve assessment and management of chemical contaminants and other diverse environmental stressors. Grand challenges to achieving sustainable management of the environment are becoming increasingly complex and structured by global megatrends, which collectively challenge existing sustainable environmental quality efforts. Transdisciplinary, systems-based approaches will be required to define and avoid adverse biological effects across temporal and spatial gradients. Similarly, coordinated research activities among organizations within and among countries are necessary to address the priority research needs reported here. Acquiring answers to these 40 research questions will not be trivial, but doing so promises to advance sustainable environmental quality in the 21st century. Environ Toxicol Chem 2019;38:1606-1624. © 2019 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals, Inc. on behalf of SETAC.

Bioavailability of Carbon Nanomaterial-Adsorbed Polycyclic Aromatic Hydrocarbons to Pimphales promelas: Influence of Adsorbate Molecular Size and Configuration.

Despite carbon nanomaterials' (CNMs) potential to alter the bioavailability of adsorbed contaminants, information characterizing the relationship between adsorption behavior and bioavailability of CNM-adsorbed contaminants is still limited. To investigate the influence of CNM morphology and organic contaminant (OC) physicochemical properties on this relationship, adsorption isotherms were generated for a suite of polycyclic aromatic hydrocarbons (PAHs) on multiwalled carbon nanotubes (MWCNTs) and exfoliated graphene (GN) in conjunction with determining the bioavailability of the adsorbed PAHs to Pimphales promelas using bile analysis via fluorescence spectroscopy. Although it appeared that GN adsorbed PAHs indiscriminately compared to MWCNTs, the subsequent bioavailability of GN-adsorbed PAHs was more sensitive to PAH morphology than MWCNTs. GN was effective at reducing bioavailability of linear PAHs by ∼70%, but had little impact on angular PAHs. MWCNTs were sensitive to molecular size, where bioavailability of two-ringed naphthalene was reduced by ∼80%, while bioavailability of the larger PAHs was reduced by less than 50%. Furthermore, the reduction in bioavailability of CNM-adsorbed PAHs was negatively correlated with the amount of CNM surface area covered by the adsorbed-PAHs. This study shows that the variability in bioavailability of CNM-adsorbed PAHs is largely driven by PAH size, configuration and surface area coverage.

Trophic transfer of microplastics in aquatic ecosystems: Identifying critical research needs.

To evaluate the process of trophic transfer of microplastics, it is important to consider various abiotic and biotic factors involved in their ingestion, egestion, bioaccumulation, and biomagnification. Toward this end, a review of the literature on microplastics has been conducted to identify factors influencing their uptake and absorption; their residence times in organisms and bioaccumulation; the physical effects of their aggregation in gastrointestinal tracts; and their potential to act as vectors for the transfer of other contaminants. Limited field evidence from higher trophic level organisms in a variety of habitats suggests that trophic transfer of microplastics may be a common phenomenon and occurs concurrently with direct ingestion. Critical research needs include standardizing methods of field characterization of microplastics, quantifying uptake and depuration rates in organisms at different trophic levels, quantifying the influence that microplastics have on the uptake and/or depuration of environmental contaminants among different trophic levels, and investigating the potential for biomagnification of microplastic-associated chemicals. More integrated approaches involving computational modeling are required to fully assess trophic transfer of microplastics. Integr Environ Assess Manag 2017;13:505-509. © 2017 SETAC.

Effect of natural organic matter on the photo-induced toxicity of titanium dioxide nanoparticles.

Nano-titanium dioxide (TiO2 ) is the most widely used form of nanoparticles in commercial industry and comes in 2 main configurations: rutile and anatase. Rutile TiO2 is used in ultraviolet (UV) screening applications, whereas anatase TiO2 crystals have a surface defect that makes them photoreactive. There are numerous reports in the literature of photo-induced toxicity to aquatic organisms following coexposure to anatase nano-TiO2 and UV. All natural freshwater contains varying amounts of natural organic matter (NOM), which can drive UV attenuation and quench reactive oxygen species (ROS) in aquatic ecosystems. The present research examined how NOM alters the photo-induced toxicity of anatase nano-TiO2 . Daphnia magna neonates were coexposed to NOM and photoexcited anatase nano-TiO2 for 48 h. Natural organic matter concentrations as low as 4 mg/L reduced anatase nano-TiO2 toxicity by nearly 100%. These concentrations of NOM attenuated UV by <10% in the exposure system. However, ROS production measured using a fluorescence assay was significantly reduced in a NOM concentration--dependent manner. Taken together, these data suggest that NOM reduces anatase nano-TiO2 toxicity via an ROS quenching mechanism and not by attenuation of UV. Environ Toxicol Chem 2017;36:1661-1666. © 2016 SETAC.

Nanomaterials in the aquatic environment: A European Union-United States perspective on the status of ecotoxicity testing, research priorities, and challenges ahead.

The European Union-United States Communities of Research were established in 2012 to provide a platform for scientists to develop a "shared repertoire of protocols and methods to overcome nanotechnology environmental health and safety (nanoEHS) research gaps and barriers" (www.us-eu.org/). Based on work within the Ecotoxicology Community of Research (2012-2015) the present Focus article provides an overview of the state of the art of nanomaterials (NMs) in the aquatic environment by addressing different research questions, with a focus on ecotoxicological test systems and the challenges faced when assessing NM hazards (e.g., uptake routes, bioaccumulation, toxicity, test protocols, and model organisms). The authors' recommendation is to place particular importance on studying the ecological effects of aged/weathered NMs, as-manufactured NMs, and NMs released from consumer products in addressing the following overarching research topics: 1) NM characterization and quantification in environmental and biological matrices; 2) NM transformation in the environment and consequences for bioavailability and toxicity; 3) alternative methods to assess exposure; 4) influence of exposure scenarios on bioavailability and toxicity; 5) development of more environmentally realistic bioassays; and 6) uptake, internal distribution, and depuration of NMs. Research addressing these key topics will reduce uncertainty in ecological risk assessment and support the sustainable development of nanotechnology.

Interactions of metal-based engineered nanoparticles with aquatic higher plants: A review of the state of current knowledge.

The rising potential for the release of engineered nanoparticles (ENPs) into aquatic environments requires evaluation of risks to protect ecological health. The present review examines knowledge pertaining to the interactions of metal-based ENPs with aquatic higher plants, identifies information gaps, and raises considerations for future research to advance knowledge on the subject. The discussion focuses on ENPs' bioaccessibility; uptake, adsorption, translocation, and bioaccumulation; and toxicity effects on aquatic higher plants. An information deficit surrounds the uptake of ENPs and associated dynamics, because the influence of ENP characteristics and water quality conditions has not been well documented. Dissolution appears to be a key mechanism driving bioaccumulation of ENPs, whereas nanoparticulates often adsorb to plant surfaces with minimal internalization. However, few reports document the internalization of ENPs by plants; thus, the role of nanoparticulates' internalization in bioaccumulation and toxicity remains unclear, requiring further investigation. The toxicities of metal-based ENPs mainly have been associated with dissolution as a predominant mechanism, although nano toxicity has also been reported. To advance knowledge in this domain, future investigations need to integrate the influence of ENP characteristics and water physicochemical parameters, as their interplay determines ENP bioaccessibility and influences their risk to health of aquatic higher plants. Furthermore, harmonization of test protocols is recommended for fast tracking the generation of comparable data. Environ Toxicol Chem 2016;35:1677-1694. © 2016 SETAC.

The effects of bupropion on hybrid striped bass brain chemistry and predatory behavior.

Increased use of antidepressants has led to an increase in their detection in final treated wastewater effluents and receiving streams. Antidepressants are intended to modify human behavior by altering brain chemistry, and because of the high functional conservation of antidepressant target receptors in vertebrates, aquatic organisms may be at risk. The antidepressant bupropion is designed to alter brain norepinephrine and dopamine concentrations in humans. The objective of the present study was to understand if alteration of dopaminergic neurotransmitter concentrations in the hybrid striped bass (Morone saxatilis × Morone chrysops) brain by bupropion would alter this predator's ability to capture prey. The authors exposed hybrid striped bass to bupropion in a static system for 6 d, followed by a 6-d recovery period. During the present study's 12-d experiment, each hybrid striped bass was fed 4 unexposed fathead minnows every 3 d, and the time it took the hybrid striped bass to consume each of those 4 fathead minnows was quantified. After each feeding event, hybrid striped bass brains were harvested and analyzed for changes in several brain neurotransmitter concentrations, including serotonin, norepinephrine, dopamine, and many of their metabolites. Although bupropion altered the concentration of dopamine and many of the dopaminergic neurotransmitter metabolite concentrations in the brains on day 3 of the exposure, it did not alter the time to capture prey. This suggests that alteration of dopaminergic neurotransmitter concentrations in the hybrid striped bass brain does not alter a predator's ability to capture prey. Environ Toxicol Chem 2016;35:2058-2065. © 2016 SETAC.

Effects of an antidepressant mixture on the brain serotonin and predation behavior of hybrid striped bass.

Antidepressants have been found in measurable concentrations in final treated wastewater effluent and receiving waters throughout the world. Studies have shown that these concentrations are typically not overtly toxic, but the psychotropic mode of action of these chemicals warrants examination of their behavioral effects. Exposure of hybrid striped bass to the antidepressants fluoxetine or venlafaxine alone has been shown to cause decreased brain serotonin levels and increased time to capture prey at concentrations typically 1 to 2 orders of magnitude higher than environmentally relevant concentrations. In the present study, equally effective doses of fluoxetine and venlafaxine were used to perform a mixture study, using a toxic unit approach to determine whether these antidepressants may act in an additive manner at lower concentrations. The results indicated that mixtures of these antidepressants caused decreased brain serotonin and increased time to capture prey at concentrations lower than reported in previous studies. Low concentration mixtures caused an additive effect on brain serotonin levels and time to capture prey, whereas higher concentrations were less than additive. The results were consistent with the dose addition concept, with higher concentration mixtures potentially saturating the effects on serotonin in the brain. Results from the present study indicate that antidepressants have the potential to be additive on the biochemical and individual scale, which necessitates more robust analysis of antidepressant mixtures and their potential to act together in low concentration scenarios.

Characterization of ecological risks from environmental releases of decamethylcyclopentasiloxane (D5).

Decamethylcyclopentasiloxane (D5) is used in personal care products and industrial applications. The authors summarize the risks to the environment from D5 based on multiple lines of evidence and conclude that it presents negligible risk. Laboratory and field studies show that D5 is not toxic to aquatic organisms or benthic invertebrates up to its solubility limit in water or porewater or its sorptive capacity in sediment. Comparison of lipid-normalized internal concentrations with measured concentrations in benthos indicates that field-collected organisms do not achieve toxic levels of D5 in their tissues, suggesting negligible risk. Exposure to D5 resulted in a slight reduction of root biomass in barley at test concentrations 2 orders of magnitude greater than measured D5 levels in biosolids-amended soils and more than twice as high as the maximum calculated sorptive capacity of the soil. No effects were observed in soil invertebrates exposed to similar concentrations, indicating that D5 poses a de minimis risk to the terrestrial environment. High rates of metabolism and elimination of D5 compared with uptake rates from food results in biodilution in the food web rather than biomagnification, culminating in de minimis risk to higher trophic level organisms via the food chain. A fugacity approach substantiates all conclusions that were made on a concentration basis.

Responses of Hyalella azteca to acute and chronic microplastic exposures.

Limited information is available on the presence of microplastics in freshwater systems, and even less is known about the toxicological implications of the exposure of aquatic organisms to plastic particles. The present study was conducted to evaluate the effects of microplastic ingestion on the freshwater amphipod, Hyalella azteca. Hyalella azteca was exposed to fluorescent polyethylene microplastic particles and polypropylene microplastic fibers in individual 250-mL chambers to determine 10-d mortality. In acute bioassays, polypropylene microplastic fibers were significantly more toxic than polyethylene microplastic particles; 10-d lethal concentration 50% values for polyethylene microplastic particles and polypropylene microplastic fibers were 4.64 × 10(4) microplastics/mL and 71.43 microplastics/mL, respectively. A 42-d chronic bioassay using polyethylene microplastic particles was conducted to quantify effects on reproduction, growth, and egestion. Chronic exposure to polyethylene microplastic particles significantly decreased growth and reproduction at the low and intermediate exposure concentrations. During acute exposures to polyethylene microplastic particles, the egestion times did not significantly differ from the egestion of normal food materials in the control; egestion times for polypropylene microplastic fibers were significantly slower than the egestion of food materials in the control. Amphipods exposed to polypropylene microplastic fibers also had significantly less growth. The greater toxicity of microplastic fibers than microplastic particles corresponded with longer residence times for the fibers in the gut. The difference in residence time might have affected the ability to process food, resulting in an energetic effect reflected in sublethal endpoints.

Modeling the influence of physicochemical properties on gold nanoparticle uptake and elimination by Daphnia magna.

Monitoring the distribution and subsequent effects of nanoparticle contaminants in aquatic ecosystems will be pivotal to developing regulations that minimize their environmental footprint. The present study focused on the link between nanoparticle characteristics and Daphnia magna body burden using gold nanoparticles (AuNPs) with different size, shape, and surface charge configurations as model particles. Uptake followed first-order kinetics across the entire concentration range for all particles except the cationic rods, which demonstrated 2 distinct uptake patterns. Elimination followed the 2-compartment model for all particle configurations. Multiple regression analysis identified size and surface charge as controlling influences over AuNP uptake and elimination, whereas shape was regarded as inconsequential to both processes. Examination of the lumen-microvilli interface produced no evidence to indicate assimilation of the AuNPs used in the present study. Instead, these nanoparticles were restricted to the gut lumen and the carapace, where ingestion efficiency and adsorption were the primary determinants of total body burden. Models developed from the present data predict that D. magna will amass a higher body burden of larger cationic AuNPs at high concentration exposures and larger anionic AuNPs at low concentration exposures. A survey of the nanoparticle literature revealed that these trends were consistent with observations for certain nanomaterial exposures but could not be applied indiscriminately to all nanoparticle types and species.

Influence of carbon nanotubes on the bioavailability of fluoranthene.

Concurrent with the increase in the use of carbon nanotubes (CNTs) in society is the rise of their introduction into the environment. Carbon nanotubes cause adverse effects themselves, and they have the potential to adsorb contaminants such as polycyclic aromatic hydrocarbons (PAHs). Although CNTs have a high adsorption capacity for PAHs and these contaminants can co-occur in the environment, few studies have characterized the bioavailability of CNT-adsorbed PAHs to fish. The goal of the present study was to characterize the bioavailability of fluoranthene adsorbed to suspended multiwalled-carbon nanotubes (MWNTs) in freshwater containing natural organic matter (NOM). Adsorption isotherms indicated that NOM influenced the adsorption of fluoranthene to MWNTs, although in the absence of MWNTs it did not influence the bioavailability of fluoranthene to Pimephales promelas. Pimephales promelas were exposed for 16 h in synthetic moderately hard water containing fluoranthene in the presence of different concentrations of NOM, and fluoranthene adsorbed to MWNTs in the presence of NOM. Bioavailable fluoranthene was quantified in each exposure through bile analysis using fluorescence spectrophotometry. By comparing the concentration of fluoranthene metabolites in the bile with the concentration of fluoranthene added to MWNT and NOM solutions, the relative bioavailability of fluoranthene adsorbed to MWNTs was quantified. Results indicate that approximately 60% to 90% of the fluoranthene was adsorbed to the MWNTs and that adsorbed fluoranthene was not bioavailable to P. promelas. The results also suggest that fluoranthene is not desorbed from ingested MWNT, and the bioavailable fraction is only the freely dissolved fluoranthene in the aqueous phase.

Acute and chronic response of Daphnia magna exposed to TiO2 nanoparticles in agitation system.

Aquatic toxicity of titanium dioxide nanoparticles (TiO2 NPs) to Daphnia magna was characterized using a completely stirred bioassay system intended to keep particles in suspension thereby maintaining a consistent exposure. The 48-h LC50 was 4.5 mg/L TiO2 NPs, whereas LC50 values for 7 and 14-days exposures were 2.7 and 1.9 mg/L, respectively. An exposure of 1.5 mg/L over a 21-days exposure resulted in significant reductions in fecundity. While reproduction was initially reduced in the 0.5 and 1.0 mg/L exposures, it recovered and was similar to the control by 21 days. For reproduction inhibition, NOEC was 1.0 mg/L. Exposure to 2.5 mg/L TiO2 NPs resulted in 40 % of the organisms failing to become gravid; all surviving organisms exposed to 5.0 mg/L failed to become gravid. The increased sensitivity was due to the refinement in the bioassay system that kept NP in suspension resulting in consistent exposure concentrations.

Reprint of: Effects of the antidepressant venlafaxine on fish brain serotonin and predation behavior.

Antidepressants that enter receiving waters through final treated wastewater effluent have exhibited relatively low acute toxicity in traditional fish tests at currently measured concentrations. However, the psychotropic mode of action of these compounds warrants examination of the behavioral effects these chemicals may have on aquatic organisms. Previous research has demonstrated that exposure to the antidepressant fluoxetine causes decreased brain serotonin levels in fish and results in a decreased ability to capture prey. Another antidepressant, venlafaxine, has been found at low µg/L concentrations in final treated wastewater effluent. The objective of this study was to quantify the effects of venlafaxine on fish predation behavior and determine if this effect was correlated with changes in brain neurotransmitter concentrations. The predator prey bioassay used hybrid striped bass (Morone saxatilis x Morone chrysops) as the predator and fathead minnows (Pimephales promelas) as prey. Bass were exposed to venlafaxine (0-500 µg/L) for a period of 6 days and then allowed to recover for 6 days. During both exposure and recovery, bass were fed four minnows every third day. The time to capture the minnows was quantified and compared among treatments to determine if there was an effect on predation behavior. Brain tissue was analyzed for serotonin, norepinephrine, and dopamine, to determine the relationship between exposure concentration, brain monoamine levels, and predation behavior. Results indicated that venlafaxine exposures increased time to capture prey 1 and 2 by day 6 for the 250 and 500 µg/L treatments. Time to capture prey 3 was increased for all venlafaxine treatments by day 6. Venlafaxine caused a statistically significant decrease in brain serotonin concentrations that initially decreased in a dose dependent manner before reaching a steady state by the end of exposures for all treatments. No significant, dose-dependent changes in dopamine or norepinephrine were seen. Brain serotonin alone did not adequately explain behavioral results. Serotonin response in other tissues as well as peripheral effects may have accounted for additional behavioral responses after brain serotonin reached a depressed steady state.

Effects of the antidepressant venlafaxine on fish brain serotonin and predation behavior.

Antidepressants that enter receiving waters through final treated wastewater effluent have exhibited relatively low acute toxicity in traditional fish tests at currently measured concentrations. However, the psychotropic mode of action of these compounds warrants examination of the behavioral effects these chemicals may have on aquatic organisms. Previous research has demonstrated that exposure to the antidepressant fluoxetine causes decreased brain serotonin levels in fish and results in a decreased ability to capture prey. Another antidepressant, venlafaxine, has been found at low µg/L concentrations in final treated wastewater effluent. The objective of this study was to quantify the effects of venlafaxine on fish predation behavior and determine if this effect was correlated with changes in brain neurotransmitter concentrations. The predator prey bioassay used hybrid striped bass (Morone saxatilis x Morone chrysops) as the predator and fathead minnows (Pimephales promelas) as prey. Bass were exposed to venlafaxine (0-500 µg/L) for a period of 6 days and then allowed to recover for 6 days. During both exposure and recovery, bass were fed four minnows every third day. The time to capture the minnows was quantified and compared among treatments to determine if there was an effect on predation behavior. Brain tissue was analyzed for serotonin, norepinephrine, and dopamine, to determine the relationship between exposure concentration, brain monoamine levels, and predation behavior. Results indicated that venlafaxine exposures increased time to capture prey 1 and 2 by day 6 for the 250 and 500 µg/L treatments. Time to capture prey 3 was increased for all venlafaxine treatments by day 6. Venlafaxine caused a statistically significant decrease in brain serotonin concentrations that initially decreased in a dose dependent manner before reaching a steady state by the end of exposures for all treatments. No significant, dose-dependent changes in dopamine or norepinephrine were seen. Brain serotonin alone did not adequately explain behavioral results. Serotonin response in other tissues as well as peripheral effects may have accounted for additional behavioral responses after brain serotonin reached a depressed steady state.

Tracking and quantification of single-walled carbon nanotubes in fish using near infrared fluorescence.

Detection of SWCNTs in complex matrices presents a unique challenge as common techniques lack spatial resolution and specificity. Near infrared fluorescence (NIRF) has emerged as a valuable tool for detecting and quantifying SWCNTs in environmental samples by exploiting their innate fluorescent properties. The objective of this study was to optimize NIRF-based imaging and quantitation methods for tracking and quantifying SWCNTs in an aquatic vertebrate model in conjunction with assessing toxicological end points. Fathead minnows (Pimephales promelas) were exposed by single gavage to SWCNTs and their distribution was tracked using a custom NIRF imaging system for 7 days. No overt toxicity was observed in any of the SWCNT treated fish; however, histopathology observations from gastrointestinal (GI) tissue revealed edema within the submucosa and altered mucous cell morphology. NIRF images showed strong SWCNT-derived fluorescence signals in whole fish and excised intestinal tissues. Fluorescence was not detected in other tissues examined, indicating that no appreciable intestinal absorption occurred. SWCNTs were quantified in intestinal tissues using a NIRF spectroscopic method revealing values that were consistent with the pattern of fluorescence observed with NIRF imaging. Results of this work demonstrate the utility of NIRF imaging as a valuable tool for examining uptake and distribution of SWCNTs in aquatic vertebrates.

Treatment with coated layer double hydroxide clays decreases the toxicity of copper-contaminated water.

Copper is a common pollutant found in watersheds that exerts toxic effects on both invertebrates and vertebrates. Layer double hydroxide (LDH) clays are able to adsorb a wide range of contaminants through ion-exchange mechanisms. Coating LDH clays with various materials alters the aggregation of clay particles into the nano-size range, thus increasing relative surface area and offering great potential for contaminant remediation. The goal of this study was to determine if treatment with coated LDH clays decreases the toxicity of copper-containing solutions to Daphnia magna. Four LDH clays with different coatings used to alter hydrophobicity were as follows: used: Na(+) montmorillonite, Zn-Al LDH-nitrate, Zn-Al LDH-stearate, and Zn-Al LDH-carbonate. It was determined that coated LDH clays decreased copper toxicity by decreasing bioavailability and that smaller aggregate sizes decreased bioavailability the most. 96 h LC50 values increased by as much as 4.2 times with the treatment of the solutions with 100 mg/L LDH clay. Copper analysis of the clay and solutions indicated that the clays work by decreasing copper bioavailability by way of a binding mechanism. Coated LDH clays hold promise as a small-scale remediation tool or as an innovative tool for toxicity identification and evaluation characterization of metals.

Testing the individual effective dose hypothesis.

The assumption of the individual effective dose is the basis for the probit method used for analyzing dose or concentration-response data. According to this assumption, each individual has a uniquely innate tolerance expressed as the individual effective dose (IED) or the smallest dose that is sufficient to kill the individual. An alternative to IED, stochasticity suggests that individuals do not have uniquely innate tolerance; deaths result from random processes occurring among similar individuals. Although the probit method has been used extensively in toxicology, the underlying assumption has not been tested rigorously. The goal of the present study was to test which assumption, IED or stochasticity, best explained the response of Daphnia magna exposed to multiple pulses of copper sulfate (CuSO4 ) over 24 d. Daphnia magna were exposed to subsequent age-dependent 24-h median lethal concentrations (LC50s) of copper (Cu). Age-dependent 24-h LC50 values and Cu depuration test were determined prior to the 24-d bioassay. The LC50 values were inversely related to organism age. The Cu depuration of D. magna did not depend on age or Cu concentration, and 5 d was sufficient recovery time. Daphnia magna were exposed to 4 24-h Cu exposures, and surviving organisms after each exposure were transferred to Cu-free culture media for recovery before the next exposure. Stochasticity appropriately explained the survival and reproduction response of D. magna exposed to Cu.

Microscopic investigation of single-wall carbon nanotube uptake by Daphnia magna.

The objectives of this study were to determine the extent of absorption of functionalized single-wall carbon nanotubes (SWCNTs) across the gut epithelial cells in Daphnia magna. Several microscopic techniques were utilized, including micro-Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM) and selective area diffraction (SAD). In an effort to examine the variation in uptake due to surface properties, four groups of differently functionalized SWCNTs were used: hydroxylated (OH-SWCNTs), silicon dioxide (SiO2-SWCNTs), poly aminobenzenesulfonic acid (PABS-SWCNTs) and polyethylene glycol (PEG-SWCNTs). Raman spectroscopy was able to detect OH-SWCNTs within the gut, but lacked the spatial resolution that is needed to identify lower concentrations of SWCNTs that may have been absorbed by body tissues. Initially, low-magnification imaging of exposed D. magna sections in the TEM revealed several features, which suggested absorption of SWCNTs. However, subsequent analysis with additional techniques (HRTEM, X-ray energy-dispersive spectroscopy and SAD) indicated that these features were either artifacts produced via the specimen staining process or consisted of non-graphitic, organic structures. This latter observation emphasizes the inherent difficulty in resolving SWCNTs embedded within a complex, organic matrix, as well as the care with which imaging results must be interpreted and supplemented with other, more analytical techniques.