Heterogeneity of natural organic matter amelioration of silver toxicity to Daphnia magna: effect of source and equilibration time

Despite the heterogeneity of natural organic matter (NOM) in the aquatic environment, current models that predict metal toxicity to aquatic biota treat these important metal-complexing agents in a homogeneous manner. In this investigation, the ability of 11 commercial and naturally isolated NOM samples to ameliorate silver toxicity to the freshwater crustacean Daphnia magna was examined. The commercially available Aldrich humic acid (AHA) increased the 48-h median lethal concentration for daphnid neonates from nominally NOM-free levels of 0.29 to 3.80 microg/L (at 6.9 mg C/L) in a concentration-dependent manner. Three of the tested samples exhibited similar protective effects, but the additional seven NOM samples displayed significantly stronger ameliorative actions. In fact, four samples of both commercial and naturally isolated origin demonstrated greater than fourfold increases in protection compared to that of AHA. Additional investigations showed that increased silver-AHA equilibration time resulted in decreased toxicity. Increased equilibration time also decreased whole-body silver accumulation at NOM levels less than 1 mg C/L. The present results suggest that heterogeneity of NOM and silver-NOM equilibration time will have to be accounted for in future models of silver toxicity to D. magna and that laboratory toxicity testing using NOM and metals should account for the effects of metal-NOM equilibration time.     

Testing the Underlying Chemical Principles of the Biotic Ligand Model (BLM) to Marine Copper Systems: Measuring Copper Speciation Using Fluorescence Quenching

Speciation of copper in marine systems strongly influences the ability of copper to cause toxicity. Natural organic matter (NOM) contains many binding sites which provides a protective effect on copper toxicity. The purpose of this study was to characterize copper binding with NOM using fluorescence quenching techniques. Fluorescence quenching of NOM with copper was performed on nine sea water samples. The resulting stability constants and binding capacities were consistent with literature values of marine NOM, showing strong binding with \(\log K\) values from 7.64 to 10.2 and binding capacities ranging from 15 to 3110 nmol mg \({\text {C}}^{-1}.\) Free copper concentrations estimated at total dissolved copper concentrations corresponding to previously published rotifer effect concentrations, in the same nine samples, were statistically the same as the range of free copper calculated for the effect concentration in NOM-free artificial seawater. These data confirms the applicability of fluorescence spectroscopy techniques for NOM and copper speciation characterization in sea water and demonstrates that such measured speciation is consistent with the chemical principles underlying the biotic ligand model approach for bioavailability-based metals risk assessment.     

Effects of dissolved organic matter and reduced sulphur on copper bioavailability in coastal marine environments

Copper-induced toxicity in aqueous systems depends on its speciation and bioavailability. Natural organic matter (NOM) and reduced sulphur species can complex copper, influencing speciation and decreasing bioavailability. NOM composition in estuaries can vary, depending on inputs of terrigenous, autochthonous, or wastewater source material. At a molecular level, variability in NOM quality potentially results in different extents of copper binding. The aims of this study were to measure acute copper EC₅₀ values in coastal marine and estuarine waters, and identify the relationships between total dissolved copper EC₅₀ values and measured water chemistry parameters proportional to NOM and reduced sulphur composition. This has implications on the development of marine-specific toxicity prediction models. NOM was characterised using dissolved organic carbon (DOC) concentration and fluorescence measurements, combined with spectral resolution techniques, to quantify humic-, fulvic-, tryptophan-, and tyrosine-like fractions. Reduced sulphur was measured by the chromium-reducible sulphide (CRS) technique. Acute copper toxicity tests were performed on samples expressing extreme DOC, fluorescent terrigenous, autochthonous, and CRS concentrations. The results show significant differences in NOM quality, independent of DOC concentration. CRS is variable among the samples; concentrations ranging from 4 to 40 nM. The toxicity results suggest DOC as a very good predictive measure of copper EC₅₀ in estuaries (r²=0.87) independent of NOM quality. Furthermore, for filtered samples, CRS exists at concentrations that would be saturated with copper at measured EC₅₀, suggesting that while CRS might bind Cu and decrease bioavailability, it does not control copper speciation at toxicologically relevant concentrations and therefore is not a good predictive measure of copper toxicity in filtered samples.     

Influence of natural organic matter source on acute copper, lead, and cadmium toxicity to rainbow trout (Oncorhynchus mykiss)

Natural organic matter (NOM) was concentrated from various sites across Canada using a portable reverse-osmosis unit to obtain a range of NOM types, from mainly allochthonous (terrestrially derived) to mainly autochthonous (aquatically derived) NOM. The addition of NOM to Cu exposures in ion-poor water always decreased Cu toxicity to rainbow trout (Oncorhynchus mykiss, approximately 1 g) over a 96-h period, and the degree of protection varied with respect to NOM source. A good correlation was found between the specific absorbance coefficient (SAC) and time to reach 50% mortality (LT50; p < 0.001), indicating that more optically dark, allochthonous-like NOM decreases Cu toxicity better than does optically light, more autochthonous-like NOM. A similar, good relationship between NOM source and Pb toxicity was seen (p < 0.001), once confounding effects of Ca binding to NOM were accounted for. No significant relationship between Cd toxicity and NOM optical quality was seen (p = 0.082), and in toxicity tests with Cd the presence of some of the NOM sources increased Cd toxicity compared to Cd-only controls. Specific absorbance coefficients were used as a proxy measurement of NOM aromaticity in our study, and fluorescence indices were run on some NOM samples to obtain percent aromaticity for each sample. A good correlation was found between SAC and percent aromaticity, indicating that the simple SAC measurement is a reasonable indication of NOM aromaticity and of metal binding by NOM.     

Does sulfide or water hardness protect against chronic silver toxicity in Daphnia magna? A critical assessment of the acute-to-chronic toxicity ratio for silver

The protective effects of sulfide and water hardness against acute and chronic silver toxicity in Daphnia magna were assessed in the presence of food. Results showed that both sulfide and water hardness protected against lethal acute and chronic silver toxicity in terms of mortality. However, only sulfide showed a protective effect against the sub-lethal chronic silver effects on growth and reproduction. These findings suggest that both reactive sulfide and water hardness must be taken into account in the development of a chronic version of the Biotic Ligand Model (BLM) for waterborne silver. Furthermore, acute-to-chronic ratio values for silver toxicity showed that only small increases in toxicity are seen over the chronic exposure relative to the acute toxicity. Mortality is the most sensitive endpoint in moderately hard water and in the presence of sulfide. Reproduction, measured as the number of neonates produced per adult per reproduction day, is the most sensitive one in hard water in the absence of sulfide.     

Relative Acute Toxicity of Acid Mine Drainage Water Column and Sediments to Daphnia magna in the Puckett's Creek Watershed, Virginia, USA

Acid mine drainage (AMD) is produced when pyrite (FeS₂) is oxidized on exposure to oxygen and water to form ferric hydroxides and sulfuric acid. If produced in sufficient quantity, iron precipitate, heavy metals (depending on soil mineralogy), and sulfuric acid may contaminate surface and ground water. A previous study of an AMD impacted watershed (Puckett's Creek, Powell River drainage, southwestern Virginia, USA) conducted by these researchers indicated that both water column and sediment toxicity were significantly correlated with benthic macroinvertebrate community impacts. Sites that had toxic water or sediment samples had significantly reduced macroinvertebrate taxon richness. The present study was designed to investigate the relative acute toxicity of acid mine drainage (AMD) water column and sediments to a single test organism (Daphnia magna) and to determine which abiotic factors were the best indicators of toxicity in this system. Nine sampling stations were selected based on proximity to major AMD inputs in the watershed. In 48-h exposures, sediment samples from three stations were acutely toxic to D. magna, causing 64–100% mortality, whereas water samples from five stations caused 100% mortality of test organisms. Forty-eight-hour LC50 values ranged from 35 to 63% for sediment samples and 27 to 69% for water column samples. Sediment iron concentration and several water chemistry parameters were the best predictors of sediment toxicity, and water column pH was the best predictor of water toxicity. Based on these correlations and on the fact that toxic sediments had high percent water content, water chemistry appears to be a more important adverse influence in this system than sediment chemistry. Received: 22 April 1999/Accepted: 21 October 1999     

Effects of suspended multi-walled carbon nanotubes on daphnid growth and reproduction

Relatively little is known about the potential impacts of engineered nanoparticles on aquatic biota. Particularly relevant to aquatic ecosystems are those particles, which display increased solubility either through specialized coatings or through an ability to interact with water column constituents such as natural organic matter (NOM). Previous research has indicated that grazing zooplankton (Daphnia magna) are able to ingest lipid-coated single-walled carbon nanotubes (SWNTs) from the water column during their normal feeding behavior (Roberts et al., 2007). Acute mortality was observed only at high concentrations (>5 mg/L). In this research NOM was used in place of a surfactant to stabilize suspensions. Water chemistry (ionic strength, hardness, and pH) has been shown to alter the behavior of NOM in natural systems. We hypothesized that these same variables may also affect the toxicity of multi-walled carbon nanotubes (MWNT) stabilized in NOM. The purpose of this research was to examine the potential for sublethal effects to occur following exposure to multi-walled carbon nanotubes suspended in NOM and to determine whether those effects vary with pH alterations.     

Amelioration of the toxicity of H+ to larval stoneflies by metals found in coal mine effluent

Stonefly nymphs (Acroneuria carolinensis) were used as a bioindicator of the toxicity of coal mine drainage, and of the interaction between low pH and dissolved metals at severe pollutant levels. Laboratory experiments were done to determine the mean life expectancy of stoneflies subjected to simulated mine drainage with various levels of metals and pH. Both iron and aluminum had significant protective effects against low pH toxicity (at levels found in mine drainage streams of pH 2.8–3.3). The ameliorative effect of Al and Fe was additive. At pH 2.8, the protective effect of aluminum increased with concentration, approaching a maximum near 12 mM. Increased water hardness due to Ca and Mg had a lesser protective effect. A laboratory simulation of a field site at pH 2.8 (modelling field levels of Na, K, Mg, Ca, Al, and Fe) was more toxic than the field site it was patterned after, indicating that there is an ameliorating factor that is not among the ions considered. The other metals tested (Mn, Zn, Cu, and Cd at concentrations prevalent in central Pennsylvania mine effluent) did not reduce the toxicity of H+ ions at pH 3.3. Stonefly nymphs placed in an acid mine drainage stream with a pH of 4.0, lost approximately 50% of their body sodium. However, when these field water chemistry conditions were mimicked in the laboratory there was no significant body Na loss. This indicates that some factor(s) other than the cations considered is affecting toxicity, and that field conditions were more toxic than the laboratory simulation at pH 4.0. Larval stoneflies are more tolerant of acidic mine effluent than are trout. Hence, they can be a useful bioindicator of the toxicity of severe acid and metal mixtures, especially when enough detail to understand the mechanisms is sought.     

Evaluation of the effect of reactive sulfide on the acute toxicity of silver (I) to Daphnia magna. Part 2: toxicity results

The protective effect of reactive sulfide against AgNO3 toxicity to Daphnia magna neonates was studied. Acute (48-h) toxicity tests were performed in the absence (<5 nM) and presence of low (approximately 25 nM) and high (approximately 250 nM) concentrations of zinc sulfide clusters under oxic conditions. In both the presence and the absence of sulfide, lower mean lethal concentration (LC50) values were observed when measured as opposed to nominal silver concentrations were used in calculations. This reflected the fact that measured total silver concentrations were lower than nominal concentrations due to losses of silver from solution observed during the experiment. High concentration (approximately 250 nM) of sulfide completely protected against toxicity up to the highest silver concentration tested (2 microg/L [19 nM]) with measured silver data. In the presence of environmentally realistic levels of sulfide (approximately 25 nM) in receiving waters, acute silver toxicity was reduced by about 5.5-fold. However, when filtered (0.45 microm) silver concentrations alone were considered, toxicity (48-h LC50) was similar in the absence (0.22 microg/L) and presence (0.28 microg/L) of sulfide. The difference between measured total and filtered silver was attributed to chemisorption of the metal sulfide onto the membrane filter and provides evidence that the toxic fraction of silver is that which is unbound to sulfide. Accumulation of silver was greater in daphnids exposed to silver in the presence of sulfide than in its absence, even though a toxic effect was not observed under these conditions. In this case, silver appears to be incorporated by daphnids rather than merely adsorbed on the surface. Our results point out the need to incorporate sulfide into the acute biotic ligand model and to assess its potentially large role in preventing chronic toxicity.     

Influence of natural organic matter source on copper toxicity to larval fathead minnows (Pimephales promelas): implications for the biotic ligand model

The influence of dissolved natural organic matter (NOM) source on copper toxicity was investigated with larval fathead minnows (Pimephales promelas) in reconstituted moderately hard water. Ninety-six-hour static renewal toxicity tests were conducted to investigate an assumption of the biotic ligand model (BLM) that NOM source does not need to be considered to adequately predict copper toxicity. The nine different NOM isolates used in these toxicity tests were chemically well-characterized substances that were obtained by reverse osmosis as part of an NOM typing project based in southern Norway. Three median lethal concentration (LC50) values were estimated for toxicity tests conducted with each NOM, at nominal dissolved organic carbon (DOC) concentrations of 2, 5, and 10 mg/L. Tests also were conducted in dilution waters in which no NOM was added. Regression analyses were conducted to compare NOM-specific (specific NOM source) LC50s versus DOC concentration relationships to each other, as well as to the overall LC50 versus DOC concentration relationship. Statistical differences were found regarding the effects of NOM source on copper toxicity. Similar analyses were conducted with humic acid (HA) concentrations and spectral absorbance, and differences in the effect of NOM source on copper toxicity were similarly concluded. These results do not support the assumption that copper toxicity can be adequately predicted by utilizing DOC concentration, regardless of NOM source. Evaluation of relationships between LC50 values and other NOM characteristics revealed that despite significant differences due to NOM source on copper toxicity, DOC and HA concentrations were the most effective parameters in explaining variability in LC50 values. When BLM-predicted LC50 values were compared to observed LC50 values, predicted values showed reasonable agreement with observed values, but some deviations occurred due to NOM source and DOC concentration.     

Effects of water hardness on toxicological responses to chronic waterborne silver exposure in early life stages of rainbow trout (Oncorhynchus mykiss)

Rainbow trout embryos and larvae were exposed to 0, 0.1, and 1 microg/L total silver (as AgNO3) in water of three different hardnesses (soft water [2 mg/L as CaCO3], moderately hard water [150 mg/L], and hard water [400 mg/L]) in a flow-through system from fertilization to swim-up (64 d). The objective of the study was to investigate the effects of water hardness on chronic silver toxicity. In the absence of silver, elevating hardness had a positive effect on early life stage survival and development, significantly decreasing mortality and accelerating time to 50% swim-up. Following hatch, exposure to 1 microg/L Ag significantly increased mortality relative to exposure to 0 microg/L Ag. No significant effects of silver on time to 50% hatch were observed; however, time to 50% swim-up was delayed, and 50% swim-up was not achieved over the course of the experiment during some exposures to 1 microg/L Ag. These results suggest that the current Canadian Water Quality Guideline (http://www.ccme.ca/assets/pdf/e1_062.pdf) of 0.1 microg/L Ag is sufficient in preventing mortality and altered development in early life stages of rainbow trout. Increasing water hardness from 2 to 150 or 400 mg/L was modestly protective against the mortality and delays in time to 50% swim-up associated with exposure to 1 microg/L Ag. The 150- and 400-mg/L hardnesses were equally protective against mortality, but 150-mg/L was more protective than 400-mg/L hardness against the delays in time to 50% swim-up. Overall, the protective effects of hardness on chronic silver toxicity in early life stages of rainbow trout are modest but similar to the protection afforded to acute silver toxicity in juvenile and adult rainbow trout.     

Effect of dissolved organic matter source on acute copper toxicity to Daphnia magna

The protective effect of dissolved organic matter (DOM) on metal toxicity to aquatic organisms has been reported by numerous authors. Bioavailability models such as the biotic ligand model (BLM) thus account for this factor to predict metal toxicity to aquatic organisms. Until now, however, few attempts have been made to assess the effect of the DOM source on metal speciation and toxicity and, accordingly, on BLM predictions. The aims of this study were to investigate to what extent DOMs differ in their ability to decrease acute copper toxicity to the cladoceran Daphnia magna and to evaluate if ultraviolet (UV) absorbance measurements may be a simple and effective method to incorporate DOM variability into the acute Cu-BLM for D. magna. Acute toxicity tests were carried out in artificial test water enriched with DOMs isolated from six locations in Europe and North America and in seven natural European surface waters. The acute Cu-BLM for D. magna was then used to estimate the copper complexing capacity of each DOM (expressed as % active fulvic acid, %AFA). A factor of 6 difference was observed between the lowest and the highest copper complexing capacity. A significant linear relationship was observed between the UV-absorbance coefficient at 350 nm (epsilon350) and the %AFA. Linking this relationship to the acute Cu-BLM resulted in a significant improvement of the predictive capacity of this BLM. Without accounting for this relationship, 90% of the predicted 48-h 50% effective concentrations (EC50) were within a factor of 2 of the observed EC50s; taking this relationship into account, 90% of the EC50s were predicted with an error of less than factor 1.3. The present study and other studies seem to indicate that UV absorbance may be a good measure of biologically and toxicologically relevant differences in copper binding behavior of DOM.     

Toxicity and metal speciation in acid mine drainage treated by passive bioreactors

Sulfate-reducing passive bioreactors treat acid mine drainage (AMD) by increasing its pH and alkalinity and by removing metals as metal sulfide precipitates. In addition to discharge limits based on physicochemical parameters, however, treated effluent is required to be nontoxic. Acute and sublethal toxicity was assessed for effluent from 3.5-L column bioreactors filled with mixtures of natural organic carbon sources and operated at different hydraulic retention times (HRTs) for the treatment of a highly contaminated AMD. Effluent was first tested for acute (Daphnia magna and Oncorhynchus mykiss) and sublethal (Pseudokirchneriella subcapitata, Ceriodaphnia dubia, and Lemna minor) toxicity. Acute toxicity was observed for D. magna, and a toxicity identification evaluation (TIE) procedure was then performed to identify potential toxicants. Finally, metal speciation in the effluent was determined using ultrafiltration and geochemical modeling for the interpretation of the toxicity results. The 10-d HRT effluent was nonacutely lethal for O. mykiss but acutely lethal for D. magna. The toxicity to D. magna, however, was removed by 2 h of aeration, and the TIE procedure suggested iron as a cause of toxicity. Sublethal toxicity of the 10-d HRT effluent was observed for all test species, but it was reduced compared to the raw AMD and to a 7.3-d HRT effluent. Data regarding metal speciation indicated instability of both effluents during aeration and were consistent with the toxicity being caused by iron. Column bioreactors in operation for more than nine months efficiently improved the physicochemical quality of highly contaminated AMD at different HRTs. The present study, however, indicated that design of passive treatment should include sufficient HRT and posttreatment aeration to meet acute toxicity requirements.     

Biotic ligand model of the acute toxicity of metals. 2. Application to acute copper toxicity in freshwater fish and Daphnia

The biotic ligand model (BLM) was developed to explain and predict the effects of water chemistry on the acute toxicity of metals to aquatic organisms. The biotic ligand is defined as a specific receptor within an organism where metal complexation leads to acute toxicity. The BLM is designed to predict metal interactions at the biotic ligand within the context of aqueous metal speciation and competitive binding of protective cations such as calcium. Toxicity is defined as accumulation of metal at the biotic ligand at or above a critical threshold concentration. This modeling framework provides mechanistic explanations for the observed effects of aqueous ligands, such as natural organic matter, and water hardness on metal toxicity. In this paper, the development of a copper version of the BLM is described. The calibrated model is then used to calculate LC50 (the lethal concentration for 50% of test organisms) and is evaluated by comparison with published toxicity data sets for freshwater fish (fathead minnow, Pimephales promelas) and Daphnia.     

Evaluation of the effect of reactive sulfide on the acute toxicity of silver (I) to Daphnia magna. Part 1: description of the chemical system

Experiments were designed to assess the potential protective effect of the presence of sulfide against the acute (48-h) toxicity of silver(I) to Daphnia magna. Tests were conducted in borosilicate glass beakers (250 ml) in moderately hard synthetic water. Toxicity solutions were replaced after 24 h by static renewal method. This paper describes the chemical system, and the acute toxicity results are presented in a companion paper. Sulfide was below detection limit (<5 nM) in controls with no sulfide added. Sulfide, added as zinc sulfide clusters at approximately 35- or approximately 350-nM concentration, dropped in concentration to approximately 25 and 250 nM, respectively, over the 24-h period of measurements. Silver also decreased in concentration during the experiment (up to 59%), and the rate of loss was greater in the absence of sulfide compared with the presence of sulfide. A filtration experiment indicated a 1:1 binding ratio of silver to sulfide and a conditional stability constant for the Ag(I)-zinc sulfide complex of log K' = 8.9. The losses of sulfide and silver during the experiments highlighted the need for regular monitoring of the important chemical components of the system, even during short (48-h) toxicity tests.     

An evaluation of biotic ligand models predicting acute copper toxicity to Daphnia magna in wastewater effluent

The toxicity of Cu to Daphnia magna was investigated in a series of 48-h immobilization assays in effluents from four wastewater treatment works. The assay results were compared with median effective concentration (EC50) forecasts produced by the HydroQual biotic ligand model (BLM), the refined D. magna BLM, and a modified BLM that was constructed by integrating the refined D. magna biotic ligand characterization with the Windermere humic aqueous model (WHAM) VI geochemical speciation model, which also accommodated additional effluent characteristics as model inputs. The results demonstrated that all the BLMs were capable of predicting toxicity by within a factor of two, and that the modified BLM produced the most accurate toxicity forecasts. The refined D. magna BLM offered the most robust assessment of toxicity in that it was not reliant on the inclusion of effluent characteristics or optimization of the dissolved organic carbon active fraction to produce forecasts that were accurate by within a factor of two. The results also suggested that the biotic ligand stability constant for Na may be a poor approximation of the mechanisms governing the influence of Na where concentrations exceed the range within which the biotic ligand stability constant value had been determined. These findings support the use of BLMs for the establishment of site-specific water quality standards in waters that contain a substantial amount of wastewater effluent, but reinforces the need for regulators to scrutinize the composition of models, their thermodynamic and biotic ligand parameters, and the limitations of those parameters.     

Effect of coexposure to DDT and manganese on freshwater invertebrates: pore water from contaminated rivers and laboratory studies

An environmental survey of several rivers of the southern Huasteca area of Mexico revealed high concentrations of manganese (Mn) and the presence of DDT in the sediments and pore water. Therefore, acute (48-h) toxicities of Mn and DDT were assessed both independently and as a combination on 24-h-old neonates of Daphnia magna Strauss and Lecane quadridentata Ehrenberg. Daphnia magna showed high sensitivity to both toxicants, whereas L. quadridentata was highly resistant to DDT and less susceptible to Mn. For D. magna, the Mn and DDT coexposure was significantly more toxic than any of the singly tested compounds. When D. magna was exposed to sediment pore water, no association was found between the Mn content in the samples and the observed toxicity. Preliminary particle analysis of pore water showed different compounds of Mn, which apparently were not in bioavailable form.     

Effects from filtration, capping agents, and presence/absence of food on the toxicity of silver nanoparticles to Daphnia magna

Relatively little is known about the behavior and toxicity of nanoparticles in the environment. Objectives of work presented here include establishing the toxicity of a variety of silver nanoparticles (AgNPs) to Daphnia magna neonates, assessing the applicability of a commonly used bioassay for testing AgNPs, and determining the advantages and disadvantages of multiple characterization techniques for AgNPs in simple aquatic systems. Daphnia magna were exposed to a silver nitrate solution and AgNPs suspensions including commercially available AgNPs (uncoated and coated), and laboratory-synthesized AgNPs (coated with coffee or citrate). The nanoparticle suspensions were analyzed for silver concentration (microwave acid digestions), size (dynamic light scattering and electron microscopy), shape (electron microscopy), surface charge (zeta potentiometer), and chemical speciation (X-ray absorption spectroscopy, X-ray diffraction). Toxicities of filtered (100 nm) versus unfiltered suspensions were compared. Additionally, effects from addition of food were examined. Stock suspensions were prepared by adding AgNPs to moderately hard reconstituted water, which were then diluted and used straight or after filtration with 100-nm filters. All nanoparticle exposure suspensions, at every time interval, were digested via microwave digester and analyzed by inductively coupled argon plasma-optical emission spectroscopy or graphite furnace-atomic absorption spectroscopy. Dose-response curves were generated and median lethal concentration (LC50) values calculated. The LC50 values for the unfiltered particles were (in μg/L): 1.1 ± 0.1-AgNO(3) ; 1.0 ± 0.1-coffee coated; 1.1 ± 0.2-citrate coated; 16.7 ± 2.4 Sigma Aldrich Ag-nanoparticles (SA) uncoated; 31.5 ± 8.1 SA coated. LC50 values for the filtered particles were (in μg/L): 0.7 ± 0.1-AgNO(3) ; 1.4 ± 0.1-SA uncoated; 4.4 ± 1.4-SA coated. The LC50 resulting from the addition of food was 176.4 ± 25.5-SA coated. Recommendations presented in this study include AgNP handling methods, effects from sample preparation, and advantages/disadvantages of different nanoparticle characterization techniques.     

Challenges for the development of a biotic ligand model predicting copper toxicity in estuaries and seas

An effort is ongoing to develop a biotic ligand model (BLM) that predicts copper (Cu) toxicity in estuarine and marine environments. At present, the BLM accounts for the effects of water chemistry on Cu speciation, but it does not consider the influence of water chemistry on the physiology of the organisms. We discuss how chemistry affects Cu toxicity not only by controlling its speciation, but also by affecting the osmoregulatory physiology of the organism, which varies according to salinity. In an attempt to understand the mechanisms of Cu toxicity and predict its impacts, we explore the hypothesis that the common factor linking the main toxic effects of Cu is the enzyme carbonic anhydrase (CA), because it is a Cu target with multiple functions and salinity-dependent expression and activity. According to this hypothesis, the site of action of Cu in marine fish may be not only the gill, but also the intestine, because in this tissue CA plays an important role in ion transport and water adsorption. Therefore, the BLM of Cu toxicity to marine fish should also consider the intestine as a biotic ligand. Finally, we underline the need to incorporate the osmotic gradient into the BLM calculations to account for the influence of physiology on Cu toxicity.     

Use of atropine-treated Daphnia magna survival for detection of environmental contamination by acetylcholinesterase inhibitors

The toxicity of cholinesterase-inhibiting compounds (e.g., carbamates and organophosphates) is due to a decrease in acetylcholine metabolism, which results in a continuous stimulation of cholinergic receptors (muscarinic and nicotinic) that can be fatal. The goal of this study was to evaluate the protective effect of atropine (muscarinic receptor antagonist) against paraoxon-induced toxicity to Daphnia magna using its survival rate for the detection of environmental contamination by cholinesterase-inhibiting compounds. As expected, paraoxon was lethal to D. magna in a concentration-dependent manner. Noteworthy, the pretreatment of these organisms with atropine dramatically increased their survival against paraoxon. These results indicate that muscarinic stimulation plays an important role in paraoxon-induced lethality in D. magna. Therefore, simply by using the survival of atropine-treated and nontreated D. magna, water contamination by cholinesterase-inhibiting compounds may be rapidly and specifically detected.