Mechanism of acute silver toxicity in Daphnia magna

Daphnids (Daphnia magna) were exposed to AgNO3 at 0.303 +/- 0.017 microg silver/L (46.9% as Ag+), in the absence of food, in moderately hard synthetic water under static conditions for up to 48 h. Results from accumulation experiments demonstrated that silver body burden was inversely related to body mass. Daphnids exposed to silver exhibited ionoregulatory disturbance, which was characterized by decreases in whole-body sodium concentration. This ionoregulatory disturbance was explained, at least in part, by a competitive inhibition of the whole-body sodium uptake (six- to sevenfold increase in the Michaelis constant with no change in maximal velocity), which was complete by 1 h of exposure, and resulted in approximately 40% inhibition of sodium influx from the water. A rapidly developing inhibition of whole-body Na+,K(+)-dependent adenosine triphosphatase (Na+,K(+)-ATPase) activity, significant by 2 h and complete at 90% blockade by 12 h, also was observed during exposure to AgNO3. Therefore, these findings clearly demonstrate that the key mechanism involved in acute Ag+ toxicity in D. magna, the most sensitive freshwater organism tested to date, resembles that described for freshwater fish--that is, inhibition of active sodium uptake by blockade of Na+,K(+)-ATPase. Furthermore, the results showed that Na+,K(+)-ATPase inhibition was directly related to silver accumulation in the whole body of D. magna. However, the nature of the sodium uptake inhibition (competitive vs noncompetitive in fish) and the fact that whole-body chloride concentration was not disturbed in daphnids was different from fish. With regard to the biotic ligand model (BLM) for silver, our results yielded a log K value of about 8.9. However, the current version of the BLM uses a rainbow trout log K value (7.3) but achieves the correct sensitivity of the model for daphnids by reducing the saturation of toxic sites needed to cause toxicity. An alternative way may be to use the log K value derived from the present results.     

Acute toxicity of sodium nitrate, potassium nitrate, and potassium chloride and their effects on the hemolymph composition and gill structure of early juvenile blue swimmer crabs(Portunus pelagicus Linnaeus, 1758) (Decapoda, Brachyura, Portunidae)

Various nutrients, including K+ and NO3-, are increasingly being discharged into aquatic systems via anthropogenic sources, which may impact marine organisms. The present study was conducted on blue swimmer crab (Portunus pelagicus) early juveniles to determine the acute toxicity of NaNO3, KNO3, and KCl; if a toxicity interaction exists between K+ and NO3-; the hemolymph Na+, K+, and Ca2+ changes; and the gill histopathological alterations following exposure to elevated NaNO3, KNO3, and KCl levels. A total of 20 replicate crabs were exposed to each of the five NaNO3, KNO3, and KCl concentrations for 96 h. After 96 h, the surviving crabs were sampled for hemolymph Na+, K+, and Ca2+ levels and fixed for histological examination of the anterior gills. The 96-h median lethal concentration of NaNO3-N, KNO3-N, KNO3-K, and KCl-K was 3,452, 112, 312, and 356 mg/L, respectively, for early P. pelagicus juveniles. The toxicity of NaNO3-N was significantly less (p < 0.01) than that of KNO3-N. Furthermore, at the same K+ levels, KNO3-K was significantly (p < 0.05) more toxic than KCl-K, indicating a toxicity interaction between K+ and NO3-. Following exposure to elevated KNO3 and KCl levels, the crabs had significantly higher (p < 0.01) hemolymph K+ levels compared to the control. Conversely, following exposure to elevated NaNO3 concentrations, the crabs had significantly higher (p < 0.01) hemolymph Na+ levels but significantly lower (p < 0.01) hemolymph K+ levels. Despite the markedly different hemolymph ionic changes following NaNO3 and KNO3/KCl exposure, the histopathological changes to the anterior gill lamellae of the crabs appeared to be similar, including lamellae swelling, epithelial thickening, pillar cell disruption, necrosis, and distortion.     

Influence of dissolved organic matter source on silver toxicity to Pimephales promelas

In the environment, the formation of organic and inorganic silver complexes can decrease Ag bioavailability (toxicity) to aquatic organisms. However, current water quality regulations do not consider the protective effects of water quality parameters such as dissolved organic carbon (DOC) concentration. To determine the effect of DOC concentration and source on silver toxicity, nine different natural organic matter isolates were used in 96-h static-renewal toxicity tests with fathead minnow (Pimephales promelas). The 96-h dissolved silver median lethal concentrations (LC50) among different sources of dissolved organic matter varied by up to fivefold (4.5-23.3 microg/L). Further, toxicity tests with organic matter from the site with the lowest 96-h LC50 value suggested only limited additional attenuation of silver toxicity when DOC concentration was increased from 5.1 to 14.0 mg/L. With this site excluded, we found little more than a twofold difference among 96-h dissolved Ag LC50s for the remaining sources (10.1-23.3 microg/L). However, significant toxicological differences among sites remained. It was apparent that organic matter from different sources varied both chemically and toxicologically, but no conclusions could be drawn that related compositional variation to observed Ag toxicity for these isolates.     

Physiological responses to acute silver exposure in the freshwater crayfish (Cambarus diogenes diogenes)--a model invertebrate?

Adult crayfish (Cambarus diogenes diogenes) exposed to 8.41 +/- 0.17 microg silver/L (19.4% as Ag+) in moderately hard freshwater under flow-through conditions for 96 h exhibited ionoregulatory disturbance, elevated metabolic ammonia (T(amm)) production and substantial silver accumulation in the gills, hemolymph, and hepatopancreas. The ionoregulatory disturbance included both a generally reduced unidirectional Na+ influx and an increased unidirectional Na+ efflux, leading to a substantial net loss of Na+ from the silver-exposed crayfish. The Na+ uptake in silver-exposed crayfish differed overall from controls, while the increased Na+ efflux recovered to control values 48 h into the 96 h of exposure. The general inhibition of Na+ uptake could be explained by a reduced sodium/potassium-adenosine triphosphatase (Na/K-ATPase) activity in terminally obtained gill samples from the silver-exposed crayfish. The silver-induced effect on Na+ uptake and loss translated to reduced hemolymph Na+ concentrations but not significantly reduced hemolymph Cl- concentrations. Hemolymph T(anim) and T(amm) efflux both increased in silver-exposed crayfish, indicating an increased metabolic T(amm) production. The present study demonstrates that the toxic mechanism of waterborne silver exposure in freshwater crayfish resembles that of freshwater teleost fish. The crayfish might therefore be a useful model system for extending current environmental regulatory strategies, currently based on teleost fish, to invertebrates.     

Biologically incorporated dietary silver has no ionoregulatory effects in American red crayfish (Procambarus clarkii)

Two silver-contaminated diets were prepared by exposing juvenile rainbow trout for 8 d to waterborne silver thiosulfate as Ag at either 0.1 microg/L (low-Ag diet) or 80 mg/L (high-Ag diet). The level of total Ag accumulated in whole low-Ag fish was below the detection limit of analysis. Whole high-Ag fish accumulated Ag at 21.3 nmol/g. The livers of the low- and high-Ag fish accumulated Ag at 0.43 nmol/g and 1.01 micromol/g, respectively. The Ag-contaminated fish were then fed whole to adult crayfish in an 80-d dietary study to determine the effects of long-term trophic accumulation of Ag. In a second experiment, the livers of the high-Ag trout were fed to juvenile crayfish for either one or five weeks. Accumulation of Ag was demonstrated in both adult and juvenile crayfish. Silver accumulation in juvenile crayfish peaked at approximately 650 nmol/g at three weeks, after which Ag depuration occurred. In adult crayfish that consumed the high-Ag diet, the hepatopancreas accumulated more than 90% of assimilated Ag, rising 1,000-fold over control animals to approximately 740 nmol/g at 80 d. Crayfish that consumed the low-Ag diet had small, statistically insignificant elevations of Ag in some tissues. Dietary Ag had no effect on juvenile crayfish growth or adult mortality. Disturbances in osmoregulation, which are normally associated with acute waterborne Ag exposure, were not detected. Dietary Ag also had no effect on hemolymph concentrations of Na+, Cl-, Ca2+, Mg2+, or Cu; did not affect the concentration kinetics of Na+ or Cl- influx; and had no effect on the activity of gill Na+/K+-dependent adenosine triphosphatase. Hemolymph concentrations of glucose and lactate were similarly unaffected, indicating an absence of stress-related metabolic disturbance. However, a disproportionately low number of ecdysis events occurred among crayfish that consumed the high-Ag diet.     

Effects of ligand-bound silver on Ceriodaphnia dubia

In aqueous media, ionic silver concentrations are low and transport occurs in the colloidal phase. In the aquatic environment, silver forms 1:1 complexes with thiol-containing compounds such as cysteine and glutathione. In order to quantitatively characterize the risk associated with silver in aquatic ecosystems, the bioavailabilities and toxicities of silver cysteinate and silver glutathionate were characterized. Static renewal bioassays were conducted with Ceriodaphnia dubia to estimate chronic toxicity, using mortality and reproduction as endpoints. Silver nitrate was the most lethal compound, with a median lethal concentration (8-d LC50) of 0.32 microg Ag/L (95% confidence interval [CI] = 0.19-0.54). The 48-h LC50 for AgNO3 was 0.5 microg/L and did not change significantly through 8 d. The presence of food in the bioassay did not change the 48-h LC50 for AgNO3. Silver glutathionate (AgGSH) and silver cysteinate (AgCys) induced less mortality during the 8-d bioassay. Silver cysteinate appeared to have the greatest effect on fecundity, with a no-observable-effect concentration (NOEC) less than 0.001 microg/L. Silver nitrate and AgGSH had lowest-observable-effect concentration (LOEC) values (nominal concentrations) of 0.01 and 0.6 microg/L, respectively. Results indicate that the ligand-bound silver in these laboratory studies is bioavailable and impairs reproduction of C. dubia at low aqueous concentrations.     

Acute silver toxicity in the euryhaline copepod Acartia tonsa: influence of salinity and food

The euryhaline copepod Acartia tonsa was exposed to silver (AgNO(3)) in either the absence or the presence of food (diatom Thalassiosira weissflogii; 2 x 10(4) cells/ml). Standard static-renewal toxicity tests that included a fixed photoperiod of 16: 8 h light:dark and temperature (20 degrees C) were run in three different salinities (5, 15, and 30 ppt) together with measurements of pH, ions (Na(+), Cl(-), K(+), SO(4)(2-), Mg(2+), and Ca(2+)), alkalinity, dissolved organic carbon, and total and dissolved (0.45 microm) silver concentrations in the experimental media. In the absence of food, the 48-h EC50 (concentration causing effect to 50% of the individuals tested) values based on total and dissolved silver concentrations were 11.6, 87.2, and 163.2 microg Ag/L and 7.1, 79.2, and 154.6 microg Ag/L at salinities 5, 15, and 30 ppt, respectively. In the presence of food, they were 62.1, 98.5, and 238.4 microg Ag/L and 48.4, 52.3, and 190.9 microg Ag/L, respectively. In all experimental conditions, most of the toxic silver fraction was in the dissolved phase, regardless of salinity or the presence of food in the water. In either the absence or the presence of food, acute silver toxicity was salinity dependent, decreasing as salinity increased. Data indicate that changes in water chemistry can account for the differences in acute silver toxicity in the absence of food, but not in the presence of food, suggesting that A. tonsa requires extra energy to cope with the stressful conditions imposed by acute silver exposure and ionoregulatory requirements in low salinities. These findings indicate the need for incorporation of both salinity and food (organic carbon) in a future biotic ligand model (BLM) version for estuarine and marine conditions, which could be validated and calibrated using the euryhaline copepod A. tonsa.     

Predicting acute zinc toxicity for Daphnia magna as a function of key water chemistry characteristics: development and validation of a biotic ligand model

The individual effect of different major cations (Ca2+, Mg2+, Na+, K+, and H+) on the acute toxicity of zinc to the waterflea Daphnia magna was investigated. The 48-h median effective concentration (EC50) in the baseline test medium (i.e., a standard medium with very low ion concentrations) was about 6 microM (Zn2+). An increase of Ca2+ (from 0.25 mM to 3 mM), Mg2+ (from 0.25 mM to 2 mM), and Na+ activity (from 0.077 mM to 13 mM) reduced zinc toxicity by a factor of 6.3, 2.1, and 3.1, respectively. No further toxicity reduction was observed when Ca2+ and Mg2+ activities exceeded 3.0 and 2.0 mM, respectively. Both K+ and H+ did not significantly alter zinc toxicity (expressed as Zn2+ activity). From these data, conditional stability constants for Ca2+ (log K = 3.24), Mg2+ (log K = 2.97), Na+ (log K = 2.16), and Zn2+ (log K = 5.31) were derived and incorporated into a biotic ligand model (BLM) predicting acute zinc toxicity to D. magna in surface waters with different water quality characteristics. Validation of the developed BLM using 17 media with different pH, hardness, and dissolved organic carbon (DOC) content resulted in a significant correlation coefficient (R2 = 0.76) between predicted and observed 48-h EC50. Eighty-eight percent of the predictions were within a factor of 1.3 of the observed 48-h EC50.     

Chronic toxicity of silver to the sea urchin (Arbacia punctulata)

The chronic toxicity of silver to the sea urchin (Arbacia punctulata) was determined in 30 per thousand salinity seawater during a three-part study: A fertilization test (1-h sperm exposure), a 48-h embryo test, and a 30-d adult test. Combined data from the three tests resulted in a lowest-observed-effect concentration of 19 microg/L, a no-observed-effect concentration of 8.6 microg/L, and a maximum acceptable toxicant concentration of 13 microg/L, based on measured concentrations of dissolved silver. The 96-h median effective concentration was 40 microg/L, and the acute to chronic toxicity ratio was 3.1. During the tests, measured concentrations of free ionic silver (Ag+) were only 0.0027 to 0.0046% of dissolved silver concentrations, as predicted by ion-speciation theory. Some measured Ag+ concentrations were lower than predicted, indicating the presence of other ligands in the seawater test media. These strong sulfide ligands were exuded by the exposed sea urchins into the seawater (where Ag-sulfide complexes formed) in amounts that increased in direct proportion to the silver concentration during the toxicity test. This suggests a toxicity-defense mechanism that functioned by modifying the chemistry of the surrounding external medium.     

Comparison of acute and chronic toxicity of silver nanoparticles and silver nitrate to Daphnia magna

Silver nanoparticles (AgNP) are now widely used as antibacterial products, and their potential toxicities in aquatic organisms are a matter of increasing concern. In the present study, we conducted experiments to reveal the acute and chronic toxicities of AgNP and its bioaccumulation from both aqueous and dietary sources in a model freshwater cladoceran, Daphnia magna. No mortality was observed in 48-h acute toxicity testing when the daphnids were exposed up to 500?μg Ag/L as AgNP. The AgNP accumulation reached as high as 22.9?mg Ag/g dry weight at the highest AgNP concentration tested (500?μg/L). In contrast, D. magna was extremely sensitive to free Ag ion (Ag(+) , added as AgNO(3) ), with a measured 48-h 50% lethal concentration of 2.51?μg/L. Thus, any AgNP potential acute toxicity may be caused by the release of Ag(+) into the solution. During the 21-d chronic exposure, dietborne AgNO(3) had the most significant influence on reproduction, whereas waterborne AgNP had the most significant inhibition on growth. Significant delay and decrease of reproduction in daphnids exposed to dietborne AgNO(3) occurred at a dissolved Ag concentration of 0.1?μg/L added to the algae. Significant inhibitions of growth and reproduction were also found for the AgNP exposure, with the lowest observed effective concentration of 5?μg/L and 50?μg/L, respectively. Chronic effects of AgNP were probably caused by the low food quality of algae associated with AgNP and the low depuration of ingested AgNP. Environmental risk assessments of AgNP should therefore include tests on the chronic toxicity to aquatic organisms as well as the direct and indirect effects of AgNP resulting from the release of Ag(+) into the environment.     

Chronic effects of silver exposure on ion levels, survival, and silver distribution within developing rainbow trout (Oncorhynchus mykiss) embryos

Rainbow trout embryos were chronically exposed to silver (as AgNO3) in moderately hard water (120 mg CaCO3/L, 0.70 mM Cl-, 1.3 mg/L dissolved organic matter. 12.3+/-0.1 degrees C) at nominal concentrations of 0.1, 1, and 10 microg/L (measured = 0.117+/-0.008, 1.22+/-0.16, and 13.51+/-1.58 microg/L, respectively) to investigate the effects on mortality, ionoregulation, and silver uptake and distribution of the embryo. Mortalities in the low concentrations (0.1 and 1.2 microg/L) were not significantly different from controls throughout embryonic development (days 1-32 postfertilization). Mortalities of embryos in the 13.5-microg/L treatment reached 56% by day 32 postfertilization (33% when accounting for control mortality), by which time more than 50% of surviving embryos had hatched. Accumulation of silver in whole embryos of 1.2- and 13.5-microg/L treatments reached the highest concentrations of 0.13 and 0.24 microg/g total silver, respectively, by day 32, but whole embryo silver burden was not correlated with mortality. Silver concentrations in different compartments of the whole embryo (chorion, dissected embryo, and yolk) were greatest just before hatch and were higher in the chorion for all experimental treatments. Up to 85% of total whole embryo silver content was bound to the chorion, which acts as a protective barrier during silver exposure. Whole embryo Na+ concentration in the 13.5-microg/L treatment was significantly reduced relative to controls from days 23 to 32 postfertilization, and levels in the embryo were reduced by 40% at day 32 postfertilization, indicating that silver toxicity in the whole embryo is associated with an ion regulatory disturbance that is similar to the acute effect of AgNO3 in juvenile and adult trout.     

Evaluation of the protective effects of reactive sulfide on the acute toxicity of silver to rainbow trout (Oncorhynchus mykiss)

Acute 96-h toxicity tests were performed with juvenile rainbow trout (Oncorhynchus mykiss) exposed to AgNO3 in either the absence or the presence of 100 nM reactive sulfide to evaluate the protective effect of aqueous sulfides against ionic Ag toxicity. The sulfide was presented in the form of zinc sulfide (ZnS) clusters under oxic conditions. Silver was lost from the water column during the course of the experiment, so mean measured Ag concentrations were used to generate all median lethal concentration (LC50) data. The system was complicated in that Ag2S precipitated because of the need for large amounts of Ag to obtain lethal effects in the presence of ZnS. Some of the losses of Ag could be explained by complexation with ZnS and formation of solid Ag2S. Other losses were probably the result of partial adsorption to exposure-chamber walls or to complexation with ligands or functional groups within organic material produced by the fish. The LC50 (95% confidence interval) values generated using measured concentrations for total Ag were 139 (122-162) nM in the absence of sulfide and 377 (340-455) nM in the presence of 100 nM sulfide. The LC50 values generated using measured concentrations from filtered (pore size, 0.45 microm) water samples were 122 (105-145) nM in the absence of sulfide and 225 (192-239) nM in the presence of 100 nM sulfide. These results suggest a stoichiometric protection of sulfides up to a 2:1 ratio of Ag:sulfide. Greater accumulation of Ag at the gills was measured in fish exposed to AgNO3 in the presence of sulfide.     

Effects of insecticide exposure on feeding inhibition in mayflies and oligochaetes

The present study examined the effects of pulse exposures of the insecticide imidacloprid on the mayfly, Epeorus longinmanus Eaton (Family Heptageniidae), and on an aquatic oligochaete, Lumbriculus variegatus Miller (Family Lumbriculidae). Pulse exposures of imidacloprid are particularly relevant for examination, because this insecticide is relatively soluble (510 mg/L) and is most likely to be at effect concentrations during runoff events. Experiments examined the recovery of organisms after a 24-h pulse exposure to imidacloprid over an environmentally realistic range of concentrations (0, 0.1, 0.5, 1, 5, and 10 microg/L). Effects on feeding were measured by quantifying the algal biomass consumed by mayflies or foodstuffs egested by oligochaetes. Imidacloprid was highly toxic, with low 24-h median lethal concentrations (LC50s) in early mayfly instars (24-h LC50, 2.1 +/- 0.8 microg/L) and larger, later mayfly instars (24-h LC50, 2.1 +/- 0.5 microg/L; 96-h LC50, 0.65 +/- 0.15 microg/L). Short (24-h) pulses of imidacloprid in excess of 1 microg/L caused feeding inhibition, whereas recovery (4 d) varied, depending on the number of days after contaminant exposure. In contrast to mayflies, oligochaetes were relatively insensitive to imidacloprid during the short (24-h) pulse; however, immobility of oligochaetes was observed during a 4-d, continuous-exposure experiment, with 96-h median effective concentrations of 6.2 +/- 1.4 microg/L. Overall, imidacloprid reduced the survivorship, feeding, and egestion of mayflies and oligochaetes at concentrations greater than 0.5 but less than 10 microg/L. Inhibited feeding and egestion indicate physiological and behavioral responses to this insecticide.     

Ion-release kinetics and ecotoxicity effects of silver nanoparticles

The environmental toxicity associated with silver nanoparticles (AgNPs) has been a major focus in nanotoxicology. The Ag(+) released from AgNPs may affect ecotoxicity, although whether the major toxic effect is governed by Ag(+) ions or by AgNPs themselves is unclear. In the present study, we have examined the ecotoxicity of AgNPs in aquatic organisms, silver ion-release kinetics of AgNPs, and their relationship. The 48-h median effective concentration (EC50) values for Daphnia magna of powder-type AgNP suspensions were 0.75?μg/L (95% confidence interval [CI]?=?0.71-0.78) total Ag and 0.37?μg/L (95% CI?=?0.36-0.38) dissolved Ag. For sol-type AgNP suspension, the 48-h EC50 values for D. magna were 7.98?μg/L (95% CI?=?7.04-9.03) total Ag and 0.88?μg/L (95% CI?=?0.80-0.97) dissolved Ag. The EC50 values for the dissolved Ag of powder-type and sol-type AgNPs for D. magna showed similar results (0.37?μg/L and 0.88?μg/L) despite their differences of EC50 values in total Ag. We observed that the first-order rate constant (k) of Ag(+) ions released from AgNPs was 0.0734/h at 0.05?mg/L total Ag at 22°C within 6?h. The kinetic experiments and the toxicity test showed that 36% and 11% of sol-type AgNPs were converted to the Ag(+) ion form under oxidation conditions, respectively. Powder-type AgNPs showed 49% conversion rate of Ag(+) ion from AgNPs. We also confirmed that Ag(+) ion concentration in AgNP suspension reaches an equilibrium concentration after 48?h, which is an exposure time of the acute aquatic toxicity test.     

Eel ATPase activity as biomarker of thiobencarb exposure

European eels (Anguilla anguilla) were exposed to a sublethal thiobencarb concentration of 0.22 mg/L in a flow-through system for 96 h. Mg(2+) and Na(+)-K(+) adenosine triphosphatase (ATPase) activities were evaluated in gill and muscle tissues at 2, 12, 24, 48, 72, and 96 h of thiobencarb exposure. Gill ATPase activities were rapidly inhibited from 2h of contact onward. Highest inhibition was registered for Na(+), K(+)-ATPase (85%) from 2 to 12h. Both Mg(2+) and total ATPase were inhibited (>73%) during the first hours of toxicant exposure. At the end of the exposure period (96 h) ATPase activities were still different from those of the controls (>50%). Significant inhibition was detected in Na(+), K(+)-ATPase activity (80%) in muscle tissue after 2h and it was maintained over the entire exposure time. However, Mg(2+)-ATPase and total ATPase showed only perturbations after 2 h of exposure. Eels were exposed to 0.22 mg/L of thiobencarb for 96 h and then a recovery period in herbicide-free water was allowed for 192 h. Gill and muscle samples were removed at 8, 24, 72, 96, 144, and 192 h and ATPase activity was evaluated. Following 144 h of recovery, Mg(2+)- and Na(+), K(+)-ATPase activities, as well as total ATPase activity, in gills of those animals previously exposed to 0.22 mg/L of thiobencarb were still significantly different compared to controls. Thiobencarb seems to act to alter the ionic profiles. Since ion-dependent ATPases are known to regulate the influx and efflux of ions across the membrane to maintain the physiological requirements of the cells, the inhibition of Na(+), K(+)-ATPase probably induced osmoregulatory perturbations. On the other hand, thiobencarb exposure causes increases in the muscle water content of A. anguilla. The results indicated that water content increased significantly (>100% higher than the controls) during the first 24 h of exposure.     

Acute and chronic toxicity of nickel to rainbow trout (Oncorhynchus mykiss)

Of the fish species tested in chronic Ni exposures, rainbow trout (Oncorhynchus mykiss) is the most sensitive. To develop additional Ni toxicity data and to investigate the toxic mode of action for Ni, we conducted acute (96-h) and chronic (85-d early life-stage) flow-through studies using rainbow trout. In addition to standard toxicological endpoints, we investigated the effects of Ni on ionoregulatory physiology (Na, Ca, and Mg). The acute median lethal concentration for Ni was 20.8 mg/L, and the 24-h gill median lethal accumulation was 666 nmol/g wet weight. No effects on plasma Ca, Mg, or Na were observed during acute exposure. In the chronic study, no significant effects on embryo survival, swim-up, hatching, or fingerling survival or growth were observed at dissolved Ni concentrations up to 466 microg/L, the highest concentration tested. This concentration is considerably higher than the only other reported chronic no-observed-effect concentration (<33 microg/L) for rainbow trout. Accumulation of Ni in trout eggs indicates the chorion is only a partial barrier with 36%, 63%, and 1% of total accumulated Ni associated with the chorion, yolk, and embryo, respectively. Whole-egg ion concentrations were reduced by Ni exposure. However, most of this reduction occurred in the chorion rather than in the embryos, and no effects on hatching success or larval survival were observed as a result. Plasma ion concentrations measured in swim-up fingerlings at the end of the chronic-exposure period were not significantly reduced by exposure to Ni. Nickel accumulated on the gill in an exponential manner but plateaued in trout plasma at waterborne Ni concentrations of 118 microg/L or greater. Consistent with previous studies, Ni did not appear to disrupt ionoregulation in acute exposures of rainbow trout. Our results also suggest that Ni is not an ionoregulatory toxicant in long-term exposures, but the lack of effects in the highest Ni treatment precludes a definitive conclusion.     

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.     

Validation study of the acute biotic ligand model for silver

An important final step in development of an acute biotic ligand model for silver is to validate predictive capabilities of the biotic ligand model developed for fish and invertebrates. To accomplish this, eight natural waters, collected from across North America, were characterized with respect to ionic composition, pH, dissolved organic carbon, and sulfide. Tests were conducted with the cladoceran Ceriodaphnia dubia (48-h static) and the fish Pimephales promelas (96-h static renewal) to determine the concentrations causing lethality to 50% of the organisms (LC50s) for silver in each of these waters. Overall, the biotic ligand model adequately predicted silver toxicity to C. dubia; however, in some cases, predicted LC50 values exceeded measured values. The accuracy of the biotic ligand model predictions was less convincing for silver toxicity to P. promelas with pronounced problems in low-ionic strength waters. Another issue was the use of acclimated organisms in toxicity studies because the biotic ligand model has been developed with the use of a mix of studies with acclimated and nonacclimated test organisms of varying ages and sizes. To evaluate whether effects of acclimation to test waters influence biotic ligand model predictions, a subset of the natural waters were also tested with P. promelas that had been acclimated to the natural water for 7 d before testing. These experiments revealed no differences in toxicity between acclimated and nonacclimated P. promelas. To determine the influence of organism size, which has been previously correlated to Na(+) turnover and acute silver toxicity across multiple species, Na(+) and Cl(-) influx rates were measured in P. promelas of different sizes. Our results show that Na(+) and Cl(-) influx rates were inversely related to fish mass and positively correlated with silver sensitivity.     

Aquatic toxicity of triclosan

The aquatic toxicity of triclosan (TCS), a chlorinated biphenyl ether used as an antimicrobial in consumer products, was studied with activated-sludge microorganisms, algae, invertebrates, and fish. Triclosan, a compound used for inhibiting microbial growth, was not toxic to wastewater microorganisms at concentrations less than aqueous solubility. The 48-h Daphnia magna median effective concentration (EC50) was 390 microg/L and the 96-h median lethal concentration values for Pimephales promelas and Lepomis macrochirus were 260 and 370 microg/L, respectively. A no-observed-effect concentration (NOEC) and lowest-observed-effect concentration of 34.1 microg/L and 71.3 microg/L, respectively, were determined with an early life-stage toxicity test with Oncorhynchus mykiss. During a 96-h Scenedesmus study, the 96-h biomass EC50 was 1.4 microg/L and the 96-h NOEC was 0.69 microg/L. Other algae and Lemna also were investigated. Bioconcentration was assessed with Danio rerio. The average TCS accumulation factor over the five-week test period was 4,157 at 3 microg/L and 2,532 at 30 microg/L. Algae were determined to be the most susceptible organisms. Toxicity of a TCS-containing wastewater secondary effluent to P. promelas and Ceriodaphnia was evaluated and no observed differences in toxicity between control and TCS-treated laboratory units were detected. The neutral form of TCS was determined to be associated with toxic effects. Ionization and sorption will mitigate those effects in the aquatic compartment.     

Toxicity of Synthetic Musks to Early Life Stages of the Freshwater Mussel Lampsilis cardium

Polycyclic musk fragrances are common additives to many consumer products. As a result of their widespread use and slow degradation rates, they are widely found in aquatic environments. This study reports on the lethal and sublethal toxicity of the polycyclic musks AHTN (Tonalide) and HHCB (Galaxolide) to glochidial (larval) and juvenile life stages of the freshwater mussel Lampsilis cardium (Rafinesque, 1820). In glochidia, 24-h median lethal concentrations (LC50s) ranged from 454 to 850 microg AHTN/L and from 1000 to >1750 microg HHCB/L (water solubility). Results for 48-h tests were similar to the 24-h tests. In 96-h tests with juveniles, we did not observe a dose-response relation between mortality and either musk. However, the growth rate was reduced by musk exposure. The median effective concentrations (EC50s, based on growth) were highly variable and ranged from 108 to 1034 microg AHTN/L and 153 to 831 microg HHCB/L. While all adverse effects occurred at concentrations that are much greater than those reported in natural waters (low microg/L to ng/L), these results indicate the potential for adverse effects on these long-lived organisms from exposure to synthetic musk fragrances.