Development of a chronic zinc biotic ligand model for Daphnia magna

The individual effects of the cations Ca(2+), Mg(2+), Na(+), and H(+) on the chronic toxicity of Zn to the waterflea Daphnia magna were investigated in different series of univariate experiments, resulting in the development of a chronic Zn biotic ligand model (BLM) for this species. Using the mathematical approach based on a linear relationship between cation activity and metal activity at the EC(x) level, the following stability constants for binding of competing cations to the biotic ligand (BL) were derived: logK(CaBL) = 3.22, logK(MgBL) = 2.69, logK(NaBL) = 1.90, and logK(HBL) = 5.77. With the derived constants and a logK(ZnBL) of 5.31, two different BLMs that predict chronic zinc toxicity (EC(50), no observed effect concentration (NOEC)) for D. magna as a function of water characteristics were developed. Fractions of binding sites occupied by Zn at the considered effect levels EC(50) and NOEC were 0.127 and 0.084, respectively. The NOEC-based model predicts the Zn toxicity within a factor of 2, while the chronic EC(50) could be predicted within a factor of 1.5. In the future, these chronic Zn BLMs for D. magna can improve the ecological relevancy of zinc risk assessments by decreasing the bioavailability-related uncertainty of zinc toxicity.     

Development and field validation of a biotic ligand model predicting chronic copper toxicity to Daphnia magna

In this study, we developed a toxicity model predicting the long-term effects of copper on the reproduction of the cladoceran Daphnia magna that is based on previously reported toxicity tests in 35 exposure media with different water chemistries. First, it was demonstrated that the acute copper biotic ligand model (BLM) for D. magna could not serve as a reliable basis for predicting chronic copper toxicity. Consequently, BLM constants for chronic exposures were derived by multiple regression analysis of 21-d median effective concentrations (EC50s; expressed as Cu2+ activity) versus physicochemistry from a large toxicity dataset and the results of an additional experiment in which the individual effect of sodium on copper toxicity was investigated. The effect of sodium on chronic toxicity (log K NaBL = 2.91) seemed to be similar to its effect on acute toxicity (log K NaBL = 3.19). However, in contrast to the acute BLM, no significant calcium, magnesium, or combined competition effect was observed, and an increase in proton competition and bioavailability of CuOH+ and CuCO3 complexes was noted. Some indirect evidence was also found for some limited toxicity of complexes of copper with two of three tested types of dissolved organic matter. Because the latter was only a minor effect, this factor was not included in the chronic Cu BLM. The newly developed model performed well in predicting 21-d EC50s and no-observed-effect concentrations in natural water samples: 79% of the toxicity threshold values were predicted within a factor of two of the observed values. It is clear, however, that more research is needed to provide information on the exact mechanisms that have resulted in different BLM constants for chronic exposures (as opposed to acute exposures). It is suggested that the developed model can contribute to the improvement of risk assessment procedures of copper by incorporating bioavailability of copper in these regulatory exercises.     

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.     

Application of an acute biotic ligand model to predict chronic copper toxicity to Daphnia magna in natural waters of Chile and reconstituted synthetic waters

The objective of the present study was to assess the predictive capacity of the acute Cu biotic ligand model (BLM) as applied to chronic Cu toxicity to Daphnia magna in freshwaters from Chile and synthetic laboratory-prepared waters. Samples from 20 freshwater bodies were taken, chemically characterized, and used in the acute Cu BLM to predict the 21-d chronic Cu toxicity for D. magna. The half-maximal effective concentration (EC50) values, determined using the Organisation for Economic Co-operation and Development (OECD) 21-d reproduction test (OECD Method 211), were compared with the BLM simulated EC50 values. The same EC50 comparison was performed with the results of 19 chronic tests in synthetic media, with a wide range of hardness and alkalinity and a fixed 2 mg/L dissolved organic carbon (DOC) concentration. The acute BLM was modified only by adjustment of the accumulation associated with 50% of an effect value (EA50). The modified BLM model was able to predict, within a factor of two, 95% of the 21-d EC50 and 89% of the 21-d half-maximal lethal concentrations (LC50) in natural waters, and 100% of the 21-d EC50 and 21-d LC50 in synthetic waters. The regulatory implications of using a slightly modified version of an acute BLM to predict chronic effects are discussed.     

Predicting sediment metal toxicity using a sediment biotic ligand model: methodology and initial application

An extension of the simultaneously extracted metals/acid-volatile sulfide (SEM/AVS) procedure is presented that predicts the acute and chronic sediment metals effects concentrations. A biotic ligand model (BLM) and a pore water-sediment partitioning model are used to predict the sediment concentration that is in equilibrium with the biotic ligand effects concentration. This initial application considers only partitioning to sediment particulate organic carbon. This procedure bypasses the need to compute the details of the pore-water chemistry. Remarkably, the median lethal concentration on a sediment organic carbon (OC)-normalized basis, SEM*(x,OC), is essentially unchanged over a wide range of concentrations of pore-water hardness, salinity, dissolved organic carbon, and any other complexing or competing ligands. Only the pore-water pH is important. Both acute and chronic exposures in fresh- and saltwater sediments are compared to predictions for cadmium (Cd), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn) based on the Daphnia magna BLM. The SEM*(x,OC) concentrations are similar for all the metals except cadmium. For pH = 8, the approximate values (micromol/gOC) are Cd-SEM*(xOC) approximately equal to 100, Cu-SEM*(x,OC) approximately equal to 900, Ni-SEMoc approximately equal to 1,100, Zn-SEM*(x,OC) approximately equal to 1,400, and Pb-SEM*(x,OC) approximately equal to 2,700. This similarity is the explanation for an empirically observed dose-response relationship between SEM and acute and chronic effects concentrations that had been observed previously. This initial application clearly demonstrates that BLMs can be used to predict toxic sediment concentrations without modeling the pore-water chemistry.     

Accumulation and regulation of zinc in Daphnia magna: links with homeostasis and toxicity

Zinc accumulation in Daphnia magna was investigated, and the results were linked to the previously established optimal concentration range for zinc and D. magna. It was observed that organisms cultured in this optimal range (300-600 microg Zn/L) contained 212 +/- 57 to 254 +/- 79 microg Zn/g dry weight. Lower and higher zinc contents were obtained after acclimation to previously established culture concentrations inducing deficiency and toxicity, respectively. The calculation of bioconcentration factors indicated that zinc was actively regulated, at least up to a concentration of 600 microg Zn/L. Zinc uptake and elimination are rapid processes; major increases and decreases in body content occurred within 1 day. Zinc concentrations in daphnids exposed to 600 microg Zn/L fluctuated with 2- to 3-day intervals, suggesting a role of molting in the regulation and elimination of zinc.     

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.     

The water flea (Daphnia magna) as a sensitive indicator for the assessment of toxicity of synthetic detergents

The water flea (Daphnia magna) was used as a sensitive indicator for assessing the toxicity due to synthetic detergents. Acute and chronic toxicity of detergents to the water flea was studied under laboratory conditions by following the median tolerance limit (TLM) at 48 hr and the rate of survival. A significant decrease in the rate of reproduction (number of hatching and neonates produced) were found at 21 days. During acute toxicity studies behavioural changes were also noticed.     

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.     

Age and exposure duration as a factor influencing Cu and Zn toxicity toward Daphnia magna

Standardized toxicity tests are generally performed with juvenile test organisms, e.g., in Daphnia magna assays neonates<24 h old are used. The purpose of this research was to investigate the influence of a delayed exposure to Cu and Zn on population parameters and toxicity values derived from these endpoints. Juveniles (<24 h; T0) and 7-d old daphnids (T7) were exposed for 21 and 14 d, respectively. For Cu, juveniles were significantly more sensitive than 7d old organisms following acute (48 h) as well as chronic (14 d) exposure. After 14 d of exposure to 130 microg/L Cu, mortality was 80% and 10% in T0 and T7, respectively. Juveniles per surviving female at this concentration decreased by 78% and 14% compared to the control. 14 d-NOEC and LOEC values (based on juveniles per surviving female) were 75 and 90 microg/L Cu for T0 and both>130 microg/L for T7. For Zn, survival in T0 and T7 was similar. Although T7 organisms produced significantly more offspring, 14 d-NOEC and LOEC values were equal to those of T0, i.e., 80 and 115 microg/L, respectively. For Cu as well as for Zn effect concentrations based on 14 and 21 d exposure were similar (results from T0). It can be concluded that acute and chronic toxicity data obtained from juvenile D. magna are more sensitive or equally sensitive than obtained from 7d old organisms. As differences are observed between the two metals extrapolations of these conclusions to other toxicants and other aquatic species cannot be made without further investigation.     

The effects of 4-nonylphenol and ethanol on acute toxicity,embryo development,and reproduction in Daphnia magna

The mean 48-h EC(50) (n=3) of 4-nonylphenol (NP) using ethanol as the carrier solvent was 155 microg/L, compared to a mean 48-h EC(50) (n=3) of 281 microg/L without ethanol. The 96-h EC(50)'s for embryo lethality (arrested egg development) and deformities (curved or unextended shell spines and undeveloped second antennae) were 738 and 263 microg/L, respectively. Reproduction studies were conducted using conditions that stimulate male production (i.e., reduced photoperiod and food levels). An increase in neonate deformities was observed at 50 microg/L (without ethanol), but no changes were observed in fecundity or sex ratios. A decrease in sex ratios was observed at 25 and 50 microg/L (with ethanol) compared to the ethanol control. However, an increase in sex ratios was observed in the ethanol control compared to media controls. The use of ethanol as a solvent carrier confounds the effects of 4-NP on acute toxicity and male production in daphnids.     

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.     

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.     

Predicting effects of cations on copper toxicity to lettuce (Lactuca sativa) by the biotic ligand model

A biotic ligand model (BLM) was developed to estimate Cu toxicity to lettuce (Lactuca sativa) in terms of root elongation after 4 d of exposure. Effects of Na(+), K(+), Ca(2+), and Mg(2+) on Cu toxicity were examined. The addition of these cations resulted in a 50-fold difference in the copper median effective activity (EC50 cu2+). However, these variations could not be interpreted entirely as a function of the concentrations of these cations alone. In particular, only the relationship between EC50 cu2+ and the activity of protons was found to be significant in the whole range of pH examined from 5.0 to 7.0. The addition of K(+), Na(+), Ca(2+), and Mg(2+) at concentrations up to 20 mM resulted in a 16-fold difference in EC50 cu2+ values. This difference was significant, as indicated by non-overlapping standard deviations of the negative logarithm of EC50 cu2+ pEC50 cu2+) obtained with (7.37 ± 0.22) and without (6.76 ± 0.22) additions of K(+), Na(+), Ca(2+), and Mg(2+). The variations were not statistically significantly related to concentrations of these cations; therefore, only protons can be integrated in the BLM predicting Cu toxicity to lettuce L. sativa with the important parameters: log K(HBL) =6.27, log K(CuBL) =7.40, and [formula in text]. The lack of significant relationships between EC50 cu2+ and concentrations of the cations was not in line with the main assumption of the BLM about the competition between cations for binding sites.