Scott J. Markich

pH-dependent toxicity of copper and uranium to a tropical freshwater alga (Chlorella sp.).

Copper (Cu) and uranium (U) are of potential ecotoxicological concern to tropical freshwater organisms in northern Australia as a result of mining activity. No local data on the toxicity of these metals to tropical freshwater algae are currently available. The aim of this study was to investigate the effect of pH (5.7 and 6.5) on the toxicity of Cu and U to the green alga Chlorella sp. in a synthetic softwater representative of fresh surface waters in sandy-streams of tropical northern Australia. The effects of Cu and U on algal growth (cell division) rate after a 72-h exposure were determined. Intracellular and extracellular (membrane-bound) metal concentrations at the two selected pH values were also compared. Based on the 72-h minimum detectable effect concentrations (MDEC), Chlorella sp. was approximately 20-fold more sensitive to Cu (0.7 and 1.4 µg l(-1) at pH 6.5 and 5.7, respectively) than U (13 and 34 µg l(-1) at pH 6.5 and 5.7, respectively), and more sensitive than other Australian tropical freshwater organisms. The toxicity of Cu and U was highly pH-dependent. Copper concentrations required to inhibit growth (cell division) rate by 50% (72-h EC(50)) increased from 1.5 to 35 µg l(-1) as the pH decreased from 6.5 to 5.7. Similarly, the 72-h EC(50) values for U increased from 44 to 78 µg l(-1) over the same pH range. Calculation of Cu and U speciation using the geochemical model HARPHRQ, showed that differences in the concentrations of the free metal ions (Cu(2+) and UO(2)(2+)) were only minimal (<10%) between pH 5.7 and 6.5. The decreased toxicity at pH 5.7 was due to lower concentrations of cell-bound and intracellular Cu and U compared to those at pH 6.5. These results are explained in terms of the possible mechanism of competition between H(+) and the metal ion at the cell surface.

Uranium Speciation and Bioavailability in Aquatic Systems: An Overview

The speciation of uranium (U) in relation to its bioavailability is reviewed for surface waters (fresh- and seawater) and their sediments. A summary of available analytical and modeling techniques for determining U speciation is also presented. U(VI) is the major form of U in oxic surface waters, while U(IV) is the major form in anoxic waters. The bioavailability of U (i.e., its ability to bind to or traverse the cell surface of an organism) is dependent on its speciation, or physicochemical form. U occurs in surface waters in a variety of physicochemical forms, including the free metal ion (U or UO2) and complexes with inorganic ligands (e.g., uranyl carbonate or uranyl phosphate), and humic substances (HS) (e.g., uranyl fulvate) in dissolved, colloidal, and/or particulate forms. Although the relationship between U speciation and bioavailability is complex, there is reasonable evidence to indicate that UO2 and UO2OH are the major forms of U(VI) available to organisms, rather than U in strong complexes (e.g., uranyl fulvate) or adsorbed to colloidal and/or particulate matter. U(VI) complexes with inorganic ligands (e.g., carbonate or phosphate) and HS apparently reduce the bioavailability of U by reducing the activity of UO2 and UO2OH. The majority of studies have used the results from thermodynamic speciation modeling to support these conclusions. Time-resolved laser-induced fluorescence spectroscopy is the only analytical technique able to directly determine specific U species, but is limited in use to freshwaters of low pH and ionic strength. Nearly all of the available information relating the speciation of U to its bioavailability has been derived using simple, chemically defined experimental freshwaters, rather than natural waters. No data are available for estuarine or seawater. Furthermore, there are no available data on the relationship between U speciation and bioavailability in sediments. An understanding of this relationship has been hindered due to the lack of direct quantitative U speciation techniques for particulate phases. More robust analytical techniques for determining the speciation of U in natural surface waters are needed before the relationship between U speciation and bioavailability can be clarified.

Influence of genetic and environmental factors on the tolerance of Daphnia magna Straus to essential and non-essential metals.

The ultimate aim of ecotoxicological studies is to predict how natural populations respond to contaminant exposure. Thus, it is crucial to understand how genetic and environmental factors in the field modify responses measured in the laboratory. In the present study the authors determine the genetic and environmental components of variability in acute responses among four Daphnia magna clones exposed to both essential (Zn and Cu) and non-essential (Cd and U) metals in waters with varying water hardness. The authors postulate that genotype acute responses to physiologically non-essential metals may be more variable than responses to essential metals and that this may be explained by hypothesizing that acute responses to non-essential substances are subject to intermittent selection (since the substances may not always be present in biologically significant amounts), whereas responses to essential substances are subject to continuous directional selection (since essential substances by definition co-occur with biota in biologically significant amounts). D. magna clones were exposed to single-metal solutions of varying concentration at two or three levels of water hardness (soft, moderate–hard and hard) for periods ranging from 12–96 h (12 h increments). LC50 values for each metal×genotype×water hardness×exposure period combination were determined for (i) total metal concentration and (ii) the free hydrated metal ion concentration (predicted using geochemical speciation modeling) and compared using analysis of covariance with environment (water hardness) and genotype (clone) as fixed factors and exposure time as a covariate. The results showed that Zn–Cd were consistent, but Cu–U inconsistent, with the essentiality hypothesis. In addition, the small, or even non-existent, genotype-environment interaction effect on the inheritance of metal tolerance supports the argument that genetic variability in resistance to trace metals is inherited through single major genes. Implications of these results for the prediction of population tolerance ranges are discussed in relation to the problem of predicting metal speciation and the biological effects of metals in fresh waters with varying hardness.

Evaluation of the free ion activity model of metal-organism interaction: extension of the conceptual model

The present study integrates the concepts of the free ion activity model (FIAM) into biological receptor theory (BRT; i.e. pharmacodynamic principles) to obtain a more rigorous conceptual model; one that more precisely quantifies the interaction of chemical species at biological receptor sites. The developed model, which is viewed as an extended FIAM, explains the conditions under which the FIAM will be effective in explaining biological response (BR). It establishes that BR is directly proportional to the activity of the free metal ion in the linear regions of concentration-response curves only. Additionally, it indicates that [X-cell], the activity of free surface sites on the cell membrane, does not need to be constant in the region of BR, as assumed by the original FIAM. The extended FIAM was tested by re-examining concentration-response data from the literature on aquatic organisms exposed to several ecotoxicologically-relevant trace metals. These data, which would be considered exceptions to the original FIAM, were found to be consistent with the extended FIAM. Due to its more rigorous conceptual basis, the extended FIAM is capable of modelling concentration-response experiments from a wider range of water chemistry conditions (i.e. varying pH, hardness and dissolved organic matter) than the original model and, as such, potentially provides a more useful tool for evaluating metal-organism interactions. This study proposes, for the first time, a quantitative method of uncoupling the biological effects of a metal hydroxide (1:1) complex from that of amelioration of the free metal ion (M(z+)) by H(+). Since the activities of H(+) and metal-hydroxide cannot be independently varied, it has been previously very difficult to evaluate whether metal-hydroxide species contribute to eliciting a BR. Furthermore, the extended FIAM can directly derive fundamental information from concentration-response curves, such as the binding constants of H(+) or the hardness cations (Ca(2+) and/or Mg(2+)) to the cell membrane surface of aquatic organisms.

Absorption of divalent trace metals as analogues of calcium by Australian freshwater bivalves: an explanation of how water hardness reduces metal toxicity

Abstract A competitive inhibition experimental design, incorporating radiotracer labelling of metals and the geochemical simulation of their speciation at two varying Ca water concentrations, was employed to conclusively demonstrate that the divalent trace metals Pb, Mn, Cd and Co, were absorbed from the aquatic medium as metabolic analogues of Ca by two species of Australian freshwater bivalves (Hyridella depressa and Velesunio ambiguus). Several important implications stem from this mechanistic interpretation of metal uptake by aquatic organisms. Because of the general positive empirical relationship established between metal uptake/accumulation and acute/sub-chronic toxicity, the ameliorative effect of an increased water hardness on metal toxicity most likely results from the competitive binding of Ca ( > Mg) at the Ca channels of the cell membrane. This conclusion is consistent with empirical studies and also with the basic chemical properties of Ca and Mg, that are relevant to their behaviour at the Ca channel. It follows that Ca water concentration, rather than total water hardness, should be utilised in water quality guidelines as the variable that governs the maximum permissible concentration of certain trace metals that can be sustained by freshwater life.

Evaluation of Criteria Used to Assess the Quality of Aquatic Toxicity Data

Abstract Good quality toxicity data underpins robust hazard and risk assessments in aquatic systems and the derivation of water quality guidelines for ecosystems. Hence, an objective scheme to assess the quality of toxicity data forms an important part of this process. The variation of scores from 2 research papers using the Australasian ecotoxicity database (AED) quality assessment scheme was evaluated by 23 ecotoxicologists. The results showed that the quality class that the assessors gave each paper varied by less than 10% when compared with a quality score agreed a priori between the authors of this study. It was determined that the majority of the variation in each assessment was due to ambiguous or poorly written assessment criteria, information that was difficult to find, or information in the paper that was overlooked by the assessor. This led to refinements of the assessment criteria in the AED, which resulted in a 16% improvement (i.e., reduction) in the mean variation of scores for the 2 papers when compared with the a priori scores. The improvement in consensus among different assessors evaluating the same research papers suggests that the data quality assessment scheme proposed in this article provides a more robust scheme for assessing the quality of aquatic toxicity data than methods currently available.

The relative importance of water and food as cadmium sources to Daphnia magna Straus

Knowledge of the transport pathways of metals into aquatic organisms is paramount in determining the metals potential mechanism of toxicity. To determine the relative importance of water and food as cadmium (Cd) sources for the cladoceran Daphnia magna grazing on the algae Chlorella vulgaris, we measured cadmium accumulation and toxicity (feeding inhibition and survival) in three genetically different clones of D. magna subsequent to water, food, and water and food exposures. We found that Cd uptake from water and food was independent of source and additive in effect, with D. magna juveniles accumulating twice as much Cd from water than from food (algae). However, the efficiency with which Cd was assimilated by D. magna from its algal diet was much higher (10%) than from water (0.3%). Uptake and toxic responses were inversely related: tolerant clones accumulated more Cd. As a consequence, models based on uptake of metals from the combined routes of water and food may be reliable to predict metal dynamics in the field, but may fail to predict toxic effects since tolerance to metals is not necessarily linked to reduced total uptake of metals.

The effect of water hardness on the toxicity of uranium to a tropical freshwater alga Chlorella sp.

Uranium (U) derived from mining activities is of potential ecotoxicological concern to freshwater biota in tropical northern Australia. Few data are available on the effects of water hardness (Ca and/or Mg), which is elevated in U mine wastewaters, on the toxicity and bioavailability of U to freshwater biota, particularly algae. This study determined the effect of water hardness (8, 40, 100 and 400 mg CaCO(3) x l(-1), added as calcium (Ca) and magnesium (Mg) sulphate) on the toxicity (72 h growth rate inhibition) of U to the unicellular green alga, Chlorella sp., in synthetic freshwater, at constant pH (7.0) and alkalinity (8 mg CaCO(3) x l(-1)), similar in chemical composition to sandy coastal streams in tropical northern Australia. A 50-fold increase in water hardness resulted in a 5-fold decrease (P<or =0.05) in the toxicity of U to Chlorella sp. (i.e. the 72 h EC(50) increased from 56 to 270 micro g U l(-1)). Possible explanation for the ameliorative effect of water hardness includes: (i) competition between U and Ca and/or Mg for binding sites on the cell surface; and (ii) a change in U speciation, and hence, bioavailability. Results showed that extracellular (cell-surface) and intracellular U concentrations significantly (P<0.05) decreased (2-5-fold) as water hardness increased from 8 to 400 mg CaCO(3)x l(-1). Calculation of U speciation using the geochemical model HARPHRQ showed that there were no significant (P>0.05) differences in the predicted speciation (% distribution) of U amongst the four water hardness levels. The reduction in U toxicity with increasing water hardness was most likely due to competition between U and Ca and/or Mg for binding sites on the algal cell surface. The minimum detectable effect concentrations of U were approximately 3 and 24 times higher (at 8 and 400 mg CaCO(3)x l(-1) hardness, respectively) than the national interim U guideline value (0.5 micro g x l(-1)) for protecting aquatic ecosystems. Overall, the results reinforce the need for a more flexible U guideline based on a hardness-dependent algorithm, which may allow environmental managers to relax the national guideline for U on a site-specific basis.

Influence of water chemistry on the acute toxicity of copper and zinc to the cladoceran Ceriodaphnia cf dubia.

This study determined the influence of key water chemistry parameters (pH, alkalinity, dissolved organic carbon [DOC], and hardness) on the aqueous speciation of copper and zinc and its relationship to the acute toxicity of these metals to the cladoceran Ceriodaphnia cf dubia. Immobilization tests were performed for 48-h in synthetic or natural waters buffered at various pH values from 5.5 to 8.4 (other chemical parameters held constant). The toxicity of copper to C. cf dubia decreased fivefold with increasing pH, whereas the toxicity of zinc increased fivefold with increasing pH. The effect of DOC on copper and zinc toxicity to C. cf dubia was determined using natural fulvic acid in the synthetic water. Increasing DOC was found to decrease linearly the toxicity of copper, with the mean effect concentration of copper that immobilized 50% of the cladocerans (EC50) value 45 times higher at 10 mg/L, relative to 0.1 mg/L DOC at pH 6.5. In contrast, the addition of 10 mg/L DOC only resulted in a very small (1.3-fold) reduction in the toxicity of zinc to C. cf dubia. Copper toxicity to C. cf dubia generally did not vary as a function of hardness, whereas zinc toxicity was reduced by a factor of only two, with an increase in water hardness from 44 to 374 mg CaCO3/L. Increasing bicarbonate alkalinity of synthetic waters (30-125 mg/L as CaCO3) decreased the toxicity of copper up to fivefold, which mainly could be attributed to the formation of copper-carbonate complexes, in addition to a pH effect. The toxicity of copper added to a range of natural waters with varying DOC content, pH, and hardness was consistent with the toxicity predicted using the data obtained from the synthetic waters.

Divalent metal accumulation in freshwater bivalves: an inverse relationship with metal phosphate solubility.

Whole soft tissue concentrations of Mn, Co, Ni, Cu, Zn, Pb, Cd and U were measured in two species of freshwater (unionid) bivalves (Hyridella depressa and Velesunio ambiguus) from a minimally polluted site in the Hawkesbury-Nepean River, south-eastern Australia. Although the mean concentrations of metals in the tissue were similar for each bivalve species, their patterns of accumulation were dissimilar. For each metal, positive linear relationships between tissue concentration and shell length (r2 = 0.37-0.77; P < or = 0.001) and tissue dry weight (r2 = 0.29-0.51; P < or = 0.01) were found in H. depressa, but not in V. ambiguus. However, for both species, positive linear relationships were found between the tissue concentration of each divalent metal and Ca tissue concentration (r2 = 0.59-0.97; P < or = 0.001). For both bivalve species, the normalised rates of accumulation of the metals relative to increasing Ca concentration and/or size, were U approximately = Cd > or = Pb > or = Mn > Co > or = Zn > Cu > Ni. The differential rates of accumulation of divalent metals are interpreted as being predominantly governed by their varying loss rates, which are controlled by the differing solubilities (log Ksp values) of the metals in the phosphatic extracellular granules, the demonstrated major sites of metal deposition in the tissue of H. depressa and V. ambiguus. The rates of accumulation of Mn, Co, Zn, Cu and Ni were linearly and inversely related (r2 = 0.91-0.97; P < or = 0.001) to their solubilities as hydrogen phosphates, a finding consistent with the bioaccumulation model previously developed for the alkaline-earth metals. However, for U, Cd and Pb, this linear inverse relationship did not continue to hold, i.e. their rates of accumulation did not increase with decreasing solubility. However, these results are still consistent with the model if U, Cd and Pb are so insoluble in the granules of H. depressa and V. ambiguus over their lifetime (up to approx. 50 years) that there is effectively no loss of these metals, and hence, no differential between their rates of accumulation. The present results reaffirm the use of Ca tissue concentration to predict the tissue concentrations of other divalent metals by explaining up to 94 and 97% of the variability between individual bivalves of H. depressa and V. ambiguus, respectively. The use of Ca tissue concentration to effectively minimise the inherent variability between individuals in their metal tissue improves the ability of an investigator to discern smaller spatial and/or temporal differences in the metal tissue concentrations of these bivalves, and thus to detect metal pollution.