Combined Effects of Dissolved Organic Material and Water Hardness on Toxicity of Cadmium to Daphnia Magna

Abstract

Abstract-The interaction between dissolved organic material (DOM) and water hardness and their effects on the acute toxicity of cadmium (Cd) to Daphnia magna was studied. At an original hardness (0.1 mmol Ca ϩ Mg per liter) of humic lake water (DOC 19.6 mg/L), Cd was significantly less toxic in the humic than in the reference (DOC Ͻ 0.2 mg/L) water. Furthermore, after dilution down to 10% (dissolved organic carbon [DOC] 2.0 mg/L), the humic water still decreased the lethality significantly. The results suggest that the reduced toxicity of Cd in the lake water is due to complexation with DOC. An increase in water hardness (from 0.1 to 0.5 or 2.5 mmol Ca ϩ Mg per liter) decreased the measured binding coefficient of Cd to DOM. In addition, the acute toxicity of Cd decreased, and the difference between the reference and humic water disappeared. As a conclusion, DOM in the soft lake water had a protective effect against Cd toxicity. In hard water, obviously, the added hardness cations, especially Ca 2ϩ , effectively competed with Cd 2ϩ for available binding sites in DOM. Simultaneously, Ca 2ϩ ions interfered also with the uptake of Cd 2ϩ either by competing in transport through cell membranes or by reducing membrane permeability.

Performance of standard media in toxicological assessments with Daphnia magna: chelators and ionic composition versus metal toxicity

“…, metal uptake occurs at a lower rate, because of competition for binding and transport sites (biotic ligands) at the organism-water interface (Penttinen et al 1998;Jackson et al 2000;Di Toro et al 2001;De Schamphelaere and Janssen 2002). This reduces the bioavailability of metals, and calcium seems to play the most significant role in this protective mechanism when Cd is concerned (Penttinen et al 1998;Jackson et al 2000). We have no explanation for the absence of this protective effect against Cu in ADaM, as could be expected from the conceptual and experimental data of Di Toro et al (2001) and De Schamphelaere and Janssen (2002).…”

“…and other cations. Having no specific uptake pathways, Cd and other metals enter cells via calcium-or sodium-specific transmembranar channel proteins or undefined biotic ligands (Penttinen et al 1998;Di Toro et al 2001;De Schamphelaere and Janssen 2002). At high concentrations of cationic trace elements such as Ca 2?…”

“…and Na ? , metal uptake occurs at a lower rate, because of competition for binding and transport sites (biotic ligands) at the organism-water interface (Penttinen et al 1998;Jackson et al 2000;Di Toro et al 2001;De Schamphelaere and Janssen 2002). This reduces the bioavailability of metals, and calcium seems to play the most significant role in this protective mechanism when Cd is concerned (Penttinen et al 1998;Jackson et al 2000).…”

Effects of water hardness and Cu and Zn on LC50 in Gambusia holbrooki

“…Therefore, by competing with heavy metals and blocking their access to aquatic organisms, calcium and magnesium levels are important considerations with respect to the toxic effects of heavy metals upon biota in aquatic systems (Kim et al, 2001;. According to Penttinen et al (1998) the uptake of calcium and magnesium ions by the cell membrane causes it to stabilize, and this reduces its permeability to metal ions. Water hardness reduces metal toxicity by saturating gill surface binding sites with Ca 2 þ to the exclusion of metal cations (Pyle et al, 2002).…”

Sorption of cadmium(II) and copper(II) by soil humic acids: temperature effects and sorption heterogeneity

“…Rate effect to that in soils was found in an aquatic system when humic acid, but not fulvic acid, reduced Cd and Zn toxicity to a freshwater unicellular alga [7]. In other aquatic and marine ecosystems, complexation by humic substances can reduce the toxicity of Cd [8] or Cu [9,10] to invertebrate fauna, although complexes of Cd with humic acid may be bioavailable to some (mollusc) species [9,11]. In addition, the response of humic substance-metal ion complexation reactions to changes in temperature is potentially important with respect to seasonal temperature changes in soils and aquatic systems, complexation of metal ions in hydrothermal systems and in the context of global climate change.…”