A Biotic Ligand Model Predicting Acute Copper Toxicity for Daphnia magna:  The Effects of Calcium, Magnesium, Sodium, Potassium, and pH

Abstract

The extent to which Ca2+, Mg2+, Na+, K+ ions and pH independently mitigate acute copper toxicity for the cladoceran Daphnia magna was examined. Higher activities of Ca2+, Mg2+, and Na+ (but not K+) linearly increased the 48-h EC50 (as Cu2+ activity), supporting the concept of competitive binding of these ions and copper ions to toxic action or transport sites at the organism-water interface (e.g. fish gill, the biotic ligand). The increase of the EC50 (as Cu2+ activity) with increasing H+, however, seemed to suggest cotoxicity of CuOH+ rather than proton competition. Based on the biotic ligand model (BLM) concept, we developed a methodology to estimate stability constants for the binding of Cu2+, CuOH+, Ca2+, Mg2+, Na+, and H+ to the biotic ligand, solely based on toxicity data. Following values were obtained: log K(CuBL) = 8.02, log K(CuOHBL)= 7.45, log K(CaBL) = 3.47, log K(MgBL) = 3.58, log K(NaBL) = 3.19, and log K(HBL) approximately 5.4. Further, we calculated that on average 39% of the biotic ligand sites need to be occupied by copper to induce a 50% acute effect for D. magna after 48 h of exposure. Using the estimated constants, a BLM was developed that can predict acute copper toxicity for D. magna as a function of water characteristics. The presented methodology can easily be applied for BLM development for other organisms and metals. After validation with laboratory and natural waters (including DOC), the developed model will support efforts to improve the ecological relevance of presently applied risk assessment procedures.

Temporal assessment of copper speciation, bioavailability and toxicity in UK freshwaters using chemical equilibrium and biotic ligand models: Implications for compliance with copper environmental quality standards

“…This is in accordance with the commonly reported effects of these major cations on metal toxicity to aquatic organisms and probably reflect the competition between these ions and the free copper ion for binding on the cell surfaces (Playle et al, 1993;Campbell, 1995;Di Toro et al, 2001;De Schamphelaere and Janssen, 2002). Occupation of the sites by calcium and/or magnesium would result in normal membrane function, while occupation of the sites by copper would disrupt function as toxicity is considered to occur when a critical proportion of the sites are occupied by copper.…”

Predicting availability of mineral elements to plants with the DGT technique: a review of experimental data and interpretation by modelling

“…The BLM has been successfully applied to predict metal toxicity for a variety of aquatic organisms and metals (e.g. refs [40,41]). …”

Modeling toxicity of binary metal mixtures (Cu2+–Ag+, Cu2+–Zn2+) to lettuce, Lactuca sativa, with the biotic ligand model

“…According to the BLM, f M is determined based on assumed independence of the complexation capacity of ions on water quality characteristics [14,16]. The calculation of f M following exposure to metal mixtures was based on the stability constant K MBL determined from the toxicological data following exposure to single metals as described below.…”