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PT09 Biotic Ligand Model
(PT162) Applicability and limitations of the biotic ligand model to freshwater microalgae.
Stauber, J1, Wilde, K2, Markich, S2, Franklin, N3, 1 CSIRO Sydney, NSW, Australia2 ANSTO Sydney, NSW, Australia3 McMaster University, Hamilton, Canada
ABSTRACT- To provide a more mechanistic basis for predicting the toxicity of metals to aquatic biota, the biotic ligand model (BLM) has been developed and applied to fish, invertebrates and more recently, microalgae. Several key assumptions of the BLM, including that metal-cell binding does not alter the characteristics of the biotic ligand and that toxicity is proportional to membrane-bound metal, may not hold true for some metals and metal mixtures and algae. As part of a larger study to test the BLM with unicellular algae, the effect of pH (6.0 – 7.5), hardness (40–400 mg CaCO3/L) and dissolved organic carbon on the toxicity and accumulation of Cu and Zn in the freshwater alga Chlorella sp was investigated. To avoid the problems of metal depletion in solution in standard static bioassays due to adsorption losses to algae at high cell densities, low initial cell densities (103 cells/mL) were used, with counting by flow cytometry. Measurements of intracellular and extracellular Cu confirmed that metal cell binding and growth inhibition increased as the pH increased, supporting current BLM theory of competition between copper and protons (H+) at the cell surface. For zinc however, the binding characteristics were not independent of the test medium, suggesting that the conventional BLM is not valid for all metals. In addition, mixtures of Cu and Cd were shown to alter algal membrane permeability, thereby violating the BLM assumption that metal-cell binding does not alter the nature of the biotic ligand itself.
Key words: metals, algae, biotic ligand model
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