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(70-44) Biotic Ligand Model with zinc for the algae Raphidocelis subcapitata: possibilities and limitations. Heijerick, Dagobert*,1, De Schamphelaere, Karel1, Janssen, Colin1, 1 Laboratory of Environmental Toxicology, J. Plateaustraat 22, Ghent, Belgium ABSTRACT- Biotic ligand models have been developed for various metals (Cu, Ag, Zn) and organisms (fish and invertbrates). These models incorporate the effect of major cations (Ca, Mg, Na) and physico-chemical water characteristics (pH, dissolved organic carbon) on metal bioavailability and toxicity. In this study, the individual effects of calcium, magnesium, potassium, sodium and pH on zinc toxicity to the green alga Raphidocelis subcapitata were studied. Stability constants for binding to algal cells (KBL) were derived for those cations affecting zinc toxicity, using the mathematical approach that was proposed by De Schamphelaere and Janssen (2001). Potassium proved to be the only cation tested that did not alter zinc toxicity to algae significantly. Initial derived Log(KBL) for Ca, Mg and Na to the biotic ligand at pH 7.5 were 3.2, 3.9 and 2.8, respectively. The results of the toxicity tests at different pH-levels (5.5 to 8.0) indicate that competition beween H+ and Zn2+ reduces zinc toxicity. However, the observed relationship between (H+) and the 72h-EbC50 (expressed as Key words: raphidocelis subcapitata, biotic ligand model, zinc toxicity |
M (Zn2+)) is not linear and suggests that pH affects the physiology of the biotic ligand (cell wall). Although this study indicates that zinc toxicity to algae can be modelled based on key water characteristics, the results also suggest that the part of the conventional BLM-hypothesis, i.e. that the binding characteristics of the biotic ligand (cell wall) are independent of the test medium characteristics, is not valid for algae. This (pH-dependent) change of stability contants and number of binding sites should therefore be incorporated in future BL-modelling efforts with algae.