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(79-13) Importance of pH in metal toxicity testing with algae: chemical and physiological considerations. Heijerick, Dagobert*,1, De Schamphelaere, Karel1, Janssen, Colin1, 1 Laboratory of Environmental Toxicology, J. Plateaustraat 22, Ghent, Belgium ABSTRACT- Metal toxicity in the aquatic environment is strongly determined by the pH and the corresponding metal speciation. Since the free ion is considered to be the most bioavailable species, increased pH-levels are expected to decrease metal toxicity. Recent studies with zinc and copper, however, have demonstrated that this is not the case for the algae Raphidocelis subcapitata: an increase of pH from 6 to 8 increased the toxicity of dissolved zinc and copper in ISO-medium with a factor of 10 and 4, respectively. (Heijerick et al., 2002; De Schamphelaere et al., 2002). In this study the effect of pH (from 5 to 9) on the toxicity of various metals is investigated: cadmium, cobalt, nickel, lead, copper and zinc, with the emphasis on the latter two metals. Observed 72h-EbC50s (in Key words: raphidocelis subcapitata, metal toxicity, pH, binding capacity |
g/L Medissolved) decreased when pH increased from 5 to 8. The observed relationships between the free metal activity at the 72h-EbC50s and the pH indicate that this phenomenom cannot be attributed to proton-competition only: additional adsorption experiments suggests that an elevated number of binding sites on the cell wall at higher pH-levels also contributes to the observed changes in sensitivity. These findings, together with initial observations that binding constants of metals and competing cations (Ca, Mg) are also pH-dependent, are in accordance with previously reported results. At pH 9, the speciation effect (further decrease of the free metal ion fraction) becomes more important than the biological effects (increased number of binding sites, increased binding constants), resulting in decreased metal toxicity. The results of this study demonstrate that pH affects metal toxicity to algae through chemical and biological processes which should be taken into account when toxicity data from this group of organisms are used in risk assessments.