Author(s): E Fulladosa
The Microtox® bioassay, based upon the fading of light emitted by the luminescent bacteria Vibrio fischeri when exposed to noxious substances, was used for studying the changes in speciation and the related changes in toxicity of two metals known as environmental pollutants. It was verified that modifications of pH and of ionic composition of the incubation medium did not affect the standard toxicity of phenol. By contrast, Cr(VI) toxicity was found to decrease as pH increased, underscoring that hydrogenchromate anion, the dominant species at low pH, is the most harmful. Cr(VI) toxicity was not modified when changing the medium composition, as this metal does not form chloro-complexes in the presence of sodium chloride. Conversely, Cd(II) toxicity was almost unaffected by pH within the 5.0–7.0 range. Replacing sodium chloride either by sodium nitrate or by sodium perchlorate resulted in changes of the measured cadmium toxicity, due to changes in speciation. Free Cd2+ ion was found to be the most harmful toward the Vibrio fischeri bacteria. In conclusion, both pH and ionic composition are factors that strongly influence the measured toxicity of environmental samples containing hexavalent chromium and/or cadmium when using the Microtox® bioassay.