Influence of Complexation and pH on Individual and Combined Heavy Metal Toxicity to a Freshwater Green Alga

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Abstract
The effect of complexation and pH on heavy metal (Cu, Zn, Pb) toxicity to a freshwater green alga, Scenedesmus quadricauda, was investigated. Extracellular ligands produced by S. quadricauda were capable of binding heavy metals and reducing their single and combined toxicities. Apparent complexing capacities and the ability of the sediment humics and artificial complexing agents such as ethylenediaminetetraacetic acid (EDTA), citric acid, and glycolic acid to ameliorate Cu, Zn, or Pb toxicity were also assessed. The toxicity of metals to algal growth was enhanced at acidic pH. Combined toxicity of these metals was significantly greater at pH 4.5 than at pH 8.5 or pH 6.5. Synergistic effects (between Cu, Zn, and Pb) towards algal growth increased at low pH. Specific heavy metals, their respective concentrations, the presence of complexing ligands, and pH influence both individual and combined heavy metal toxicities.