Effect of pH on Cu(II) Toxicity in Rapid Bioassays for Paramecium caudatum Ehrenberg, 1838 and Escherichia coli Migula, 1895

Abstract The effect of pH (neutral and slightly alkaline) on the copper ions toxicity in the concentration range of 0.005–0.15 mg/L was studied for two test objects: Escherichia coli (bioluminescence reaction) and Paramecium caudatum (chemotaxis reaction). It was shown that the toxic effect depends on the pH level. A slightly alkaline medium enhances the toxic effect on E. coli with inhibition of bioluminescence occurring at 0.05 mg/L. In a neutral environment, inhibition of the test response occurs only at 0.1 mg/L, while stimulation is recorded at low concentrations (0.005–0.025 mg/L) for both types of water. For P. caudatum, a neutral medium proved to be significantly more toxic: in the 0.01–0.15 mg/L range, the toxicity index was 1.4 to 3.9 times higher than in a slightly alkaline medium. At high concentrations (0.1–0.15 mg/L), each test showed increased toxicity in neutral water compared to slightly alkaline water. The study results highlight the need to consider the pH of the medium when selecting a test object for copper-contaminated waters bioassay.