Evaluating combined toxicity of binary heavy metals to the cyanobacterium Microcystis: A theoretical non-linear...

A theoretical non-linear combined toxicity assessment method is proposed and evaluated using Microcystis aeruginosa as the test organism. The combined toxicity of binary heavy metals was evaluated by comparing the actual inhibitory rates shown from the experiments with the theoretically calculated inhibitory rates. It was identified that the binary mixtures of Cu²⁺+ Cd²⁺, Cu²⁺+ Cr³⁺ and Zn²⁺+ Cr³⁺ had the synergistic effects when the combined concentrations were low, but exhibited the antagonistic effects with the higher combined concentrations. Furthermore, the toxic effect of Pb²⁺ was not influenced by the addition of Cu²⁺ when combined concentration was low, but it was enhanced by Cu²⁺ at the high combined concentration. The binary mixtures of Zn²⁺+ Cd²⁺, Pb²⁺+ Cr³⁺, Pb²⁺+ Cd²⁺, Pb²⁺+ Zn²⁺, and Cr³⁺+ Cd²⁺ always presented antagonistic effects, while the synergistic toxicity effect on M. aeruginosa was observed for the binary mixtures of Cu²⁺+ Zn²⁺ regardless of combined concentration. The proposed assessment method was also validated by the antioxidant enzyme activity, which showed synergistic or antagonistic effects under different binary mixtures of heavy metals.