Toxicity of four metals and their mixtures to Pseudomonas fluorescens: An assessment using fixed ratio ray design

The toxicities of binary (Ni(II) + Co(II) and Zn(II) + Cd(II)), ternary (Zn(II) + Cd(II) + Ni(II) and Ni(II) + Co(II) + Cd(II)) and quaternary (Ni(II) + Co(II) + Zn(II) + Cd(II)) mixtures of metal ions to Pseudomonas fluorescens were assessed by using inhibition of dehydrogenase activity as an endpoint. Uniform design concentration ratio (UDCR) and equieffect concentration ratio (EECR) mixtures were designed to evaluate the combined toxicities of these heavy metal ions. All the dose-response relationships of the UDCR and EECR mixtures and the individual metals could be described by logistic function. Toxicities predicted by concentration addition (CA) and independent action (IA) models were compared with the observed toxicities. The CA and IA models predicted similar toxicities of the binary and Zn(II) + Cd(II) + Ni(II) ternary mixtures. Generally, Zn(II) + Cd(II) + Ni(II) ternary mixtures were synergistic while Ni(II) + Co(II) + Cd(II) ternary mixtures were antagonistic. The equieffect and quaternary mixtures were generally synergistic. Synergistic, antagonistic and additive effects of the mixtures are possible from the viewpoint of these analyses.