Synergism between nickel and copper in their toxicity to microbes: mediation by pH.

The toxicity of a combination of nickel (Ni) and copper (Cu) toward growth of heterotrophic microorganisms was greater than the sum of the toxicity of each metal individually. This synergism between Ni and Cu was potentiated at acidic pH levels, with the potentiation of the toxicity being, for some organisms, an effect of the acidic pH on the toxicity of the Ni, rather than of the Cu, component. For others, however, the potentiation was an effect of acidic pH on both the Ni and Cu components. The potentiation of the Ni-Cu synergistic interaction at acidic pH levels has relevance to the deposition of acid precipitation into environments contaminated with these metals. Furthermore, the occurrence of such synergistic interactions and their mediation by pH should be considered in the methodologies used to establish criteria for tolerable levels of metals in the environment.