Effect of pH on the toxicity of nickel and other divalent metals to Burkholderia cepacia PR1(301).

Nickel (Ni) is a common cocontaminant at many waste sites where the soils and sediments often are acidic, thereby influencing metal availability. Growth of Burkholderia cepacia PR1(301) was not affected at 3.41 mM Ni at pH 5, but was inhibited by 73.2% at pH 6 and inhibited completely at pH 7 compared to growth without Ni. This pH effect was not observed in the Ni-resistant strains, Ralstonia metallidurans CH34 and 31A. Predicted Ni speciation did not explain the observed toxicity trends. Sorption of Ni to PR1 increased with increasing pH (1.49, 1.12, and 3.88 mg Ni/g dry weight at pH 5, 6, and 7, respectively), but was low at all three pH values, and most likely does not explain the observed pH effect. Growth inhibition of PRI with increasing pH also was observed for other divalent cations, with growth observed at 4.24 mM Co, 2.22 mM Cd, and 3.82 mM Zn at pH 5 and 6, but totally inhibited at pH 7. These studies suggest that, at circumneutral pH, PRI would be considered sensitive to Ni and other divalent cations, in spite of the ability to grow in higher concentrations at lower pH values.