Effects of ferrous sulfate, inoculum history, and anionic form on lead, zinc, and copper toxicity to Acidithiobacillus caldus strain BC13.

The current study reports the single and combined toxicities of Pb, Zn, and Cu to Acidithiobacillus caldus strain BC13. The observed half-maximal inhibitory concentrations (IC50), ± 95% confidence intervals, for Pb, Zn, and Cu were 0.9 ± 0.1 mM, 39 ± 0.5 mM, and 120 ± 8 mM, respectively. The observed minimum inhibitory concentrations (MIC) for Pb, Zn, and Cu were 7.5 mM, 75 mM, and 250 mM, respectively. When metals were presented in binary mixtures, the toxicities were less than additive. For example, when 50% of the Pb MIC and 50% of the Cu MIC were presented together, the specific growth rate was inhibited by only 59 ± 3%, rather than 100%. In addition, the presence of ferrous iron in the growth media decreased Pb and Zn toxicity to A. caldus strain BC13. The importance of inoculum history was evaluated by pre-adapting cultures through subsequent transfers in the presence of Pb, Zn, and Cu at their respective IC50s. After pre-adaptation, cultures had specific growth rates 39 ± 11, 32 ± 7, and 28 ± 12% higher in the presence of Pb, Zn, and Cu IC50s, respectively, compared with cultures that had not been pre-adapted. In addition, when cells exposed to the MICs of Pb, Zn, and Cu were harvested, washed, and re-inoculated into fresh, metal-free medium, they grew, showing that the cells remained viable with little residual toxicity. Finally, metal chlorides showed more toxicity than metal sulfates, and studies using sodium chloride or a mixture of metal sulfates and sodium chloride suggested that this was attributable to an additive combination of the metal and chloride toxicities.