The significance of pH in dictating the relative toxicities of chloride and copper to acidophilic bacteria

The ability of acidophilic bacteria to grow in the presence of elevated concentrations of cationic transition metals, though varying between species, has long been recognized to be far greater than that of most neutrophiles. Conversely, their sensitivity to both inorganic and organic anions, with the notable exception of sulfate, has generally been considered to be far more pronounced. We have compared the tolerance of different species of mineral-oxidizing Acidithiobacillus and Sulfobacillus, and the heterotrophic iron-reducer Acidiphilium cryptum, to copper and chloride when grown on ferrous iron, hydrogen or glucose as electron donors at pH values between 2.0 and 3.0. While tolerance of copper varied greatly between species, these were invariably far greater at pH 2.0 than at pH 3.0, while their tolerance of chloride showed the opposite pattern. The combination of copper and chloride in liquid media appeared to be far more toxic than when these elements were present alone, which was thought to be due to the formation of copper–chloride complexes. The results of this study bring new insights into the understanding of the physiological behaviour of metal-mobilising acidophilic bacteria, and have generic significance for the prospects of bioleaching copper ores and concentrates in saline and brackish waters.