Toxic effects of Hg 2+ to a Hg 2+-resistant Pseudomonas in the presence of halide ions

Abstract

A Hg 2+-resistant Pseudomonas which can utilize glycine as a sole carbon and nitrogen source was isolated from a municipal sewage treatment plant, and the toxicity of Hg 2+ in the presence of halide ions was examined. In glycine medium without halide ions, bacterial growth was scarcely influenced even at an initial Hg 2+ concentration of 25 μM. The viable cell count decreased when adsorbed Hg 2+ was more than 300 μmol·g-cell −1. The toxicity of Hg 2+ was enhanced when the halide ion concentration was such that the [HgX 2] 0 complex ( X=Br − or I −) in the solution became dominant. In the presence of [HgBr 2] 0 complex, complete growth inhibition of unacclimatized cells was observed at 2–3 μM Hg 2+. A sudden decrease in the viable cell count was evident when adsorbed Hg 2+ was more than 15 μmol·g-cell −1. Growth of the isolate pre-incubated in glycine medium containing 25 μM Hg 2+ (acclimatized cells) was also inhibited completely at 10 μM Hg 2+ in the presence of Br −. The adsorption and toxicity of Hg 2+ decreased with increasing halide ion concentration in which [HgBr 3] − or [HgBr 4] 2− complexes exist dominantly. These results indicate that toxicity of Hg 2+ was enhanced by the formation of [HgX 2] 0 complex and was not governed by the amount of Hg 2+ adsorbed to the cells.