Isolation and some properties of Thiobacillus ferrooxidans strains with differing levels of mercury resistance from natural environments

Fifty iron-oxidizing bacteria isolated from natural environments were screened for resistance to mercuric ions (Hg²⁺). Thiobacillus ferrooxidans Funis 2-1, the strain found to show the greatest resistance to Hg²⁺ among the fifty isolates, gave a cell yield of 7.0 x 10⁷ cells/ml after 8 d cultivation in an Fe²⁺-medium (pH 2.5) containing 0.7 μM Hg²⁺. Funis 2-1 volatilized 80% of the total mercury added to the medium over 8 d of cultivation. T. ferrooxidans AP19-3, more sensitive to Hg²⁺ than Funis 2-1, could not grow in an Fe²⁺-medium (pH 2.5) containing 0.7 μM Hg²⁺ even over a 28 d cultivation period. When resting cells of strains Funis 2-1 and AP19-3 were incubated for 3 h in a salt solution containing 0.7 μM Hg²⁺ (pH 3.0), 14.3% and 7.9% of the total mercury added to the reaction mixtures respectively, were volatilized. The activity of the mercuric reductase from Funis 2-1 was only 2.8 times higher than that of the enzyme from AP19-3. Since the markedly higher mercury resistance of Funis 2-1 compared with that of AP19-3 cannot be explained only by the level of the mercuric reductase activity, the levels of mercury resistance of iron oxidase and cytochrome c oxidase were studied. The 1 μM mercuric ions inhibited the 35% of iron- oxidizing activity from AP19-3. In contrast, the same concentration of Hg²⁺ did not inhibit the activity of iron oxidase from Funis 2-1. In the case of the cytochrome c oxidases purified from both strains, the 0.2 μM Hg²⁺ inhibited approximately 40% of cytochrome c oxidizing activity from AP19-3, on the contrary, the activity of the enzyme from Funis 2-1 was activated 1.8- and 1.2-fold, respectively, in the presence of 0.08 and 0.2 μM Hg²⁺. Since cytochrome c oxidase is one of the most important components of the iron- oxidizing system, these results indicate that both the existence of cytochrome c oxidase resistant to Hg²⁺ as well as that of mercuric reductase in the cells is responsible for the more rapid growth of Funis 2-1 than that of in an Fe²⁺-medium containing 0.7 μM Hg²⁺.