Ferrous iron-dependent volatilization of mercury by the plasma membrane of Thiobacillus ferrooxidans

Of 100 strains of iron-oxidizing bacteria isolated, Thiobacillus ferrooxidans SUG 2-2 was the most resistant to mercury toxicity and could grow in an Fe2+ medium (pH 2.5) supplemented with 6 mu M Hg2+. In contrast, T. ferrooxidans AP19-3, a mercury-sensitive T. ferrooxidans strain, could not grow with 0.7 mu M Hg2+. When incubated for 3 h in a salt solution (pH 2.5) with 0.7 mu M Hg2+, resting cells of resistant and sensitive strains volatilized approximately 20 and 1.7%, respectively, of the total mercury added. The amount of mercury volatilized by resistant cells, but not by sensitive cells, increased to 62% when Fe2+ was added. The optimum pH and temperature for mercury volatilization activity were 2.3 and 30 degrees C, respectively. Sodium cyanide, sodium molybdate, sodium tungstate, and silver nitrate strongly inhibited the Fe2+-dependent mercury volatilization activity of T, ferrooxidans. When incubated in a salt solution (pH 3.8) with 0.7 (mu M Hg2+ and 1 mM 1 Fe2+ plasma membranes prepared from resistant cells volatilized 48% of the total mercury added after 5 days of incubation. However, the membrane did not have mercury reductase activity with NADPH as an electron donor. Fe2+-dependent mercury volatilization activity was not observed with plasma membranes pretreated with 2 mM sodium cyanide. Rusticyanin from resistant cells activated iron oxidation activity of the plasma membrane and activated the Fe2+-dependent mercury volatilization activity of the plasma membrane.