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

Of 100 strains of iron-oxidizing bacteria isolated, Thiobacillus ferrooxida ns 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. ferroox idans AP19-3, a mercury-sensitive T. ferrooxidans strain, could not grow wi th 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 am ount of mercury volatilized by resistant cells, but not by sensitive cells, increased to 62% when Fe2+ was added. The optimum pH and temperature for m ercury volatilization activity were 2.3 and 30 degrees C, respectively. Sod ium cyanide, sodium molybdate, sodium tungstate, and silver nitrate strongl y inhibited the Fe2+-dependent mercury volatilization activity of T, ferroo xidans. 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 membran e 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 activate d the Fe2+-dependent mercury volatilization activity of the plasma membrane .