Volatilization of mercury by an iron oxidation enzyme system in a highly mercury-resistant acidithiobacillus ferrooxidans strain mon-1

Abstract

A highly mercury-resistant strain Acidithiobacillus ferrooxidans MON-1, was isolated from a culture of a moderately mercury-resistant strain, A. ferrooxidans SUG 2-2 (previously described as Thiobacillus ferrooxidans SUG 2-2), by successive cultivation and isolation of the latter strain in a Fe2+ medium with increased amounts of Hg2+ from 6 μM to 20 μM. The original stain SUG 2-2 grew in a Fe2+ medium containing 6 μM Hg2+ with a lag time of 22 days, but could not grow in a Fe2+ medium containing 10 μM Hg2+. In contrast, strain MON-1 could grow in a Fe2+ medium containing 20 μM Hg2+ with a lag time of 2 days and the ability of strain MON-1 to grow rapidly in a Fe2+ medium containing 20 μM Hg2+ was maintained stably after the strain was cultured many times in a Fe2+ medium without Hg2+. A similar level of NADPH-dependent mercury reductase activity was observed in cell extracts from strains SUG 2-2 and MON-1. By contrast, the amounts of mercury volatilized for 3 h from the reaction mixture containing 7 μM Hg2+ using a Fe2+-dependent mercury volatilization enzyme system were 5.6 nmol for SUG 2-2 and 67.5 nmol for MON-1, respectively, indicating that a marked increase of Fe2+-dependent mercury volatilization activity conferred on strain MON-1 the ability to grow rapidly in a Fe2+ medium containing 20 μM Hg2+. Iron oxidizing activities, 2,3,5,6-tetramethyl-p-phenylenediamine (TMPD) oxidizing activities and cytochrome c oxidase activities of strains SUG 2-2 and MON-1 were 26.3 and 41.9 μl O2 uptake/mg/min, 15.6 and 25.0 μl O2 uptake/mg/min, and 2.1 and 6.1 mU/mg, respectively. These results indicate that among components of the iron oxidation enzyme system, especially cytochrome c oxidase activity, increased by the acquisition of further mercury resistance in strain MON-1. Mercury volatilized by the Fe2+-dependent mercury volatilization enzyme system of strain MON-1 was strongly inhibited by 1.0 mM sodium cyanide, but was not by 50 nM rotenone, 5 μM 2-n-heptyl-4-hydroxy-quinoline-N-oxide (HQNO), 0.5 μM antimycin A, or 0.5 μM myxothiazol, indicating that cytochrome c oxidase plays a crucial role in mercury volatilization of strain MON-1 in the presence of Fe2+. © 2003 by Japan Society for Bioscience, Biotechnology, and Agrochemistry.