Open AccessActive transport, substrate specificity, and methylation of Hg(II) in anaerobic bacteria
Author(s) -
Jeffra K. Schaefer,
Sara S. Rocks,
Wang Zheng,
Liyuan Liang,
Baohua Gu,
François M. M. Morel
Publication year - 2011
Publication title -
proceedings of the national academy of sciences of the united states of america
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.011
H-Index - 771
eISSN - 1091-6490
pISSN - 0027-8424
DOI - 10.1073/pnas.1105781108
Subject(s) - geobacter sulfurreducens , methylmercury , methylation , chemistry , mercury (programming language) , bacteria , geobacter , desulfovibrio , biochemistry , sulfate reducing bacteria , anaerobic bacteria , efflux , environmental chemistry , biophysics , sulfate , biofilm , biology , bioaccumulation , dna , organic chemistry , computer science , genetics , programming language
The formation of methylmercury (MeHg), which is biomagnified in aquatic food chains and poses a risk to human health, is effected by some iron- and sulfate-reducing bacteria (FeRB and SRB) in anaerobic environments. However, very little is known regarding the mechanism of uptake of inorganic Hg by these organisms, in part because of the inherent difficulty in measuring the intracellular Hg concentration. By using the FeRBGeobacter sulfurreducens and the SRBDesulfovibrio desulfuricans ND132 as model organisms, we demonstrate that Hg(II) uptake occurs by active transport. We also establish that Hg(II) uptake byG. sulfurreducens is highly dependent on the characteristics of the thiols that bind Hg(II) in the external medium, with some thiols promoting uptake and methylation and others inhibiting both. The Hg(II) uptake system ofD. desulfuricans has a higher affinity than that ofG. sulfurreducens and promotes Hg methylation in the presence of stronger complexing thiols. We observed a tight coupling between Hg methylation and MeHg export from the cell, suggesting that these two processes may serve to avoid the build up and toxicity of cellular Hg. Our results bring up the question of whether cellular Hg uptake is specific for Hg(II) or accidental, occurring via some essential metal importer. Our data also point at Hg(II) complexation by thiols as an important factor controlling Hg methylation in anaerobic environments.