Active transport, substrate specificity, and methylation of Hg(II) in anaerobic bacteria

Abstract

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 FeRB Geobacter sulfurreducens and the SRB Desulfovibrio desulfuricans ND132 as model organisms, we demonstrate that Hg(II) uptake occurs by active transport. We also establish that Hg(II) uptake by G. 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 of D. desulfuricans has a higher affinity than that of G. 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. , methylmercury, MeHg) is a potent neurotoxic compound (1). It is biomagnified in the food webs of aquatic systems, reaching high concentrations in carnivorous fish, thus posing a risk to human health (2). Understanding the mechanism of inorganic Hg methylation and the parameters that control the extent of methylation in the environment is thus essential for relating patterns of Hg pollution to human exposure. The production of MeHg has been linked to obligate anaerobic bacteria in the δ-Proteobacteria, including ironand sulfate-reducing bacteria (FeRB and SRB) that live in soil and sediments (3-6). Although mechanisms of Hg(II) methylation by methylating enzymes have been proposed for some time (7,8), the mechanism of Hg(II) uptake by the bacteria has remained obscure. The dominant view is that cellular uptake occurs by passive diffusion of neutral Hg(II) complexes, particularly sulfide complexes, through external membranes, leading to accidental methylation of some of the intracellular Hg(II) (9). However, this view is based on indirect data and modeling, as the precipitation of metal sulfides in the medium and the extensive Hg binding to the surface of the organisms (10-12) have made it difficult to directly measure Hg(II) uptake in methylating bacteria.In previous work (13), we demonstrated that the cysteine complex of Hg(II) was available to the FeRB Geobacter sulfurreducens PCA and that Hg(II) was likely transported into the cell via an unknown facilitated transport mechanism. Here we examine the energy dependence and specificity of Hg(II) uptake and methylation by both G. sulfurreducens and ...

Stoichiometry and kinetics of mercury uptake by photosynthetic bacteria

“…It has been revealed that Hg(II) uptake in anaerobic bacteria is an active transport process requiring energy and not a passive process as commonly perceived (Schaefer et al 2011). The question can be asked whether cellular Hg uptake is specific for Hg(II), or accidental, occurring via some essential metal importer.…”

Linkage between community diversity of sulfate-reducing microorganisms and methylmercury concentration in paddy soil

“…Inorganic mercury (Hg 2+ ) can be transformed into highly neurotoxic methylmercury (MeHg) via microbial processes that involve sulfate or iron reduction in anaerobic environments (Pak and Bartha 1998;Schaefer et al 2011;Yu et al 2012;Hsu-Kim et al 2013). Most Hg in soil is detected as ionic Hg 2+ species and shows relatively low toxicity, however, it still poses some environmental risk due to potential for Hg methylation followed by bioamplification in soilplant systems (Meng et al 2010;Liu et al 2012).…”

Recent advances in the study of mercury methylation in aquatic systems

“…In addition, legacy Hg deposits currently sequestered in sediments, wetland soils, permafrost, and forests may become mobile under a number of scenarios such as forest fires, and the disturbance of soils, sediment, and permafrost, thus increasing the inorganic Hg available for MeHg production. However, the uptake of this critical substrate has been found to be limiting in the methylation process, especially in the absences of certain complexing thiols (Schaefer et al 2011). Hsu-Kim et al (2013b provide an excellent review of the factors controlling bioavailable Hg and summarize four main hypotheses for the uptake through both outer and inner membranes.…”