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Chapter 8 : Microbial Mercury Reduction

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Abstract:

This chapter concentrates on selected aspects of microbial mercury reduction and other mechanisms of mercuric ion resistance which are relevant to the interaction of metals and microorganisms in the environment. Mercury (Hg) is simple to refine—the ores are roasted in a current of air, and metallic mercury is condensed from the vapor. The simplicity of refining and the unusual properties of this metal probably account for the long and fascinating human relationship with mercury. Hg has been released into the lithosphere, atmosphere, and hydrosphere over millennia by geochemical processes, and it is therefore an important toxic element in the biosphere. The high affinity of mercury for thiol and imino nitrogen groups in proteins and the diverse cellular targets of mercuric ions preclude some of the main strategies used by microorganisms to avoid, eliminate, or detoxify other toxic metals. Using the criterion that a specific metal resistance is regulated by a specific discriminatory, regulatory component, some of the mechanisms discussed are simply tolerance mechanisms. Experimental and pilot scale microbial mercury reduction and volatilization systems have been developed for removal of Hg(II). Hg is an underexploited model system for the study of the evolution of an environmental trait across different prokaryotic genera. Great progress has been made in one's understanding of the genetics and biochemistry of reductive Hg.

Citation: Hobman J, Wilson J, Brown N. 2000. Microbial Mercury Reduction, p 177-197. In Lovley D (ed), Environmental Microbe-Metal Interactions. ASM Press, Washington, DC. doi: 10.1128/9781555818098.ch8

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Figure 1

Microbial transformations in the mercury cycle. A schematic summary of microbial mercury transformations is shown. The levels of Hg(II) and MMHg are governed by the balance of reduction and oxidation by aerobic bacteria and by the rate of bacterial methylation and demethylation. Demethylation of MMHg can be reductive (broad-spectrum mer operons), generating CH as a metabolite, or oxidative, where CO is produced. Both Hg(II) and MMHg can pass into the food chain or be adsorbed by organic matter (particulate or dissolved). Hg(II) and MMHg will be released back into the mercury cycle when the biomass decays. Under anoxic conditions, the reaction of MMHg with HS (generated by sulfate-reducing bacteria from sulfates) produces dimethylmercuric sulfide. This is unstable and degrades to insoluble HgS and volatile DMHg. DMHg can degrade under mild acid conditions to form CH and Hg(II), which can be transformed to Hg(0). Modified from Baldi, 1997 ( ).

Citation: Hobman J, Wilson J, Brown N. 2000. Microbial Mercury Reduction, p 177-197. In Lovley D (ed), Environmental Microbe-Metal Interactions. ASM Press, Washington, DC. doi: 10.1128/9781555818098.ch8
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Figure 2

Generalized model of bacterial resistance to mercury. The diagram shows the model for mercuric ion resistance in gram-negative bacteria. Mercuric ions in the environment of a bacterial cell [a] pass through the porins (OmpC and OmpF) in the outer membrane, where they are [b] scavenged by the periplasmic protein, MerP and bind to the cysteine residues in each subunit of the protein, [c] The mercuric ion is then passed from the cysteines in MerP, to those in the transmembrane region of the inner membrane protein, MerT. As part of the transport mechanism, the Hg(II) ion is transferred to the cysteines on the cytoplasmic face of MerT, whence [d] they are passed to the heavy-metal associated motif in the amino-terminal MerP-like domain of mercuric reductase [e]. The mercuric ion is then bound at the active site and reduced to elemental mercury, Hg(0) [f]. The volatile product is released from the enzyme and diffuses through the bacterial membranes to the environment. MMHg can diffuse in through the cell membrane, and with broad-spectrum determinants, is cleaved by organomercurial lyase [g]. The Hg(II) so produced is proposed to bind to glutathione in the cytoplasm and be reduced by MR. Resistance in gram-positive bacteria operates by a similar mechanism, but the detailed structures of the transport proteins are different. Cysteine residues are also present in other mercury transport proteins from gram-negative sources (e.g., MerC and MerF) or from gram-positive sources and are predicted to lie in the transmembrane region. The MR from gram-negative and gram-positive sources are similar but differ in the number (0, 1, or 2) of MerP-like N-terminal domains and in their detailed amino acid sequences.

Citation: Hobman J, Wilson J, Brown N. 2000. Microbial Mercury Reduction, p 177-197. In Lovley D (ed), Environmental Microbe-Metal Interactions. ASM Press, Washington, DC. doi: 10.1128/9781555818098.ch8
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Figure 3

General organization of the Tn501 and related mer operons in gram-negative bacteria. A generalized schematic diagram of the mer genes of transposon Tn501 is shown. Additional genes from closely related mer resistances (referred to in the text), and their positions in the operon relative to merR, mer-T, mer-P, merA, and mer-D are marked. The merR and structural gene transcripts are shown as arrows from the mer operator/promoter (merO/P) site. The terminal inverted repeat (IR) of the transposon (Tn501) is marked. There are a number of other gram-negative bacterium mer operons whose organizations do not conform to this generalized structure yet whose genes are clearly closely related to Tn501 and Tn21 ( ).

Citation: Hobman J, Wilson J, Brown N. 2000. Microbial Mercury Reduction, p 177-197. In Lovley D (ed), Environmental Microbe-Metal Interactions. ASM Press, Washington, DC. doi: 10.1128/9781555818098.ch8
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