Box 14.15: Biogeochemical Mercury Cycle
Re ´my Guyoneaud
Mercury, whose chemical symbol Hg comes from the
Latin name Hydrargyrum (liquid silver) has different
chemical forms in the environment. Main species are
elemental mercury (Hg
0
, volatile), bivalent inorganic
mercury (Hg
2+ or Hg(Il)), mostly combined with
organic matter, chloride, hydroxydes or sulfide (cinabar,
HgS),
monomethylmercury
(CH 3 Hg
+
)
and
dimethylmercury (organic volatile species, (CH 3 ) 2 Hg).
Mercury toxicity, is due to its capacity to combine with
thiol groups in proteins. In contrast to biogeochemical
cycles of carbon, nitrogen, or sulfur, toxic mercury in
not an essential element for life. Micro-organisms participate in the mercury biogeochemical cycle but it is not
used through energetic metabolisms (electron donor or
acceptor) nor assimilation/degradation mechanisms.
Biological transformations of mercury are linked to
resistance mechanisms (transformation of the toxic
forms to the less toxic volatile elemental mercury) and
to mercury methylation mechanisms that enhance mercury accumulation in food webs (Box Fig. 14.25).
The reduction of inorganic mercury to volatile Hg
0
is a prokaryotic resistance mechanism whose genetic
determinism is well known (Barkay et al. 2003).
(continued)
Box 14.15 (continued)
Proteins involved in mercury reduction are encoded by
genes organizsed in the inducible mer operon, present
in many Bacteria. In the absence of mercury (or at low
concentrations) MerR transcriptional regulatory protein is linked to the mer promoter (PmerT) and
represses the genes expression. In the presence of
inorganic mercury or methylmercury, the modification
of the complex MerR-operator-promoter allows the
operon transcription and thus the synthesis of transport proteins, catalytic enzymes, and regulation protein (MerD) involved in the repression when mercury
concentration decreases. Mercury reduction is not
linked to an energetic metabolism (energy conservation). After its transport inside the cell, inorganic
mercury (Hg(II)) is reduced to volatile elemental mercury (Hg(0)) by a mercury reductase (MerA). The mer
operon may comprise one particular gene (merB)
encoding for an organomercurial lyase involved in
methylmercury detoxication. The presence of this
gene and its related enzyme allows a broad spectrum
resistance (inorganic mercury and methylmercury).
The methylmercury is first demethylated to CH 4 and
inorganic mercury and the later is afterward reduced
to elemental mercury through MerA reductase. This
(continued)
Periplasm
Cytoplasm
CH 3 Hg
+
Hg
2+
CH 4
Hg
0
Internal membrane
MerT
MerB
MerP
External membrane
Hg
0
Porin
RSH
2RSH
Cys
Cys
Cys
Cys
Cys
Cys
Cys
Cys
Cys
Cys
Cys
Cys MerA
Box Fig. 14.25 Resistance mechanisms to inorganic mercury and
methylmercury through proteins encoded on Tn501 transposon
(merT, merP, merA) as well as organomercurial lyase (MerB).
Mercury binds to proteins through cysteine residues, cytoplasmic
transport occurs through small peptidic thiols (RSH) (Redrawn from
Barkay et al. 2003)
604
J.-C. Bertrand et al.
Re ´my Guyoneaud
Mercury, whose chemical symbol Hg comes from the
Latin name Hydrargyrum (liquid silver) has different
chemical forms in the environment. Main species are
elemental mercury (Hg
0
, volatile), bivalent inorganic
mercury (Hg
2+ or Hg(Il)), mostly combined with
organic matter, chloride, hydroxydes or sulfide (cinabar,
HgS),
monomethylmercury
(CH 3 Hg
+
)
and
dimethylmercury (organic volatile species, (CH 3 ) 2 Hg).
Mercury toxicity, is due to its capacity to combine with
thiol groups in proteins. In contrast to biogeochemical
cycles of carbon, nitrogen, or sulfur, toxic mercury in
not an essential element for life. Micro-organisms participate in the mercury biogeochemical cycle but it is not
used through energetic metabolisms (electron donor or
acceptor) nor assimilation/degradation mechanisms.
Biological transformations of mercury are linked to
resistance mechanisms (transformation of the toxic
forms to the less toxic volatile elemental mercury) and
to mercury methylation mechanisms that enhance mercury accumulation in food webs (Box Fig. 14.25).
The reduction of inorganic mercury to volatile Hg
0
is a prokaryotic resistance mechanism whose genetic
determinism is well known (Barkay et al. 2003).
(continued)
Box 14.15 (continued)
Proteins involved in mercury reduction are encoded by
genes organizsed in the inducible mer operon, present
in many Bacteria. In the absence of mercury (or at low
concentrations) MerR transcriptional regulatory protein is linked to the mer promoter (PmerT) and
represses the genes expression. In the presence of
inorganic mercury or methylmercury, the modification
of the complex MerR-operator-promoter allows the
operon transcription and thus the synthesis of transport proteins, catalytic enzymes, and regulation protein (MerD) involved in the repression when mercury
concentration decreases. Mercury reduction is not
linked to an energetic metabolism (energy conservation). After its transport inside the cell, inorganic
mercury (Hg(II)) is reduced to volatile elemental mercury (Hg(0)) by a mercury reductase (MerA). The mer
operon may comprise one particular gene (merB)
encoding for an organomercurial lyase involved in
methylmercury detoxication. The presence of this
gene and its related enzyme allows a broad spectrum
resistance (inorganic mercury and methylmercury).
The methylmercury is first demethylated to CH 4 and
inorganic mercury and the later is afterward reduced
to elemental mercury through MerA reductase. This
(continued)
Periplasm
Cytoplasm
CH 3 Hg
+
Hg
2+
CH 4
Hg
0
Internal membrane
MerT
MerB
MerP
External membrane
Hg
0
Porin
RSH
2RSH
Cys
Cys
Cys
Cys
Cys
Cys
Cys
Cys
Cys
Cys
Cys
Cys MerA
Box Fig. 14.25 Resistance mechanisms to inorganic mercury and
methylmercury through proteins encoded on Tn501 transposon
(merT, merP, merA) as well as organomercurial lyase (MerB).
Mercury binds to proteins through cysteine residues, cytoplasmic
transport occurs through small peptidic thiols (RSH) (Redrawn from
Barkay et al. 2003)
604
J.-C. Bertrand et al.