Mercury in the Marine Environment - Mercury in the Environment - page 179

et al., 2007), and central California (Black et al., 2009b) have
indicated that the fl ux of total mercury to the ocean via
groundwater discharge is more important than previously
believed. Elevated MMHg concentrations in groundwater
in some areas (Stoor et al., 2006) suggest that submarine
groundwater discharge (SGD) may also represent a previously
unidentifi ed source of MMHg to coastal waters. To date there
has been only one report on MMHg transport in SGD (Black
et al., 2009b).
Fluxes of SGD will usually be greatest within a few kilo-
meters of the coastline. As a result, SGD may be an impor-
tant local source of MMHg to some coastal areas, but is
unlikely to be an important source to the ocean at large.
This point is demonstrated by extrapolating measured
fl uxes of MMHg in SGD along the central California coast
to coastlines globally. Using the mean MMHg fl ux in SGD
of 10 nmol m 1 d 1 (Black et al., 2009b) and assuming a rel-
evant global coastline of 1

one site, nearly all in a methylated form. However, based
on the low MMHg concentrations in the deep ocean and
the high concentrations of mercury in sediments near this
marine geothermal system, it was hypothesized that much
of this methylmercury was deposited nearby and/or rapidly
demethylated. As a result, it was predicted that submarine
hydrothermal systems are not a dominant source of MMHg
to the oceans. It is possible that the elevated concentrations
of total Hg in marine hydrothermal waters has led to the
adaptation of microbes at deep-sea hydrothermal vents
to high Hg levels (Crespo-Medina et al., 2009; Vetriani
et al., 2005), and to the rapid demethylation of MMHg by
microbes here via the mer operon, as has been suggested
for Hg-contaminated freshwater aquatic systems (Schaefer
et al., 2004).

Internal Sources of Monomethylmercury

10 6 km (Crossland et al., 2005)
after reducing the length to exclude barrier islands, coast
covered with ice, and other areas unlikely to have active
MMHg fl uxes in SGD, it is estimated that the annual fl ux of
MMHg to the ocean via SGD is 0.004 Mmol.

INTERNAL ABIOTIC MONOMETHYLMERCURY
PRODUCTION

Abiotic mechanisms for the methylation of mercury in
aquatic environments that have been demonstrated in the
laboratory include transfer of a methyl radical or carban-
ion to inorganic Hg(II) by methylcobalamin (Filippelli and
Baldi, 1993; Nobumasa et al., 1971) and methylated tin spe-
cies (Cerrati et al., 1992). The importance of these com-
pounds in this role in the marine environment is ques-
tionable, however, because of their low concentration in
natural waters and sediments, and because their potential
for interacting with Hg(II) is likely minimal because of Hg(II)
complexation by organic and inorganic ligands (Benoit
et al., 1999, 2001b; Dyrssen and Wedborg, 1991; Haitzer
et al., 2003; Han and Gill, 2005; Lamborg et al. 2004). A more
plausible abiotic methylating agent in the environment is
humic substances, based on laboratory studies that have shown
the ability of humic matter to methylate Hg(II) to MMHg (Lee
et al., 1985; Nagase et al., 1982) and because of the substantial
concentration of humics that can complex Hg(II) in natural
aquatic systems (Benoit et al., 2001b; Haitzer et al., 2003).
Studies have reported abiotic photochemical produc-
tion of MMHg in laboratory (Akagi and Takabatake, 1973;
Akagi et al., 1976) and fi eld studies (Siciliano et al. 2005).
The work by Akagi and colleagues demonstrated the abi-
otic photomethylation of aqueous Hg 2 in the presence
of acetate, which is commonly found in aquatic systems.
Alternatively, studies by Gårdfeldt et al. (2003a) have dem-
onstrated the ability of acetate to abiotically methylate
Hg(II) in the dark. They reported that when using milli-
molar concentrations of acetic acid, the reaction proceeded
via mercury acetate complexes (most likely Hg(CHCOO) 3 - ),
and the reaction rate was fi rst order with respect to inor-
ganic mercury concentration and not affected by pH or
the concentration of acetate or other ions, as long as ace-
tate complexes dominated the Hg(II) speciation. While
marine waters and sediment pore waters can have low
micromolar and low millimolar concentrations of acetate,

MARINE HYDROTHERMAL SYSTEMS

Submarine hydrothermal vents, fl uids, deposits, fumaroles,
mud volcanoes, midocean ridges, and other marine geothermal
features are sources of inorganic and elemental mercury to both
nearshore regions and the deep ocean (Crespo-Medina et al.,
2009; Dekov, 2007; Lamborg et al., 2006; Stoffers et al., 1999;
Tomiyasu et al., 2007). But only one study to date has reported
species-specifi c measurements and quantifi ed methylmercury
concentrations at a marine hydrothermal system (Lamborg
et al., 2006); it identifi ed methylmercury as the dominant
form of mercury present, although much higher total Hg con-
cent rat ions have b een measu red i in water s at ma r i ne vents else -
where (Crespo-Medina et al., 2009). Methylmercury concen-
trations as high as 16 pM were reported for the hydrothermal
vent fl uids at Gorda Ridge (Lamborg et al., 2006), although
it is not clear whether that methylmercury was the result
of biotic or abiotic methylation. Chemosynthetic microbes
abundant at these deep-sea hydrothermal systems might be
responsible for Hg methylation, or the high temperatures and
pressures characterizing these environments might be condu-
cive to the abiotic methylation of Hg by methane or other
organic compounds, as both Hg and methane are also elevated
in these hydrothermal systems. Alternatively, Fein and Wil-
liams-Jones (1997) noted that crustal fl uids can have elevated
concentrations of carboxylic acids, including acetate, which
has been shown to methylate inorganic mercury (Gårdfeldt
et al., 2003a).
Using measurements made at one marine hydrother-
mal system and extrapolating to hydrothermal systems
elsewhere, the total input of dissolved mercury to the
oceans was estimated to be 0.1-0.4 Mmol yr 1 (Lamborg
et al., 2006). This fl ux of mercury is, at least initially at this