Environmental Engineering Reference
In-Depth Information
If the upwelling and subsequent degradation of DMHg were
an important source of MMHg to surface waters, one would
expect MMHg levels to be highest in areas with strong upwell-
ing, such as along the equator or along the western margins
of North America, South America, and Africa (Mittelstaedt,
1986). Unfortunately, data for MMHg levels in upwelling
regimes are sparse. MMHg concentrations in the equatorial
Pacifi c and Monterey Bay are not anomalously high relative
to nearby waters without upwelling, and instead, MMHg con-
centrations in these regions are often below detection limits
(Black et al., 2009a; Mason and Fitzgerald, 1990, 1993).
attempting to understand MMHg distribution and produc-
tion have explored the relationship between total mercury
and MMHg in sediments (Kelly et al., 1995), the importance
of the chemical speciation of Hg(II) on its uptake and sub-
sequent methylation (Benoit et al., 1999, 2001a; Drott et al.,
2007; Jay et al., 2002; Mason et al., 1996), or the potential
role of cobalt speciation and limitation in MMHg produc-
tion (Ekstrom and Morel, 2008). Yet other studies focus-
ing on only a few factors have suggested that either DOM
(Mason and Lawrence, 1999; Lambertsson and Nilsson,
2006) or sulfate-reduction rates and microbial commu-
nity (King et al., 2001) are the most important controls
on MMHg production or distribution. These studies have
proved very valuable in demonstrating the importance of
different individual factors in controlling net MMHg pro-
duction. There has at times, however, been a propensity to
use results from such limited studies to make far-reaching
generalizations about what most controls MMHg produc-
tion in large regions based on a small number of relation-
ships measured at a few sites and a few points in time. A
more holistic view of MMHg production or abundance
realizes that numerous processes and factors are impor-
tant (e.g., Benoit et al., 2003; Marvin-DiPasquale
and Agee,
2003; Heyes et al., 2006), but their relative importance dif-
fers substantially both spatially and temporally.
INTERNAL BIOTIC MONOMETHYLMERCURY
PRODUCTION
SULFATE- AND IRON-REDUCING BACTERIA
The factors controlling the microbially mediated meth-
ylation of mercury and the biochemical mechanisms of
MMHg production have been reviewed elsewhere (Barkay
et al., 2003; Benoit et al., 2003). Briefl y described, the pro-
duction of MMHg is most controlled by factors that infl u-
ence: (1) the bioavailability of Hg(II), and (2) the activity of
microbes responsible for the methylation of Hg. Although
many microbes have been shown to have the ability to
methylate Hg in the laboratory, research using natural sedi-
ments and specifi c metabolic inhibitors suggests that sul-
fate-reducing bacteria are the principal Hg methylators in
natural marine ecosystems, with iron-reducing bacteria in
sediments and heterotrophic bacteria in the water column
receiving more recent attention.
Because of the abundance of sulfate in seawater, most
organic carbon utilization in coastal and some deep sea sedi-
ments is attributed to microbial sulfate reduction (Canfi eld,
1989). Numerous species of sulfate-reducing bacteria and
other microbes have been shown to be capable of methylating
mercury in pure culture or in natural sediments (Compeau
and Bartha, 1985; Dias et al., 2008; King et al., 2001), but
at widely variable rates (King et al., 2000). It is currently
unknown which species of bacteria are most responsible for
MMHg production in natural marine sediments, and there
exists substantial diversity in sulfate-reducing microbial com-
munities between different locations in nearshore, shelf, and
deep-sea sediments (Liu et al., 2003).
Some iron-reducing bacteria are also able to methylate
mercury (Fleming et al., 2006; Kerin et al., 2006). Because
utilization of organic carbon in sediments in some coastal
regions is largely carried out via iron reduction (Aller et al.,
1986; Canfi eld et al., 1993; Taillefert et al., 2007), iron-
reducing bacteria could be important in MMHg production
in some coastal sediments, although to date there is only
indirect evidence of this (Mitchell and Gilmour, 2008).
Although the biotic formation and degradation of
MMHg is controlled by a number of chemical, biologic, and
physical factors, many studies have focused on the role of
only a small number of these. For example, some studies
DISTINGUISHING BETWEEN DIFFERENT SITES OF
MICROBIAL MONOMETHYLMERCURY PRODUCTION
Based on data currently available, the microbial production
of MMHg is the principle source of MMHg in marine ecosys-
tems. Distinguishing between the different locations where
this can be carried out to determine the source of MMHg in
the ocean is a topic of ongoing debate (Fitzgerald et al., 2007;
Kraepiel et al., 2003 Sunderland et al., 2009). Unequivocally
identifying the predominant regions of MMHg production
will provide insight into the source of MMHg to marine
fi sh, which in turn will help answer the question of whether
the mercury found in marine organisms is predominantly
derived from natural or anthropogenic sources.
Knowing the dominant source or location of MMHg pro-
duction in the oceans is also necessary to predict how changes
in sediment, organic carbon, nutrient, and total mercury loads
will affect MMHg production and accumulation by marine
biota. For example, in freshwater systems the atmospheric
fl ux of inorganic Hg(II) plays an important role in both
MMHg production and bioaccumulation (Hammerschmidt
and Fitzgerald, 2006c; Harris et al., 2007; Orihel et al., 2006,
2007; Paterson et al., 2006). These studies suggest that newly
deposited Hg(II) is more labile and bioavailable to sediment
microbes and is thus more easily methylated than mercury
from other internal or external sources.
Conversely, the atmospheric deposition of mercury
would not likely play an important role in MMHg levels
and bioaccumulation in the ocean if MMHg production
was carried out primarily in deep-sea sediments. Were
