Environmental Engineering Reference
In-Depth Information
The convergence of factors promoting high net methylation
rates gives rise to the concept of hot spots for methylation in
the landscape (Driscoll et al., 2007; Mitchell et al., 2008b).
Patterns of MeHg concentrations and methylation rates
in soils also demonstrate the importance of wetlands.
Skyllberg et al. (2003) found a progressive increase in the
MeHg:THg ratio from soil to soil solution to the stream.
They also found a greater MeHg:THg ratio in riparian soils
as compared with upland soils, suggesting riparian soils as
a source of MeHg. Schuster et al. (2008) demonstrated net
MeHg production in riparian, wetland, and headwater
stream sediment, with greater net methylation in summer
as compared with snowmelt. Marvin-DiPasquale et al. (2009)
explored controls on Hg methylation rates in stream sedi-
ments. In comparison to aquatic sediments and wetland
and riparian soils, upland soils are a more limited source
of MeHg. Allan et al. (2001) demonstrated in situ MeHg
production in upland soils of the Canadian Shield.
Hg methylation is
performed primarily by SRB, and
the rates can vary markedly (Devereux et al., 1996; King
et al., 2001). Some evidence suggests that other bacte-
rial communities also methylate Hg (Warner et al., 2003).
Demethylation is also carried out by microbes (Warner
et al., 2003; Rodriguez Martin-Doimeadios et al., 2004;
Marvin-DiPasquale et al., 2000, 2003), but may occur in
part through abiotic processes. Methylation and demethyl-
ation often take place simultaneously. In boreal wetlands,
Tjerngren et al. (2011) found that nutrient status and organic
matter quality affected the relative extent of methylation
and demethylation. Demethylation rates are generally less
variable (Lambertsson and Nilsson, 2006), and as a result,
methylation rates tend to control net methylation patterns.
The varying degree of correlation between THg and
MeHg (Sunderland et al., 2004) suggests that the presence
of Hg (Hg
2+
), while essential (Benoit et al., 2003), is only
one of several factors and conditions required for methyla-
tion. Other conditions include the availability of both sul-
fate (electron acceptor) and high-quality carbon (electron
donor) and suitable redox conditions (Harmon et al., 2004;
Jeremiason et al., 2006; Mitchell et al., 2008b). Finally,
hydrology is an important control as well, both in supply-
ing solutes, infl uencing redox, and ultimately transporting
any MeHg produced to surface waters, or to soil environ-
ments where demethylation may occur.
The tight coupling of the sulfur and Hg biogeochemi-
cal cycles make the distribution of sulfur in the landscape
a fundamental factor infl uencing net methylation. Current
knowledge suggests that the increased deposition of sulfate,
a component of acid rain, in northern boreal catchments
and upland lake systems has boosted SRB activity and hence
increased Hg methylation. Branfi reun et al. (2001) came
to this conclusion based on the immediate increase in Hg
methylation in peat porewater following sulfate additions.
A longer-term study of chronic S additions also found an
MeHg response to several years of simulated increases in S
deposition in peat mesocosms (Branfi reun et al., 2001). More
recently, Jeremiason et al. (2006) found that S addition to a
wetland greatly stimulated methylation across the wetland.
Mitchell et al. (2008a) have also elucidated how the common
presence of high-quality carbon sources and sulfate interact
to stimulate methylation. Drevnick et al. (2007) attributed a
decrease in Hg bioaccumulation to a decrease in S deposition.
In constructed wetlands, Harmon et al. (2004) found that
sulfate amendments led to signifi cantly higher MeHg con-
centrations in porewater over the course of 1 year. Similarly,
in a mesocosm experiment, Mitchell at al. (2008a) found
that sulfate additions signifi cantly increased MeHg porewa-
ter concentrations. Additions of different carbon substrates
alone had no effect, but combined sulfate and C additions
gave the largest increases, providing an explanation of why
hot spots of MeHg appear in mires where there are inputs
of both C and sulfate (Mitchell et al., 2008b).
Research has also revealed the role that neutral Hg sulfi des
have in transporting Hg across the cell membrane into bacte-
ria where methylation can take place (Benoit et al., 1999). The
presence of sulfi de thus promotes uptake and methylation of
Hg, provided sulfi de concentrations remain low. High sulfi de
concentrations generate polysulfi des, which form stable com-
plexes with Hg, making it unavailable to the SRB (Benoit
et al., 2001). This optimal or “just right” amount of sulfi de for
methylation has been called the “Goldilocks effect” (Gilmour
et al., 1992). Research indicates that FeS(s) can strongly infl u-
ence the concentration of dissolved sulfi des [Hg(SH)
2
0
, HgS
0
,
and CH
3
HgSH
0
] and thus further complicate the relationships
among S, Hg, and MeHg (Drott et al., 2007). Studies from the
marine environment suggest alternate views that methylation
extent is controlled less by sulfur than by the amount of Hg
present (Fitzgerald et al., 2007) or the organic matter concen-
tration within the sediment (Lambertsson and Nilsson, 2006).
Given the signifi cant stores of Hg in the terrestrial land-
scape from both natural and anthropogenic emissions, the
most direct linkage between human society and Hg uptake
by biota lies in how human activity may affect net methyla-
tion in the landscape through factors such as changes in land
use, climate, and S deposition. This is especially true given
the length of time (decades or centuries) before reductions
in Hg emissions will lead to reduced Hg stores in the soil as
a whole. The scope for controlling methylation is framed by
the nature of the processes controlling methylation, though
the complexity in relation to understanding makes it diffi -
cult to make simple prescriptions about how to control net
methylation in the landscape. Another key question for con-
trolling MeHg is whether contemporary deposition of Hg
is more available for methylation, meaning that deposition
reductions could reduce methylation in the landscape.
In a study to address this latter question, Branfi reun
et al. (2005) showed that in one wetland location in
METAALICUS, 6% of newly applied
202
Hg was methylated
within 1 day. After 90 days, up to 65% (average, 21%) of the
applied Hg in porewater was methylated, as compared with
a maximum of 50% (average, 36%) of native Hg, suggesting
relatively rapid methylation of the newly applied spike Hg.
