Mercury in the Marine Environment - Mercury in the Environment - page 189

discussed in detail here. Many strains of bacteria are capable
of demethylating MMHg, and this can occur in either the
water column or sediment (Barkay et al., 2003; Monperrus
et al., 2007). The microbially mediated decomposition
of MMHg in the aquatic environment can proceed via
oxidative or reductive pathways (Marvin-DiPasquale
et al., 2000; Schaefer et al., 2004). In oxidative demethyl-
ation, MMHg decomposition is believed to be unintended
and is related to C1 metabolic pathways. The more widely
studied microbial reductive demethylation pathway
involves the inducible mer operon used in mercury detoxi-
fi cation (Misra, 1992; Barkay et al., 2003). The gene mer B,
found in the plasmid-borne mer operon, encodes a organo-
mercury lyase enzyme that degrades MMHg to methane
and inorganic Hg(II), with the Hg(II) then reduced to Hg(0)
by mercuric reductase (Misra, 1992).
Both microbial demethylation pathways have been
observed under aerobic and anaerobic conditions in fresh-
water, estuarine water, and marine sediments (Barkay et al.,
2003; Benoit et al., 2003; Hines et al., 2000, 2006; Marvin-
DiPasquale et al., 2000). In mercury-contaminated areas
the mer operon is more prevalent and plays an important
role in controlling MMHg levels (Marvin-DiPasquale et al.,
2000; Schaefer et al., 2004). However, there are relatively
few locations in the marine environment with suffi ciently
high total mercury levels that induction of the mer operon
is likely to be the predominant control on MMHg concen-
trations. Such mercury-contaminated locations would be
limited to some estuaries (e.g., San Francisco Bay), coastal
environments (e.g., Gulf of Trieste), and areas infl uenced by
geothermal activity (e.g., hydrothermal vents), and would
represent only a small percentage of the oceans. As a result,
both the absolute rates and the relative importance of the
oxidative and reductive pathways for microbial degradation
of MMHg in the water column or sediments of the ocean
are poorly understood. Nonetheless, biologically mediated
degradation could be one of the largest sinks for MMHg
in the marine environment, especially if it were to occur
throughout the entire water column at rates suggested by
some studies (Monperrus et al., 2007; Whalin et al., 2007).

toplankton is associated primarily with cell-wall material,
while MMHg is associated more with the cytoplasmic com-
ponents (Mason et al., 1996). The cytoplasm is more likely
to be remineralized and released back into solution before
sediment burial than cell-wall material, so the MMHg is
less likely to undergo sediment burial than Hg(II) follow-
ing an algal bloom. In addition, the export of Hg(II) with
the fresh organic matter during the decay of a phytoplank-
ton bloom may lead to a pulse in microbial respiration and
MMHg production in sediments. Some of this newly pro-
duced MMHg could make its way into overlying waters, in
which case algal blooms may prove to be a source of MMHg
to the water column in the longer term.
Estimates for sediment burial of total mercury (Balcom
et al., 2004, 2008; Fitzgerald et al., 2007; Horvat et al. 2003)
are much more abundant than for MMHg burial in marine
sediments. This difference is, in part, due to uncertainties
in MMHg cycling and its nonconservative behavior in sedi-
ments. Despite this, it is estimated that sediment burial
comprises 8% of all sinks for MMHg in the Chesapeake Bay
(Mason et al., 1999), and particle scavenging and burial
is an important sink for MMHg in the Pettaquamscutt
Estuary (Mason et al., 1993).
We estimate sediment burial of MMHg in nearshore
regions to be ~0.19 Mmol yr 1 , assuming that the riverine
transport of MMHg to the oceans is 0.21 Mmol yr 1 and
90% of that MMHg is buried in nearshore sediments, as
suggested for total mercury (Sunderland and Mason, 2007).
Sediment burial of total Hg outside of coastal areas is esti-
mated to be approximately 1-2 Mmol yr 1 (Lamborg et al.,
2002; Mason and Sheu, 2002; Sunderland and Mason, 2007).
Because of the lower concentrations and lower particle
partition coeffi cients for MMHg as compared with Hg(II),
MMHg likely represents only ~2% of the total Hg lost to
sedimentation in the ocean, which would give a burial sink
for MMHg of ~0.02 Mmol yr 1 outside coastal areas. Thus,
the loss of MMHg from the oceanic water column due to
particle scavenging and sediment burial for both nearshore
and deep sea sediment is ~0.21 Mmol yr 1 .

REMOVAL VIA FISHING

PARTICLE SCAVENGING AND SEDIMENT BURIAL

Many mass balances of MMHg in marine ecosystems
include an estimate of the MMHg production required to
balance uptake by biota, often derived from concentrations
of MMHg in biota and estimates of primary production.
While these estimates are useful, it is unclear whether such
uptake of MMHg represents a true sink for MMHg in the
ocean or merely internal cycling. Much of this MMHg may
be recycled both because the trophic transfer of MMHg is
less than 100% at any step in the food chain, and eukary-
otes excrete MMHg as a detoxifi cation mechanism, although
rates of MMHg depuration are much slower than rates of
accumulation (Rouleau et al., 1998; Tsui and Wang, 2004;
Wang and Wong, 2003). And although some birds and
mammals can demethylate MMHg, there is little evidence

The low, or nondetectable, levels of MMHg in waters of the
open ocean may be attributed to some degree to scavenging
by particles that then sink, being remineralized at depth
or buried in sediments. This includes increased scavenging
during phytoplankton blooms, which can reduce dissolved
Hg(II) and MMHg levels in the water column due to uptake
into the cell and scavenging onto cell surfaces (Luengen
and Flegal, 2009). When the bloom subsequently crashes
or decays, the associated export of cellular components to
bottom sediments could act to export substantial amounts
of the scavenged Hg(II) and MMHg with it. However, the
longer-term effects of phytoplankton blooms on MMHg
dynamics is unknown, in part because the Hg(II) in phy-