Environmental Engineering Reference
In-Depth Information
a particularly influential impact on the biomagnification
of MeHg in terrestrial invertivore food webs. Such findings
may have important ramifications for the health of some
terrestrial songbird, bat and other invertivorous populations
in some habitats. Although LOAELs for invertivorous birds
have not been broadly developed, there is evidence that
songbirds can be relatively sensitive to MeHg exposure
in
ovo
(Heinz et al., 2008). Such sensitivity combined with the
wide diversity of invertivorous species raises concern for the
potential effects of Hg exposure on songbird conservation
in the northeastern United States and elsewhere (Evers and
Duron, 2008). Brasso and cristol (2008) reported decreased
reproductive success in tree swallows (
Tachycineta bicolor
),
especially in young breeding females, nesting near an
Hg-contaminated river; and Wada et al. (2009) reported sup-
pressed adrenocortical responses in tree swallow nestlings
from this same area. Franceschini et al. (2009) suggested
that Hg exposure depressed resting plasma corticosterone
levels in tree swallows. Future studies should better charac-
terize the food-chain transfer of MeHg in critical terrestrial
systems, identify the main at-risk species, and further inves-
tigate possible reproductive and other effects in these species.
the livers of American white pelicans (
Pelecanus erythro-
rhynchos
) from Pyramid Lake, Nevada. Eagles-Smith et al.
(2009) reported that demethylation occurred when total
Hg concentrations rose above about 8.5 µg g
-1
dry weight
in the livers of American avocets (
Recurvirostra americana
),
black-necked stilts (
Himantopus mexicanus
), caspian terns
(
Hydroprogne caspia
), and Forster's terns (
Sterna forsteri
) in
San Francisco Bay. Above the demethylation threshold,
Se concentrations were significantly correlated with inor-
ganic Hg concentrations in the liver. A similar threshold
(~9-10 µg g
-1
dry weight) was suggested by Scheuhammer
et al. (2008) for common loons, above which hepatic Hg
and Se concentrations became closely correlated. Similarly,
for a number of marine mammal species, the inflection
point below which the majority of hepatic Hg is organic
(MeHg), and above which the proportion of inorganic Hg
increases, was estimated to be about 2.2 µg g
-1
wet weight
(or ~9 µg g
-1
dry weight) (Dietz et al., 1990).
As discussed in the section on egg-injection studies, some
bird species appear to be much more sensitive than others
to MeHg exposure
in ovo
(Heinz et al., 2009), a finding that
raises questions regarding the molecular basis for interspecies
differences in sensitivity to MeHg. Scheuhammer et al. (2008)
reported significant differences between common loons and
bald eagles with respect to Hg and Se accumulation in brain
tissue. Although both species, by virtue of their high trophic
status in aquatic ecosystems, can be exposed to relatively
high dietary MeHg concentrations, their patterns of total Hg,
MeHg, and Se accumulation in brain tissue were significantly
different. Eagles showed a highly significant co-accumulation
of Se with Hg and efficient demethylation of MeHg in brain
as compared with loons. For loons, but not eagles, the molar
ratio of Hg:Se in the brain often exceeded 1:1. Ralston et al.
(2007) suggested that the ratio of Hg to Se is an important
determinant of MeHg toxicity. certain seleno-enzymes (e.g.,
thioredoxin reductase) are highly sensitive to inhibition by
low nanomolar concentrations of Hg and may be primary tar-
gets of MeHg toxicity at the molecular level (carvalho et al.,
2008). The relative inability of the loon brain to demethylate
MeHg and/or to increase the accumulation of Se as Hg con-
centrations rise may indicate a greater sensitivity to the toxic
effects of MeHg in loons as compared with eagles. Similarly,
river otters appear to demethylate MeHg (Basu et al., 2005b)
and accumulate Se in their brain more efficiently than mink
(Haines et al., 2004), findings that may help explain why
MeHg-associated neurochemical changes in the brains of
wild river otters (Basu et al., 2005b, 2006b) were different
than those in wild mink. Additional studies are needed to
better explain interspecies differences in sensitivity to MeHg
exposure and the neurotoxic mechanisms of action of MeHg,
including the role of Se.
All of the above studies support the concept that Se pro-
tects animals against MeHg poisoning, but in a controlled
dosing study in which breeding pairs of mallard ducks were
fed a diet containing a combination of 10 µg g
-1
Hg as MeHg
and 10 µg g
-1
Se as selenomethionine, reproductive success
mercury-selenium relationships
Se is generally considered to protect against MeHg poi-
soning in vertebrates (cuvin-Aralar and Furness, 1991;
Sumino et al., 1977). It has long been postulated that
demethylation of MeHg in tissues, and the subsequent
sequestration of inorganic Hg with Se in an approximate
1:1 molar ratio, represents a detoxification mechanism
for at least some animal species that are exposed to rela-
tively elevated dietary concentrations of MeHg, includ-
ing predatory marine mammals such as polar bears (Dietz
et al., 2000), seals (Smith and Armstrong, 1978), and
toothed whales (caurant et al., 1996; Palmisano et al.,
1995), as well as a variety of seabirds and other predatory
aquatic birds (Scheuhammer et al., 1998; Thompson and
Furness, 1989). Henny et al. (2002) reported an apparent
threshold-dependant tolerance to MeHg by double-crested
cormorants, black-crowned night herons (
Nycticorax nycti-
corax
), and snowy egrets from the carson River, Nevada.
In these birds, demethylation of MeHg seemed to increase
at higher levels of MeHg intake, resulting in lower percent-
ages of MeHg in the liver and kidney as total Hg concen-
trations increased. Similar findings have been reported in
seabirds (Thompson and Furness, 1989), osprey (Hopkins
et al., 2007), and common loons and common mergansers
(Scheuhammer et al., 1998). In addition, Henny et al. (2002)
reported a nearly perfect correlation (
r
2
= 0.98) between
inorganic Hg concentrations and Se concentrations in the
livers of cormorants, night herons, and egrets, suggesting
Se was important in sequestering inorganic Hg following
demethylation. Wiemeyer et al. (2007) noted a similar
inverse relationship between MeHg and total Hg concen-
trations, and a similar correlation between Hg and Se, in
