Page 56 - The Toxicology of Fishes

36 The Toxicology of Fishes

2+
FIGURE 2.13 Biogeochemical cycling of mercury. This figure shows reactions that transform airborne Hg into CH 3 Hg +
+
and the subsequent fate of CH 3 Hg . Currently, the photodegradation products of CH 3 Hg are unknown, although it has been
+
0
speculated that Hg is produced.
gold mining (also an important point source). To date, 48 states in the United States have issued fish
consumption advisories for one or more water bodies because mercury levels in fish muscle tissue exceed
state or federal guidelines, and 23 states have issued statewide advisories (U.S. EPA, 2007). Many of
these fish reside in water bodies for which there are no known point sources.
Mercury can exist in three valence states:

• Hg , metallic mercury (elemental mercury)
0
2+
• Hg , monovalent mercury (mercurous mercury)
2
• Hg , divalent mercury (mercuric mercury)
2+
Hg is unstable under most environmental conditions. The overwhelming percentage of total mercury
2+
2
2+
0
2+
in the environment therefore occurs as either Hg or Hg . Hg is commonly associated with sulfur in
the mineral cinnebar (HgS) but may complex with other inorganic ligands, including chlorine, oxygen,
2+
and hydroxyl ions. Hg can also react covalently to form a variety of organic derivatives including
+
+
CH Hg (methylmercury), (CH ) Hg (dimethylmercury), and C H Hg (phenylmercury). Of these, methyl-
6
3 2
5
3
mercury (henceforth, MeHg) is by far the most common and is the form of greatest interest to toxicol-
ogists due to its high toxicity and propensity to accumulate in aquatic biota (Wiener and Spry, 1996).
The biogeochemical cycling of mercury in freshwater systems has been extensively reviewed (Driscoll
et al., 1994; Ullrich et al., 2001; Winfrey and Rudd, 1990; Zillioux et al., 1993). Figure 2.13 shows a
subset of known reactions that result in conversions among major mercury species. The same reactions
are thought to occur in estuarine and marine environments, but their relative importance is less well
2+
known. The focus of this figure is on reactions that transform airborne Hg into MeHg, and the
subsequent fate of MeHg. Potentially important reactions that do not appear in this figure include the
2+
0
photooxidation of Hg (LaLonde et al., 2001), photoreduction of Hg (Amyot et al., 1994), microbial
conversion of MeHg to dimethylmercury (Baldi et al., 1993), and formation of soluble sulfide complexes
2+
with Hg (see below). The percentage of total mercury in freshwater that exists as MeHg varies among
systems but is generally <15% and often closer to 5% (U.S. EPA, 1997b). Contaminated sediments serve
2+
as an important reservoir for Hg , and it is likely that sediment-bound mercury can recycle back into
an aquatic ecosystem for many years (Kudo, 1992).