Page 55 - The Toxicology of Fishes

Bioavailability of Chemical Contaminants in Aquatic Systems 35

reasonable formulations for some fundamental aspects of metal toxicity: (1) the absorption of bioavailable
metal into the organism to achieve toxic metal levels at some site of action, (2) reductions in this
absorption due to the formation of less bioavailable metal species in the exposure water, and (3) reductions
in this absorption due to the effects of other cations. This section demonstrated how this approach can
form the basis for understanding and predicting the role of bioavailability in the effects of exposure
conditions on acute copper lethality to fish, as well as how some effects might be related to processes
other than bioavailability. The same general considerations will apply to other endpoints, routes of
exposure, and metals, although details will vary. Broader and more extensive discussions of metal
bioavailability in water can be found in many of the references cited herein. The possible role of dietary
metal in the overall bioavailability of metals to fish and other aquatic organisms has been recently
reviewed by Meyer et al. (2005). There also is extensive literature concerning metal bioavailability in
sediments which, although not directly impacting most fish, is nonetheless important for assessing the
broader impacts of metals on aquatic ecosystems (Ankley et al., 1996).

Organometals: Mercury
The behavior of metals in biological systems can change dramatically when they occur as organometallic
compounds. Although several metals have the potential to form organometallic compounds, organic
species of mercury, tin, and lead have attracted the most attention because of their occurrence in the
environment and demonstrated toxicity (Pelletier, 1995). For each of these metals, covalent binding to
one or more organic groups yields compounds that may accumulate in fish and other aquatic biota.
A general model for membrane diffusion of trialkyltin compounds was developed in the 1970s (Tosteson
and Wieth, 1979; Wieth and Tosteson, 1979). According to the model, positively charged trialkyltin
–
–
species diffuse across biological membranes in association with an aqueous anion, usually Cl or OH ,
and the presence of one or more organic substituents contributes to this uptake by increasing the molecule’s
relative hydrophobicity, in effect giving it a partially “organic” character. The ion pair model does not,
however, account for the fact that organometals tend to retain their metallic (i.e., electrophilic) character,
including high reactivity with protein thiols. Transport across a biological membrane is likely, therefore,
to reflect a balance between simple diffusion of the neutral ion pair and interactions with membrane
proteins (Boudou et al., 1991). Membrane flux of some organometallic compounds may also occur by
active transport of complexes formed with small organic molecules (see below).
The dual character of organometals is also reflected in factors that control their speciation in natural
waters. As discussed in the previous section regarding copper, pH, alkalinity, and hardness can have a
large effect on the speciation, accumulation, and toxicity of inorganic metals. Uptake of organometals
also may be impacted to some extent by ionic constituents of water, particularly as they affect the formation
of neutral diffusing species. In addition, binding to DOC and POC is likely to be important for controlling
the concentration of freely dissolved species, thereby influencing bioavailability in waterborne exposures.
Because organometallic compounds tend to accumulate in organisms that occupy the base of aquatic
food webs, the diet may represent the principal route of exposure for higher trophic level organisms,
including fish. An understanding of organometal accumulation by fish therefore requires that bioavail-
ability concepts be extended to the entire aquatic food web. Additional consideration must be given to
factors that control interconversions of inorganic and organic metal species. This is particularly true
when (as is often the case) the concentration of the organometal in fish is referenced to the total
concentration of parent metal in sediment or water. In this section, methylmercury accumulation by fish
is examined as a means of illustrating these points.
Historically, demonstrated impacts of methylmercury on humans due to consumption of contaminated
fish and shellfish have focused attention on point-source releases of mercury to the aquatic environment
such as mining, smelting, and wastewater treatment (U.S. EPA, 1997a). Most of the mercury released
in these cases exists as elemental or inorganic mercury. Biotic and abiotic processes then transform a
portion of this mercury to the methylated form. These releases continue to occur, particularly in countries
with emerging industrial economies. Increasingly, however, the focus of mercury research in the United
States and elsewhere has been on airborne mercury emissions. Important anthropogenic sources of
mercury to the atmosphere include the combustion of fossil fuels, municipal waste incineration, and