Page 59 - The Toxicology of Fishes

Bioavailability of Chemical Contaminants in Aquatic Systems 39

fish’s lifetime, resulting in positive correlations between MeHg concentration and fish age or length
(U.S. EPA, 1997a).
Reported assimilation efficiencies for MeHg in dietary uptake studies with fish vary considerably.
Several authors have reported dietary assimilation efficiencies ranging from 50 to 90% (Wiener and
Spry, 1996), and values in this range have been used successfully in environmental modeling efforts to
describe observed rates of MeHg accumulation (Borgmann and Whittle, 1992; Norstrom et al., 1976;
Post et al., 1996). In contrast, a low assimilation efficiency (30%) was reported when channel catfish
were fed a prepared diet spiked with MeHg (McCloskey et al., 1998), and an even lower value (20%)
was obtained when northern pike were fed MeHg-contaminated forage fish (Phillips and Gregory, 1979).
This latter finding is particularly interesting because it suggests that the bioavailability of MeHg incor-
porated into food may be lower than that of MeHg spiked into prepared diets.
The factors that control dietary MeHg uptake by fish are poorly known. Following the digestion of a
prey item, much of the MeHg in the gastrointestinal tract probably remains bound to organic compounds
and, in particular, to sulfhydryl-containing peptides and amino acids. Although speculative at this time,
it is possible that dietary assimilation efficiency may be determined by the extent to which these
complexes are actively taken up across the gastrointestinal epithelium.
In summary, the interactions between aquatic organisms and major mercury species are varied and
complex. Field surveys have repeatedly shown that MeHg concentrations in large predatory fish from
the same general region may differ by a factor of 10 or more. To understand how this can happen we
must extend the concept of bioavailability to consider factors that control MeHg production as well as
its uptake and accumulation in aquatic organisms. Three critical interactions are thought to control MeHg
production and accumulation in a wide range of systems. The first of these occurs in anaerobic sediments
2+
and involves the uptake of Hg by sulfate-reducing bacteria. Recent work suggests that in the presence
of sulfide this uptake is controlled by the formation of a neutral sulfide complex. The second interaction
involves the uptake of MeHg by lower trophic level organisms living in the water column. In saltwater
systems, this uptake is probably dominated by simple diffusion of neutral MeHgCl, while in freshwater
systems the diffusing species is more likely to be MeHgOH. Strong binding to DOC limits the amount
of MeHg available to form these complexes. Other modes of uptake into lower trophic level organisms
(e.g., active transport of MeHg complexes) have also been proposed. The third interaction involves
dietary uptake of MeHg by fish. The identity of the chemical species that is absorbed within the
gastrointestinal tract is unknown but may include MeHg complexes with small organic molecules.

Nonionic Organics: 2,3,7,8-Tetrachlorodibenzo-p-Dioxin
The compound 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) provides an excellent example of how
bioavailabilities of persistent, hydrophobic, nonpolar organic chemicals influence exposures and conse-
quent toxicity risks to aquatic organisms. With a molecular weight of 322, a log K of approximately
ow
7, and a planar molecular structure, TCDD strongly associates with organic carbon, whether it be in
soils, sediments, POC, or DOC. The hydrophobic character of TCDD not only thermodynamically drives
its partitioning in the abiotic environment but also is responsible for a strong bioaccumulation/biomag-
nification potential which causes TCDD to accumulate in the lipid-containing tissues of biota in food
webs. In most risk assessments concerned with the toxicity of TCDD, it is now recognized that the
combined contributions of all chemicals with a TCDD-like mode of action to overall risk must be
evaluated (Van den Berg et al., 1998). The identification of chemicals that act additively with TCDD is
based on their persistence in the environment and ability to act through an aryl hydrocarbon receptor
(AhR)-mediated mechanism of action as does TCDD. Typically, the contribution of specific congeners
with polychlorinated dibenzo-p-dioxin, dibenzofuran, and biphenyl structures are considered. Thus,
differences between these congeners and TCDD, in bioavailability as well as relative potencies, are
important in quantitative risk assessments for complex mixtures of these chemicals which commonly
occur in the environment. The retrospective assessment of AHR-mediated toxicity to lake trout sac fry
in Lake Ontario provides a case study that validates this predictive capability (Cook et al., 2003).
The bioavailability of TCDD to fish involves both the amount of TCDD available for direct uptake
by fish from water and sediment and the amount of TCDD in their food, which is influenced by the