Page 63 - The Toxicology of Fishes

Bioavailability of Chemical Contaminants in Aquatic Systems 43

One complication that impacts the bioavailability of BaP and other pyrogenic PAHs in aquatic systems
is an association with the black carbon phase in sediments and suspended particles. Because BaP is
derived from combustion reactions, it may largely be transported through the atmosphere to aquatic
systems in fine soot particles to which it is more strongly bound than to biogenic organic carbon in
sediment. Thus, K values measured for ambient environmental conditions may be significantly greater
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than predicted from K because the black carbon component of sediments significantly reduces the
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amount of BaP in the sediment available for desorption to the water phase. Activated carbon–water
partition coefficients have been suggested for prediction of the effect of black carbon in sediments on
PAH concentrations in pore water (Gustafsson et al., 1997). This requires that the fraction of black
carbon in the sediment be determined in addition to the fraction of biogenic organic carbon. Although
PCBs were observed to be 100% available for equilibrium partitioning to water from sediments in
desorption experiments, only a small fraction of pyrogenic PAHs in the sediments were similarly available
(McGroddy et al., 1996). This partitioning difference is probably related to whether the chemical is
sorbed on the surface (available for desorption) vs. incorporated into the particle matrix (less available
for desorption). The degree to which a chemical formed during a combustion process is locked into the
matrices of fine combustion particles probably depends on the volatility of the chemical, the combustion
temperature, and the gas/particulate emission conditions.
Another factor that may limit benthic food chain transfer of BaP to fish is biotransformation in benthic
invertebrates at the base of the food chain. Two species of deposit-feeding marine amphipods (Rhep-
oxynius abronius and Eohaustorius washingtonianus) were found to extensively metabolize BaP accu-
mulated from sediment into three different BaP–diol and two hydroxy–BaP molecules (Reichert et al.,
1985). Similarly, there was evidence of formation of phase I and phase II metabolites of BaP accumulated
from sediment by larvae of a midge (Chironomus riparius) and a fingernail clam (Spaerium corneum),
although much of the BaP remained unmetabolized after 5 days following a 5-day exposure (Borchert
et al., 1997). Shorter exposures of C. riparius to BaP in water seemed to result in faster rates of
metabolism and elimination (Leversee et al., 1982). This difference could be related to an inability to
reach an internal steady-state distribution of BaP in shorter exposures. Three species of polychaete worms
exposed for 8 days to BaP on sediment showed different rates of metabolism, with half-lives for
elimination ranging from 3.7 to 10.3 days (Driscoll and McElroy, 1996). The fraction of BaP identified
as metabolites in ten small invertebrate species ranged from 7 to 96% (McElroy et al., 2000). Despite
this metabolism in invertebrates, fish can still receive significant PAH exposure from the consumption
of benthic invertebrates. Zebra mussels (Dreissena polymorpha) in the Detroit River and western Lake
Erie were found to accumulate BaP to concentrations up to 8 ng/g. Freshwater drum (Aplodinotus
grunniens) from this area had large concentrations of PAH metabolites in their bile, indicating extensive
exposure to PAHs (Metcalfe et al., 1997). The stomach contents of bottom-feeding fish have been found
to contain substantial concentrations of PAHs, including BaP (Maccubbin et al., 1985). The bioavailability
of BaP to fish is complicated by the degree to which metabolism affects rates of BaP elimination in
organisms throughout aquatic food webs and the possibility of direct exposure through ingestion of
sediment.
Accumulation of PAHs in tissues within fish is further limited by biotransformation by the fish. Reports
of BaP metabolism in fish trace back at least to 1972 (Lee et al., 1972). The BCF for unmetabolized
BaP in bluegill sunfish (Lepomis macrochirus) was determined to be 490, which was about 60 times
less than that predicted from the K (Spacie et al., 1983). The BCF measured for unmetabolized BaP
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in gizzard shad (Dorosoma cepedianum) after a 3-day exposure via water was only 3.2, but preexposure
to the fungicide clotrimazole, which inhibits P450 activity, increased the BCF to 35 (Levine et al., 1997),
illustrating the potential for chemical interactions that might influence BaP metabolism under environ-
mental exposure conditions. BaP is extensively metabolized in fish livers, so unmetabolized BaP is
usually not detectable in liver tissue (Varanasi and Gmur, 1981). In fact, the distribution of BaP and
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metabolites in northern pike (Esox lucius) tissues, following water exposure for 8.5 days with [ H]-BaP,
was dominated by the disposition of metabolites, with 15, 40, and 1300 times more radioactivity in liver,
kidney, and gallbladder, respectively, than in adipose tissue (Balk et al., 1984). Temperature was shown
to influence both the rate of metabolism and the composition of the metabolite mixture produced by the
gulf toadfish (Opsanus beta) (Kennedy et al., 1989). Dietary exposure of fish to BaP results in metabolism