9. Bioaccumulation and Trophic Transfer of Organic Contaminants
213
9.5. Toxicity and the Role of Lipid
9.5.1. Release of Sequestered Contaminant
During Metabolism
Because persistent organic contaminants accumulate in lipid material, alteration
of the lipid pool can alter the toxicity of contaminants. For example, when lipidrich organisms such as salmon are starved to reduce the total lipid, stored toxins
are also released, which can produce deleterious results (Ewald, 1996; Bickel,
1984). Further, organisms that are rich in lipid can sequester toxic contaminants in
storage sites, thereby removing the toxin from the site of toxic action (Geyer et aI.,
1994, 1993; van den Huevel et aI., 1991; Bickel, 1984). Thus, the relative storage
capacity of organisms needs to be considered not only for bioaccumulation but
also for toxicity. Normalization to the amount of lipid in the organism is suggested
to reduce this variation when comparing toxicity values for neutral organic contaminants among species (Geyer et aI., 1994). Normalized contaminant concentration data lead to the idea of the survival of the fattest. This concept has been
incorporated into a model of exposure and toxic response for narcotic contaminants. Not only will the fattest organisms have additional storage capacity, the
higher fat content will impart additional energy reserves to help the organism
weather stresses (Lassiter and Hallam, 1990).
The significance of lipid content and composition will increase as attempts are
made to establish residue effects concentration in organisms. Residue effects
concentrations are the measured concentrations of contaminant, on a whole-body
basis, that are equated with a toxic response. Residue effects concentrations are
under development in an attempt to move aquatic toxicology from the use of the
external environment as a measure of exposure to the internal dose as the exposure
measurement (McCarty and Mackay, 1993). There have been several studies
demonstrating the reduction in overall range of doses required to produce mortality, particularly for nonpolar narcotics (e.g., polycyclic aromatic hydrocarbons
[PAHs], chlorinated benzenes, PCBs), compared with the use of external concentrations (e.g., for fish, as reviewed by McCarty and Mackay, 1993; for amphipods, Landrum et aI., 1994, 1991, and Landrum and Dupuis, 1990; for daphnids,
Pawlisz and Peters, 1993a,b). The remaining variance can be attributed t~ three
potential causes: the relative capacity for a contaminant of one species or group of
organisms over another (i.e., differences in lipid content), the inherent difference
in the sensitivity of one species or portion of the population versus another for the
contaminant, and the relative biotransformation capability of different organisms.
For contaminants that act as narcotics and are not readily biotransformed, differences in the sensitivity among and within species can be largely accounted by
adjusting for the lipid composition and content of the organisms. For example, the
variation in the lethal body burden (residue effects concentration) in fathead
minnows exposed to chlorinated hydrocarbon contaminants was reduced 50% by
accounting for the minnows' lipid content (van Wezel et aI., 1995). Specific
213
9.5. Toxicity and the Role of Lipid
9.5.1. Release of Sequestered Contaminant
During Metabolism
Because persistent organic contaminants accumulate in lipid material, alteration
of the lipid pool can alter the toxicity of contaminants. For example, when lipidrich organisms such as salmon are starved to reduce the total lipid, stored toxins
are also released, which can produce deleterious results (Ewald, 1996; Bickel,
1984). Further, organisms that are rich in lipid can sequester toxic contaminants in
storage sites, thereby removing the toxin from the site of toxic action (Geyer et aI.,
1994, 1993; van den Huevel et aI., 1991; Bickel, 1984). Thus, the relative storage
capacity of organisms needs to be considered not only for bioaccumulation but
also for toxicity. Normalization to the amount of lipid in the organism is suggested
to reduce this variation when comparing toxicity values for neutral organic contaminants among species (Geyer et aI., 1994). Normalized contaminant concentration data lead to the idea of the survival of the fattest. This concept has been
incorporated into a model of exposure and toxic response for narcotic contaminants. Not only will the fattest organisms have additional storage capacity, the
higher fat content will impart additional energy reserves to help the organism
weather stresses (Lassiter and Hallam, 1990).
The significance of lipid content and composition will increase as attempts are
made to establish residue effects concentration in organisms. Residue effects
concentrations are the measured concentrations of contaminant, on a whole-body
basis, that are equated with a toxic response. Residue effects concentrations are
under development in an attempt to move aquatic toxicology from the use of the
external environment as a measure of exposure to the internal dose as the exposure
measurement (McCarty and Mackay, 1993). There have been several studies
demonstrating the reduction in overall range of doses required to produce mortality, particularly for nonpolar narcotics (e.g., polycyclic aromatic hydrocarbons
[PAHs], chlorinated benzenes, PCBs), compared with the use of external concentrations (e.g., for fish, as reviewed by McCarty and Mackay, 1993; for amphipods, Landrum et aI., 1994, 1991, and Landrum and Dupuis, 1990; for daphnids,
Pawlisz and Peters, 1993a,b). The remaining variance can be attributed t~ three
potential causes: the relative capacity for a contaminant of one species or group of
organisms over another (i.e., differences in lipid content), the inherent difference
in the sensitivity of one species or portion of the population versus another for the
contaminant, and the relative biotransformation capability of different organisms.
For contaminants that act as narcotics and are not readily biotransformed, differences in the sensitivity among and within species can be largely accounted by
adjusting for the lipid composition and content of the organisms. For example, the
variation in the lethal body burden (residue effects concentration) in fathead
minnows exposed to chlorinated hydrocarbon contaminants was reduced 50% by
accounting for the minnows' lipid content (van Wezel et aI., 1995). Specific