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  • 18 Bioavailability and Toxicity of Metals and Hydrophobic Organic Contaminants

    John F. McCarthy

    Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6036

    The effect of humic substances on the availability and toxicity of organic and inorganic contaminants in the aquatic environment is reviewed. Organic contaminants associated with humic substances appear to be essentially unavailable for uptake by amphipods, daph-nids, and fish. Acute toxicity of these compounds is also diminished proportionally. Because the affinity of organic solutes for binding to humic substances is related to their hydrophobicity, the effect of humic substances is significant only for compounds with oc-tanol-water partition coefficients greater than 104. In most cases, association of toxic metals with humic substances reduces the uptake and toxic effects of the contaminants. However, complex interactions among the toxicants, humic ligands, other transition metals and major cations in solution, and the carrier proteins on biological membranes make it difficult to generalize and predict any reduction in accu-mulation and toxicity of metals. Humic substances may have second-ary effects on biota uptake and accumulation of toxicants through their role in altering the transport and fate of pollutants.

    BINDING OF ORGANIC OR INORGANIC CONTAMINANTS to humic substances, or to other dissolved or colloidal organic matter in aquatic systems, can alter the availability of the contaminants for uptake by biota. It is generally thought that humic substances alter bioavailability by altering the concentration of a contaminant that is in a physicochemical form capable of traversing mem-

    0065--2393/89/0219-0263$06.00/0 1989 American Chemical Society

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    In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

  • 264 AQUATIC HUMIC SUBSTANCES

    branes. Association of metal or organic solute with the humic macromolecule masks the chemical properties of the contaminant and thus alters the normal biochemical interaction of the contaminant with the membrane. This phys-icochemical interaction also changes the toxicity of the contaminant, because the toxic effect of a pollutant is directly related to the dose incorporated by the organism. The relationship between dose and adverse response is not always straightforward. Contaminants can be sequestered in pharmaco-kinetically inactive compartments within the animal (e.g., stored in lipid deposits that are isolated from sites of toxic action) or complexes can be formed with specific proteins, such as metallothioneins, which prevent metals from entering tissues and promote excretion of toxic metals.

    In this chapter, I review current knowledge of how reversible interaction of humic substances with organic and inorganic contaminants alters the biological availability and toxic effect of the pollutants. The nature of the interaction of the humic substances with the contaminants and the factors that influence the quantitative distribution of the contaminants between a bound and free form are not discussed here; these are the topics of other chapters. In much of this discussion, the general term "dissolved or colloidal organic matter" (DOM) wil l be used to identify organic components of natural systems that alter the physicochemical properties and the bioavailability of organic or inorganic contaminants. This term includes both aquatic humic and fulvic substances, although the affinity of different components of natural D O M to bind organic or inorganic solutes may be quite different. Metal contaminants are discussed first; then follows a discussion of the effects of humic substances on the availability and toxicity of organic contaminants.

    Effect of DOM on Bioavailability and Toxicity of Metals

    Because of their polyelectrolytic nature, humic and fulvic substances are capable of complex associations with metals (J, 2). The presence of D O M has often been reported to alter the bioavailability and toxic effects of metals in aquatic systems; however, interactions are complex and not easily described or generalized. Some examples of the many and often conflicting reports can illustrate the difficulties in attempting to interpret how the interactions of metals with D O M affect the accumulation and toxcity of metals. Humic acids have been reported to increase the uptake of cadmium by mussels (3) and rainbow trout (4), but to decrease accumulation in phyto-plankton (5, 6), algae (7), Daphnia magna (8), and corn roots in water culture (9). D O M slightly increased uptake of americium and plutonium by phy-toplankton (JO), but decreased uptake of several other metals, including mercury and zinc by fish (4), and zinc, chromium, and cobalt by algae (7). D O M had no measurable effect on accumulation of copper by a polychaete

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    In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

  • 18. MCCARTHY Metals and Hydrophobic Organic Contaminants 265

    (11), nor cadmium by D. magna (12). The presence of D O M also altered the toxicity of metals, but not necessarily in direct relation to the accumulation of the metals in organisms (13). Toxicity decreased for cadmium in Atlantic salmon and algae (14); for copper in Atlantic salmon (15), algae (5), and D . magna (12, 16); and for zinc in daphnids (17). Toxicity of cadmium to D . magna and copper to D . pulex (16) increased in the presence of humic acid, but only in hard water (12, 18).

    Several mechanisms have been proposed to account for the diverse and seemingly conflicting results. It is generally assumed that the free metal ion is the chemical species responsible for the biological effects of metals. The biological uptake and toxicity of the metals are expected, therefore, to be altered by interactions of the metal with inorganic and organic ligands, which alter the concentration of the ionic species (e.g., 5, 19-22). The major processes that control chemical speciation of metals in natural waters (precipitation, formation of complexes with inorganic and organic ligands, and adsorption by particulate material) can be modeled to permit calculation of the concentration of free ionic metal (23, 24). However, direct measurement of the free metal ion concentration during accumulation or toxicity experiments failed to confirm a direct relationship between free ion concentration and biological effect (e.g., 4, 8, 17).

    The models and measurements of solution chemistry may fail because they neglect possible changes in solution equilibria at the interface between the water and the biological membrane (gill or gut). Competitive effects between the DOM-meta l complex and the carrier proteins in biological membranes responsible for active transport of metals into the organism, or changes in binding affinity of these membrane ligands due to competitive binding of other transition metals or major cations in solution, could alter the amount of metal translocated across biological membranes into the organism. Enhanced uptake of metals could be due to passive diffusion of organic-metal complexes. Association of metals with relatively low-molecular-weight organic ligands could make the metal more lipid-soluble and thus increase permeability through membranes by a mechanism other than carrier-mediated transport of the free ionic metal species (3, 25).

    Interpretation of uptake and toxicity results can be further complicated by biochemical processes within the test organism. D. magna excrete organic compounds with metal-binding activity similar to that of humic and fulvic acids (26). The release of agents that form complexes may affect the results of toxicity tests by reducing metal activities during incubation. Furthermore, relationships between the accumulated body burden of metal and adverse toxic effects may be obscured by the protective action of internal metal-binding proteins, such as metallothioneins, which are produced at higher levels within organisms chronically exposed to nonlethal levels of metals (e.g., 27).

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    In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

  • 266 AQUATIC HUMIC SUBSTANCES

    Effect of DOM on Bioavailability and Toxicity of Organic Contaminants

    The effect of humic substances on the biological uptake and toxicity of organic contaminants appears much more consistent and predictable than their effect on metal toxicants. Available data suggest that ass