Bioavailability Processes

A scientific basis for predicting contaminant impacts on ecosystems requires a mechanistic understanding of bio-availability processes. A mechanism is a physical or chemical process involved in bioavailability. For example, in environmental chemistry, a reaction mechanism is the step-by-step sequence of reactions by which an overall chemical change occurs. The physical, chemical, and biological interactions that define bioavailability as the exposure dose include the bioaccessible mechanisms of contaminant association/dissociation processes within and between environmental matrices, transport processes of both free and bound contaminants to the biological membranes, and uptake/passage through biological

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Figure 2 Analytical methods used in environmental chemistry may be conceptually divided - the analytically recoverable fraction representing traditionally used rigorous extractions methods. Traditional analytical contaminant methods rarely synchronize with biological responses. Ideally an environmental chemistry method would synchronize with the biologically available fraction, discussed in the section titled 'Tools for characterization and measurements of bioavailability'. Different conceptual approaches can lead to significantly different estimates of exposure. The addition of fractions that are not bioavailable increases exposure estimates.

Bioavailability processes: 1, 2, 3, and 4

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Bioavailability processes: 1, 2, 3, and 4

Figure 3 Process 1 illustrates contaminant interactions within environmental compartments, processes 2 and 3 illustrate transport of contaminants to the organisms, and process 4 illustrates passage of contaminants across the biological membrane. Process 5 illustrates circulation within the organism where accumulation in a target organ and toxic effects may occur. Bioaccessability includes processes illustrated in steps 1-3, while bioavailability comprises the first four processes through the biological membrane.

Figure 3 Process 1 illustrates contaminant interactions within environmental compartments, processes 2 and 3 illustrate transport of contaminants to the organisms, and process 4 illustrates passage of contaminants across the biological membrane. Process 5 illustrates circulation within the organism where accumulation in a target organ and toxic effects may occur. Bioaccessability includes processes illustrated in steps 1-3, while bioavailability comprises the first four processes through the biological membrane.

membranes. Another process defining bioavailability is the distribution, metabolism, and accumulation at the target organ where toxicity may occur, although this last process does not determine whether a contaminant is bioavailable.

In Figure 3, the bioavailability process shown in 1 refers to the physical and chemical process of binding and unbinding of contaminants with other compartment components. Environmental compartments include air, water, soil, and sediments. Unbound contaminants are often described as free or labile. Contaminants may bind and unbind on different timescales; some contaminants may bind and unbind quickly while others take years. The kinetics or time frames of these processes are another component of the bioavailable process. Binding mechanisms may include sorption into/onto particles or precipitates. For example, the organic contaminant chlordane, an organochlorine insecticide, may bind and unbind with naturally occurring dissolved organic carbon in aquatic systems. The solubility of chlordane can increase by several hundredfolds in waters containing even modest amounts of dissolved organic carbon. The enhanced solubility results from partitioning of this hydrophobic insecticide into the dissolved organic carbon fraction. However, the increased water solubility does not necessarily indicate an increase in bioaccessibility for transport through an organism membrane. Therefore, dissolved organic carbon may increase transport and mobility of chlordane in the water column, but reduce bioavailability.

In another example of bioavailable process 1 (Figure 3), copper, an inorganic contaminant, may bind and unbind with organic carbon or dissolved inorganic components, such as carbonate, bicarbonate, or chloride. Metals may exist in different valence states, and forms of complexes, depending on the metal and environmental site conditions. Each metal valence state or complex will react differently depending on site conditions. The free metal ion, unbound, is considered the most likely metal species for transport through biological membranes, for example, bioavailable. When metals like copper are bound, they are generally inhibited from passing into the biological membrane and are considered less bioavailable. Binding processes may also occur within other environmental compartments such as soils and sediments. Binding may include bonding with particles or precipitation into nonsoluble fractions. Contaminants bound to particles may become unbound by many processes including reduction/oxidation reactions, as well as the processes discussed above. Contaminants bound to solids include many different types of interactions, and the strength of these interactions can vary, ultimately affecting their bioavailability or lack thereof.

Bioavailable processes 2 and 3, shown in Figure 3, involve the transport of bound and free contaminants to the biological membrane of an organism. Unbound, or free/labile, contaminants in the gas or aqueous phase are subject to transport processes, such as diffusion, dispersion, and advec-tion. Particles may be transported by moving air or water advection processes including resuspension, bioturbation, and diffusion. Contaminants bound to particles may also be immobilized via precipitation and physical entrapment.

Bioavailability

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Figure 4 Cartoon of a biological membrane. Illustrated are several features of biological membrane bioavailability, including ion channels, lipophilic character, and pore size. All of these features affect whether a contaminant will progress into the cell interior where toxic action may occur. Contaminants are idealized as the spheres in the bulk solution; some spheres are bioavailable, shown as crossing into and through the cell membrane, whereas others, illustrated by the larger spheres, are not able to cross the biological boundary.

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Figure 4 Cartoon of a biological membrane. Illustrated are several features of biological membrane bioavailability, including ion channels, lipophilic character, and pore size. All of these features affect whether a contaminant will progress into the cell interior where toxic action may occur. Contaminants are idealized as the spheres in the bulk solution; some spheres are bioavailable, shown as crossing into and through the cell membrane, whereas others, illustrated by the larger spheres, are not able to cross the biological boundary.

Bioavailable process 4 involves the mechanisms associated with the movement of contaminants through the biological membrane. There are many organisms in the environment and their physiologies differ; however, one common feature among all organisms is the presence of a cellular membrane that separates the cytoplasm, cell interior, from the external environment (Figure 4). Most contaminants must pass through the biological membrane before toxic effects on the cell or organism can occur. Processes of contaminant passage through the membrane include passive diffusion, facilitated diffusion, or active transport. Metals must generally pass through ion channels of specific type and diameter, while organic contaminants generally have sufficient lipophilic character to pass through the cell membrane relative to the physical pore size.

Bioavailable process 5 involves the mechanisms occurring after a chemical has crossed the biological membrane. These may include metabolism, storage, and elimination. The contaminant may be metabolized to a form that is less toxic, resulting in no observed effect. Conversely, exposure may result in accumulation of the contaminant to levels that are lethal. Between these two extremes, other harmful effects may occur, such as endocrine disruption, reproductive impairment, or other fitness failures. In addition to the direct effects of exposure on the organism, bioaccumulation or biomagnification through food webs may pose serious environmental consequences, discussed in the section titled 'Bioavailability in ecological risk assessment'.

Although it is useful to consider bioavailability processes in isolation (Figure 3, 1-5), it is also important to recognize that the processes occur in concert and are often interdependent. The following sections provide additional detail on the effect of environmental chemistry on contaminant bioaccessibility, processes that determine what makes a contaminant bioavailable, factors that influence bioavailability, and tools developed to characterize and measure bioavailability.

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