Regardless of the level of biological complexity, ecotox-icological mechanisms primarily begin with and depend upon the bioavailability of the environmental contaminant. As discussed earlier, the distribution of contaminants is strongly influenced by environmental parameters, thus determining bioavailability. In aquatic systems, the bioavailable fraction will be influenced by all the site parameters, such as pH, hardness, alkalinity, anions, temperature, oxygen content, and organic carbon content. Even the type of organic carbon, for example, fulvic acid, humic acid, humin, kerogen, coal, soot, and black carbon, affects the contaminant distribution and bioavailable fraction differently. In soils and sediments, contaminant bioavailability will also be affected by site-specific conditions such as organic matter, clay content, pH, and cation-exchange capacity. This section describes in more detail the effects of the environmental chemistry on each of the bioavailable processes shown in Figure 3.
Bioavailable process 1, depicted in Figure 3, is an important factor affecting environmental distribution and bioavailability. In the environment, the contaminant may interact with organic matter or particulates in waters, or organic and mineral solids in soils and sediments. These interactions are characterized as the contaminant becoming bound or free/labile. The strength of these interactions is not only characterized by the physical-chemical partitioning properties discussed in the previous section, but also by the environmental conditions. Organic processes are typically dominated by adsorption, absorption, and partitioning. Inorganic contaminants are bound to particulates via many types ofprocesses, including absorption and adsorption and precipitation. Overall, a wide variety of mechanisms exist, which results in contaminants becoming bound/unbound with solid phases, thus influencing processes 2 and 3 in Figure 3.
Transport process 3, as depicted in Figure 3, may occur within any environmental compartment: air, water, soil, and sediment. Often, transport in water or gas phase are considered most important for bioavailable exposure scenarios, but contaminants may also be transported via soil and sediment-borne particles. These particles may transport to the biological receptor, biological membrane, via suspension in air, or water. Transport process 2 considers the released, free contaminant, typically transported within the fluid phases by fluid advection processes. Even in nonflowing fluids, contaminants are still transported by the smaller-scale molecular diffusion processes. Free contaminants may also volatilize and move into the atmosphere where they are transported long distances. For example, the persistent PCB contaminants have been discovered on a global-scale distribution due to long-range atmospheric transport. It is worth noting that during transport, chemical transformation may occur and affect contaminant bioavailability. Transport transformation may include photochemical reactions, oxidation-reduction reactions, hydrolysis reactions, and acid-base reactions.
Passage of contaminants into the biological membrane is depicted as process 4 in Figure 3, and illustrated in Figure 4. Although in ecotoxicology there are many different types of receptors, such as plants, animals, and microorganisms, they may all be conceptualized by the cell membrane-cell interior. Biological systems depend in part on the presence of the biological membrane to separate the organism from the environment. Yet, the biological membrane must allow some compounds to move through it while preventing others. The selectivity of the membrane is important. In Figure 4, the biological membrane is composed of phospholipids arranged in a bilayer, as shown by the small ball with two dangling tails. The hydrophobic portion of the molecule is the tail directed toward the center of the membrane. The hydro-philic portion points toward the outward sides of the cell membrane. The surface of the cell therefore interfaces with water, that is, the bulk solution, while the centers are lipids. Proteins are embedded in the membrane, which create pores depicted as channels in the figure, where small chemicals can move into or out of the cell. The main processes by which a chemical can move across the cell membrane are passive diffusion, facilitated diffusion, and active transport.
Once contaminants have entered the organism, the fate of the contaminant is complex and may have deleterious effects. Processes that occur include accumulation, distribution, metabolism, and excretion. As descibed above, the bioavailability of contaminants is governed by a wide range of physical, chemical, and biological processes. These processes occur in concert and may be interdependent. Bioavailability ofcontaminants is a function ofsite-, chemical-, and organism-specific conditions and processes as well as climatic/time influenced.
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