Influence of the Properties of Chemicals on Their Environmental Fate - Organic Pollutants: An Ecotoxicological Perspective - page 72

resistant to such oxidative attack. Thus, polyhalogenated compounds of this type
present a general problem of persistence and have long biological half-lives in most
animals, terrestrial or aquatic. They are, therefore, liable to undergo biomagnifica-
tion with movement along food chains, and also to survive transportation over long
distances in migrating animals (e.g., fish or whales) and birds. Some organometallic
compounds (e.g., methyl mercury) are also very persistent in terrestrial vertebrates,
and are only slowly metabolized.
Some compounds that are readily and rapidly metabolized and excreted by ter-
restrial vertebrates degrade only very slowly in other species, for example, marine
invertebrates such as bivalve mollusks. The general question of major differences
between species and groups in metabolic capacity will be discussed in Chapter 4,
Section 4.1. At this stage, the critical point is that there are compounds (such as
PAHs) that tend to bioconcentrate/bioaccumulate in certain species low in aquatic
food chains that are rapidly metabolized by fish, mammals, and birds occupying
higher trophic levels. Thus, unlike many polyhalogenated compounds, they are not
biomagnified as they move to the top of the food chain. This illustrates the limi-
tations of using standard laboratory species such as rats, mice, or Japanese quail
as metabolic models in ecotoxicology. There are many chemicals that are rapidly
metabolized by these species, which are recalcitrant and consequently persistent in
certain aquatic invertebrates (Livingstone 1991; Walker and Livingstone 1992).
The biotransformation of lipophilic pollutants into water-soluble and readily
excretable products represents the main mechanism for their elimination by terres-
trial animals. Its effectiveness, however, depends on the availability of the pollutants
to the relevant enzymes. Where pollutants are stored in fat reserves, they usually
become available to the liver in course of time, with the mobilization of lipids.
Rates of elimination depend on rates of mobilization of fat reserves. The very strong
binding of pollutants to proteins can severely limit the availability of chemicals to
enzyme systems or for direct excretion, and can result in very long biological half-
lives. Examples include the binding of superwarfarins to liver proteins (Chapter 11)
and the binding of hydroxymetabolites of PCBs to plasma proteins (see Chapter 6,
Section 6.2.4). Such long-term binding is limited by the number of available binding
sites and, in the examples given, only relates to the persistence of low concentrations
of chemical. Higher concentrations are freely available for metabolism or excretion.
The refractory nature of some pollutants, notably, persistent polyhalogenated com-
pounds, has raised problems of bioremediation of contaminated sites (e.g., sediments
and dumping sites). There has been interest in the identification, or the production by
genetic manipulation, of strains of microorganisms that can metabolically degrade
recalcitrant molecules. For example, there are bacterial strains that can reductively
dechlorinate PCBs under anaerobic conditions.
3.4 Summary
The environmental fate of chemicals is determined by both chemical/physical and
biological processes; in turn, the operation of these processes is dependent on the
properties of the environmental chemicals themselves. Polarity, vapor pressure,
partition coefficients, and chemical stability are all determinants of movement and