Distribution and Effects of Chemicals in Communities and Ecosystems - Organic Pollutants: An Ecotoxicological Perspective - page 89

chemical measured by mechanistic biomarker assays, as described in this sec-
tion and in later parts of this text (note especially Chapters 15 and 16).
Unfortunately, as with many exciting new areas of science, ideas are much
easier to conceive than to deliver. There are many difficulties both in the
execution and—not least—the interpretation of results from toxicogenomic
assays. There has been a serious problem with regard to controlling data qual-
ity. Indeed, some journals, for example, Nature , have adopted strict guidelines
on data quality when assessing papers submitted to them for publication on
this subject. Also, the value of genomic data in ecotoxicology needs to be seen
in context. Genotoxic compounds represent a rather small proportion of the
pollutants that are known to have had serious ecotoxicological effects, as will
be apparent from the pages that follow. Most primary toxic interactions—or
their immediate knock-on effects—are not at the level of the gene. Changes in
gene expression bear testimony to toxic disturbances in the organism—to the
disturbance of cellular processes, to the disruption of neurotransmission, to
the ability of blood to clot, etc. They do not provide direct measures of the ini-
tial toxic interactions or the consequent cellular disturbances. Thus, genomic
techniques have great potential when used in combination with mechanistic
biomarker assays that measure the toxic process itself. They also have poten-
tial for screening where toxic effects are suspected in the environment. The
pattern of genomic response may give critical evidence about the mode of
toxic action that operates when pollution occurs.

4.5 BIomarkerS In a wIder ecoLogIcaL context

From an ecological point of view, chemicals can be seen as constituting one class of
agents that stress biological systems (“stressors”) (Van Straalen 2003). As we have
seen, organic chemicals that have harmful effects upon living organisms may be
naturally occurring as well as human made, and both may contribute to “chemical
stress” (Chapter 1). Other stressors include extremes of temperature, acidity, humid-
ity, and levels of inorganic ions not usually regarded as toxic (e.g., nitrate, phosphate,
etc.). In an ideal world, the stress caused by all of these factors should be taken into
account when considering the state—healthy or otherwise—of individuals, popu-
lations, communities, and ecosystems. It may even be argued that, in the ultimate
analysis, ecotoxicology is part of stress ecology (Van Straalen 2003).
However, there are a number of issues here. In the first place, stress itself is a
somewhat nebulous concept, and there is continuing debate about how it should be
defined. Second, even with the benefit of multivariate statistics and the techniques of
bioinformatics, measuring stress from all sources in a meaningful way is dauntingly
complex and may not be realizable in practice.
So far as the discipline of ecotoxicology goes, there have been a number of cases
where organic pollutants were shown to be the principal or sole cause of population
declines in the natural environment, and these are described in the chapters that