Biomarkers of Defense, Tolerance, and Ecological Consequences - Ecological Biomarkers: Indicators of Ecotoxicological Effects - page 64

3.5 Conclusions
Considering the general aim of this topic, we need to conclude this chapter on biomark-
ers of defense in terms of their utility as tools for the assessment of the impact of chem-
ical stress on populations and ecosystems. The ecological importance of tolerance and
underlying defense mechanisms depends on the extent of this phenomenon. Many spe-
cies belonging to most taxonomic groups are able to develop tolerance toward the major
classes of contaminants to which they have been chronically exposed (Tables 3.1 and 3.2),
and sometimes to other compounds thanks to cross-tolerance. Thus, it seems that many
species can cope with chemical stress in their environment, so contributing to the conser-
vation of biodiversity and normal or subnormal functioning of the ecosystem.
However, this positive interpretation of the information provided by biomarkers of
defense needs to be moderated. First, it is questionable whether the literature accurately
reflects the field situation, since all authors have experienced that negative results are not
as easily publishable as positive results. A number of counterexamples have shown an
increased sensitivity of the progeny of exposed parents (Bervoets et al. 1996; Villarroel
et al. 2000). Second, logistical constraints have limited scientific work to species that are
easy to collect in the field and keep in the laboratory. Because interspecific variations of
sensitivity are important (Chapter 7), the risk assessment of chemicals based on a small
number of species may be seriously biased. If species selected as test organisms argue for
a reduction of logistical constraints, it is because they are often tolerant to the nonchemical
stress generated by laboratory conditions. Athrey et al. (2007) have shown a loss of genetic
variation resulting from maintaining populations of fish Heterandria formosa in the labora-
tory. These authors underline that the potential for loss of genetic variation in laboratory
populations must be taken into consideration when extrapolating from laboratory to natu-
ral populations. For sentinel species collected from the wild where they experience impor-
tant variations of natural factors (in the intertidal zone, in estuaries), it has been thought
that this tolerance to natural stress could spread to tolerance to chemical stress, leading to
an undervaluation of risk in field situations. More recently, it has been established that, on
the contrary, species at the limit of their tolerance to natural stress are more sensitive to
any additional (chemical) stress (Hummel et al. in Amiard-Triquet et al. 2011).
For several biomarkers of defense (MXR, SOD in Figure 3.2, MT, HSP), it has been shown
that the relationship between dose and effect deviates from linearity for severe contamina-
tion. Equal concentrations (proteins) or activities (enzymes) of biomarkers of defense can
therefore correspond to doses either below or above the maximum value for induction. In
the first case, the induction of the defense mechanism is efficient in protecting the organ-
isms, whereas in the second case, the induction is thwarted and toxic effects can occur.
The protective value of tolerance mechanisms must not be overvalued since this chapter
has documented a number of secondary negative effects of being tolerant: (1) energy cost
leading to changes in energy allocation with a risk of cascading effects from individuals
to populations (Chapter 12), whatever the origin of tolerance, either physiological acclima-
tion of individuals or inheritable genetic adaptation (Chapter 14); (2) formation of metabo-
lites, which may be more toxic than parent compounds (carcinogenic, generating oxidative
stress); (3) increased sensitivity to another type of stress such as photosensitivity or temper-
ature, which may be crucial considering the reduction of the ozone layer or global warm-
ing. The status of fish populations in highly contaminated estuaries of the east coast of
North America is a good illustration of the difficulty of deciding upon the beneficial role of
tolerance (Romeo and Wirgin in Amiard-Triquet et al. 2011). Paradoxically, high prevalence