Biomarkers of Defense, Tolerance, and Ecological Consequences - Ecological Biomarkers: Indicators of Ecotoxicological Effects - page 58

et al. 2006a). As already mentioned for other biomarkers of defense (see Section 3.3.1 and
Figure 3.1), significant linear relationships exist between ABCB1 protein expression in mus-
sels Mytilus galloprovincialis from the French coast and the body burdens of contaminants,
up to a concentration limit of ca. 1.2 mg Cd kg −1 dw cadmium and 1 mg PCB kg −1 dw. This
could indicate that the mussels were then relying on an increased transport activity or on
another defense mechanism. Alternatively, the organisms' health might have already been
compromised so that they were unable to further intensify their MXR defense mechanism.
The protective role of MXR also showed a limit in the freshwater mussel Dreissena poly-
morpha , for, in the Seine estuary in France downstream of Rouen (390,000 inhabitants), a
decrease in lysosomal stability and a reduction in condition index were observed despite
increased levels of MXR proteins (Minier et al. 2006b).
The protective role of MXR proteins may be hampered by exposure to so-called chemosen-
sitizers (synthetic musk fragrances studied by Luckenbach et al. 2004; emerging contaminants,
natural substances produced by certain invasive species studied by Smital et al. 2004; and oth-
ers reviewed by Bard 2000). At environmentally realistic doses, they are able to inhibit the nor-
mal functioning of the MXR system, thus enhancing the accumulation of xenobiotics that are
normally transported from the cell. The role of chemosensitizers as environmental pollutants
and the ecotoxicological consequences of transporter inhibition have been highlighted (Bard
2000). Because biotransformation activities (phases I and II) are generally not observed in early
development stages, Damiens and Minier (in Amiard-Triquet et al. 2011) suggest that embryos
may rely on other defense mechanisms such as the ABC system, which appears as a first line
of defense, and that inhibition of MXR activity may have dramatic consequences.
3.4 Ecological Consequences of Tolerance
3.4.1 Conservation of Biodiversity
In a number of cases, defense responses are called upon only for a limited period, for instance,
when an animal is able to avoid exposure after it has detected the presence of contaminants
(Chapter 10). This type of response is interesting for the conservation of a population in the
case of a short-term pollution (accident, occasional discharge, possibly cyclic discharges).
Thus, Lotts and Stewart (1995) have shown a temporary acclimation to residual chlorine in
several species of minnows, enabling the presence of the fish in areas where concentrations
are generally considered lethal. In the fish Catostomus commersoni living in metal-contami-
nated lakes, tolerance provided to larvae by a maternal yolk factor disappeared when larvae
began feeding, 24 days after hatching (Munkittrick and Dixon 1988). At the other extreme,
genetic adaptation to chemical stress is responsible for the transmission of tolerance to the
progeny (Chapter 14), and in this case, the protective effect will last in the long term.
Moving from tolerance at the populational level to the intrinsic relative insensitivity
of each species, there is evidence that acute contamination resulting from accidents can
cause the local extinction of sensitive species. This is particularly well documented in the
case of oil spills that can cause selective mortality of the benthic meiofauna (Ernst et al.
2009; Martínez-Colon et al. 2009) and the macrofauna (Gomez-Gesteira and Dauvin 2005).
Similarly, in a given environment, increasing chronic contamination will lead to the local
extinction of sensitive species, followed by that of less sensitive species. The new com-
munity as a whole is more tolerant to the toxicant responsible than another community,