Biomarkers of Defense, Tolerance, and Ecological Consequences - Ecological Biomarkers: Indicators of Ecotoxicological Effects - page 51

3.3 Biomarkers of Defense
Biomarkers of defense reveal mechanisms that allow aquatic organisms to cope with the
presence of pollutants in their environment, at least when they remain at “reasonable”
levels, but with an energy cost.
3.3.1 Mechanisms of Defense against Metals
The relative efficiency of different mechanisms of defense used by organisms exposed to
chemical stress, governs the interindividual, interpopulational, or interspecific variabil-
ity of tolerance. Strategies to prevent contaminant toxicity include the limitation of bioac-
cumulation (controlled uptake, increased excretion) and, when the chemical compound
is internalized, its storage in nontoxic physicochemical form (Mason and Jenkins 1995;
Marigomez et al. 2002; Amiard et al. 2006; Perales-Vela et al. 2006; Sigel et al. 2009).
MTs and related sulfur-rich chelators are recognized as important in metal ion homeo-
stasis owing to their metal binding capacity. In addition, MT antioxidant properties are
frequently evoked (Falfushynska et al. 2012) even though several conflicting experimental
studies about the antioxidant protection conferred by MTs have been reported (Moreau
et al. 2008 and literature cited therein). These authors have shown that different isoforms
of MT, present in different taxa from bacteria to mammals, exhibit different properties.
A recent topic has been devoted to these ligands in many different taxa including verte-
brates and invertebrates from marine and freshwater ecosystems (Sigel et al. 2009). In ver-
tebrates, MTs are considered the major ligand for metal detoxification. In fish originating
from a site polluted for decades by Cd and Zn, increased resistance to Cd in acute toxic-
ity tests by comparison with “naïve” individuals was probably attributable to liver MT
induction (Knapen et al. 2004). Similarly, MTs were involved in resistance to Cd acquired
over several generations in laboratory contaminated fish Heterandria formosa but, at the
maximum, 26.5% of bioaccumulated Cd was associated with MTs, indicating that a large
fraction of this metal was not detoxified by this means (Xie and Klerks 2004). In inverte-
brates, different detoxification processes can be activated in response to metal stress. In
different species and different populations within the same species (depending on their
adaptation to contaminated environments), the respective roles of MTs and biomineral-
ization of metals as metal-rich granules (MRG) may be more or less important (Wallace
et al. 1998; Berthet et al. 2003; Mouneyrac et al. 2003). As exemplified in zebra mussels
from clean and polluted (Cd, Cu, Zn) field locations, in more polluted specimens the con-
tributions of MRGs and MTs become more important, but metal detoxification was not
sufficient to prevent metal binding to low molecular weight (LMW) proteins (Voets et al.
2009). In another freshwater bivalve ( Pyganodon grandis ) translocated from a control site
to a contaminated site, the cytosolic distribution of Cd in the gills was strongly modi-
fied, and the presence of Cd bound to LMW compounds was associated with toxicity
symptoms including lipid peroxidation, decreased condition index and delayed growth
(Couillard et al. 1995). According to the findings of Ivanina et al. (2008) on Crassostrea
virginica exposed to cadmium, MT expression may provide sufficient protection against
Cd-induced damage to intracellular proteins in the digestive gland. In contrast, Cd detoxi-
fication mechanisms appear to be insufficient to fully prevent protein damage in gill cells,
thus necessitating induction of HSPs as a secondary line of cellular defense. Gills appear
to be Cd-sensitive tissues in oysters, with possible important implications for impaired
oxygen uptake contributing to energy misbalance. In crustaceans, the saturation of the