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exposure generated increased food intake, lower swimming speed, and high oxygen con-
sumption, thus involving a metabolic cost (McGeer et al. 2000 and quoted literature).
It is noteworthy that chronic exposure to xenobiotics does not systematically involve
increased acquisition of tolerance in populations, as shown by the reduction of diversity
commonly observed in contaminated environments. Theory suggests that individuals
tolerant to one particular type of stress may have reduced performance when confronted
with another stressor. The cost of resistance, which can be associated with physiologi-
cal acclimatization as well as genetic adaptation, could originate from increased alloca-
tion of energy and resources to defense mechanisms. However, other processes have also
been reported in literature, such as an alteration in the function of some protein targets
or a reduction of physiological plasticity or evolution (Meyer and Di Giulio 2003 and
literature cited therein). Indeed, in the cyanophycean
Microcystis aeruginosa
the acqui-
sition of tolerance to dinitrophenol reduces variability in growth when the blue green
bacterium is subsequently exposed to a concentration gradient of this molecule (Genoni
et al. 2001). In a PCB-resistant strain of the marine diatom
Ditylum brightwellii
, growth
in the presence of this contaminant is better than that of a sensitive strain. In other dia-
toms (
Asterionella glacialis, Thalassiosira nordenskioldii
), the growth of resistant clones origi-
nating from contaminated estuaries is enhanced by the addition of PCB in the culture
medium. Similar observations were made in the case of polynuclear hydrocarbons with
low molecular weight. Nevertheless, in
D. brightwellii
, resistance to PCB reduces tolerance
to lower salinity and nitrogen restriction, but increases tolerance to lower temperatures
(Cosper et al. 1987). These findings corroborate previous research on terrestrial plants or
bacterial strains resistant to antibiotics, revealing that resistant organisms are favored in
the presence of the toxin, but in contrast are at a disadvantage in its absence (Cosper et
al. 1988 and literature cited therein). In F1 and F2 offspring of fish (
Fundulus heteroclitus
)
exposed for decades to a mixture of contaminants (mainly creosote) in the field, there was
enhanced sensitivity to photodegradation products of anthracene and fluoranthene, and
to hypoxia (Meyer and Di Giulio 2003).
3.4.3 Contamination of Food Webs
Tolerance is responsible for the survival of organisms in polluted environments, but tol-
erant individuals/populations/species may constitute contaminated links in food webs.
This risk is more or less critical, depending on the physiological mechanisms used by
organisms along a food chain to cope with chemical exposure: particularly elimination
or storage (Figure 3.2). The influence of tolerance on the trophic transfer of contaminants
has been recently reviewed (Amiard-Triquet and Rainbow in Amiard-Triquet et al. 2011).
If the metal tolerance mechanism of an invertebrate involves increased storage detoxifi-
cation, there is a real risk of increased trophic transfer. In Cu-resistant bacteria
Vibrio
sp.,
important bioaccumulation of this metal was observed. In the presence of these bacteria,
the larvae of the bivalve
Argopecten purpuratus
accumulated Cu to very high levels. Thus,
bacterial copper accumulation could be very significant in marine environments, increas-
ing copper transfer at the base of marine food chains (Miranda and Rojas 2006). The eco-
toxicological significance of trophic transfer has been documented in some species. Thus,
decapod crustaceans
Palaemonetes varians
fed on metal-rich Restronguet Creek polychaetes
Nereis diversicolor
showed significant mortality (Rainbow et al. 2006). Zebrafish
Danio rerio
also fed on Restronguet Creek
N. diversicolor
in the laboratory showed reduced reproduc-
tive outputs, attributed by the authors to the trophic transfer of arsenic from these worms
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