Origin of Energy Metabolism Impairments - Ecological Biomarkers: Indicators of Ecotoxicological Effects - page 298

of amylase (digestive gland, crystalline style) was observed in mussels exposed to vari-
ous chemical contaminants and thus potentially facing an increase in their energy expen-
diture (General Adaptation Syndrome; Selye 1976). However, this upregulation has not
been recorded at all contaminated sites, lower levels of pollution or higher levels of food
availability probably explaining the low need for organisms to “invest” in their digestive
activities (least energy requirement). These results highlight the adjustment of digestive
enzymatic activities according to intake and energy requirements of the organism, factors
which, if not taken into account, constitute potential sources of confoundment in the inter-
pretation of responses measured.
The benefit of such biochemical biomarkers partly resides in their sensitivity in relation
to the effects at higher levels of biological organization (individual, population, or com-
munity). Different studies have thus clarified the precocious character of the response of
digestive enzymes in comparison to the emergence of effects at individual (growth, death)
or community (biodiversity) levels. In the brine shrimp, Artemia salina , inhibitions of tryp-
sin activity were observed after 24 and 72 h of exposure to copper (2 ppm) and zinc (5 ppm),
respectively, and after 72 h (copper) and 24 h (zinc) in the case of amylase activity (Alayse-
Danet et al. 1979). Inhibition of these enzyme activities (24 h for the two elements according
to the enzyme under examination) occurred earlier than the effect on growth, which was
only visible after 3 and 6 days of exposure to zinc and copper, respectively. During expo-
sure carried out in the laboratory on the zebra mussel D. polymorpha , significant induction
of digestive enzymatic activities (endocellulase in digestive gland and crystalline style)
was recorded after 3 days at the highest copper exposure concentration (600 μg L -1 ), this
response having been accompanied by a high mortality rate in the exposed organisms
(Palais 2011). During the exposure of another freshwater bivalve, the clam Corbicula flu-
minea , to different concentrations of cadmium, (3 to 25 ppb) over a period of 3 to 4 weeks,
Barfield et al. (2001) observed significant reduction in cellulase activity before significant
mortality. In these different examples, unrealistic exposure conditions (i.e., unrealistically
high concentrations of toxic metals) were probably the origin of the induced or suppressed
activities measures in the exposed organisms. These results bring to mind that an increase
in digestive enzymatic activities is just as likely to take place as a decrease in these activi-
ties during exposure to trace metals. The response as such (increase or decrease) is never-
theless difficult to anticipate, since it varies relatively according to the enzyme, exposure
(metal, exposure concentration, or duration), or the species under examination (Chen et
al. 2002). The precocity of response was also highlighted during studies undertaken in
natural conditions. Thus, during the transplantation of C. fluminea to different sites in a
river receiving factory wastewater, cellulolytic activity emerged as a highly sensitive stress
indicator (Farris et al. 1988). The decrease in activity was more sensitive than the reduction
of macroinvertebrate diversity (Farris et al. 1988). During active biomonitoring carried out
with the amphipod crustacean Gammarus fossarum in a water course strongly contami-
nated by various trace metals (L'Amous, France), significant repression of digestive enzy-
matic activities (amylase, endocellulase, esterase, galactosidase, glucosidase in the whole
organism) was recorded after 7 days at the most contaminated site, this response having
preceded a high mortality rate in the exposed amphipods (Dedourge-Geffard et al. 2009).
However, whatever the exposure conditions, multibiomarker approaches are necessary
when attempting to define risk for an individual or its population. De Coen et al. (2001)
have shown that the relationship between the enzyme activity parameters of carbohydrate
metabolism, including digestive enzymes, and the observed effect at population level in
Daphnia magna, was specific to the toxin being studied, but also that a single enzyme of
carbohydrate metabolism cannot on its own predict changes at population level.