Energy reserve modification in different age groups of Daphnia schoedleri (Anomopoda: Daphniidae) exposed to hexavalent chromium

Caloric content is a reliable biomaker of effect since it is modified by exposure to toxicants that can alter basal metabolism. Since organisms’ age modifies how energy resources are allocated and modifies the activity of antioxidant enzymes, the response to toxic agents could be altered with age. Seven age groups of Daphnia schoedleri (0, 3, 5, 7, 14, 21, and 28-day-old) were exposed for 24 h to two sublethal concentrations of Cr(VI): 1/25 and 1/5 of the 48 h EC50 of each age group, to determine the age at which susceptibility to Cr(VI) is highest. To evaluate energy content, carbohydrate, protein and lipid reserves were quantified and antioxidant enzymes activity was assessed (SOD, CAT, GPx, and GR). Furthermore, an integrative approach was applied to evaluate both sets of responses and interpret them as a whole in a simply visual way, achieved by the integrated biomarker response approach. Results indicate that Cr(VI) induced significant differences in all age groups. Seven and 14-day-old organisms were exposed to the highest concentrations (based on their EC50) and showed greater tolerance to this metal. Susceptibility to the toxicant was highest in younger specimens in which energy requirements are greater due to high growth rates (basal metabolism), as a result of which more energy reserves are expended to satisfy demands in terms of growth and response to toxicants.

► Protein, lipids, carbohydrates, and caloric content were evaluated in Daphnia schodleri.
► Different D. schodleri age groups were exposed to fractions of the LC50 of Cr(VI) for each age.
► Cr(VI) induced significant differences in energy reserves in all age groups.
► Tolerance was higher in specimens aged 7 and 14, which were exposed to the highest concentrations.
► Younger specimens showed a greater reduction in energy reserves, induced by exposure to Cr(VI).