The use of biomarkers in Daphnia magna toxicity testing. IV. Cellular Energy Allocation: a new methodology to assess the energy budget of toxicant-stressed Daphnia populations

Abstract

The Cellular Energy Allocation (CEA) methodology wasdeveloped as biomarker technique to assess the effectof toxic stress on the energy budget of testorganisms. This short-term assay is based on thebiochemical assessment of changes in the energyreserves (total carbohydrate, protein and lipidcontent) and the energy consumption (electrontransport activity). The CEA methodology was evaluatedusing Daphnia magna juveniles exposed for 96hto sublethal lindane and mercury chlorideconcentrations. The ecological relevance of the CEAassay was assessed by comparing the sub-organismalresponse with population level parameters (obtainedfrom 21 day life table experiments) such as theintrinsic rate of natural increase (rm) and themean total offspring per female. Two differentmethodologies were used to assess the effect levels:the no (lowest) observed effect level (NOAECs-LOAECs)approach and the regression-based approach. Bothtoxicants caused a significant decrease in the netenergy budget of D. magna, with a LowestObserved (Adverse) Effect Concentration (LOAEC) of0.18 mg/l and 5.6 µg/l for lindane andHgCl2,respectively. Changes in the lipid content of theorganisms were detected at toxicant concentrationslower than those affecting the total carbohydrate andprotein content. Toxicant specific effects wereobserved on the electron transport activity. Comparison of the CEA results with those of thepopulation level tests revealed that for mercury theCEA based LOAEC was a three times lower than thatbased on rm and the total brood size(18 µg/l). For lindane the CEA based LOAEC was twotimes lower than the LOAEC based on rm(0.32 mg/l) but was higher than that based on thetotal number of offspring produced (0.1 mg/l). Using the regression-based approach, EC10 valueswere calculated using three parameter sigmoid orlogistic models. Comparison between the CEA andrm based EC10 values demonstrates that forboth chemicals similar effect concentrations areobtained: the CEA-based EC10 (0.20 mg/l) forlindane is 1.5 times higher than the rm-basedEC10 threshold (0.13 mg/l), while for mercury thebiomarker-based EC10 value (9 µg/l) was 1.4times lower than the population-based EC10 value(12.5 µg/l). From these results, we suggest that the short-term CEAassay may be useful for predicting long-term effectsat the population level. The consequences of theobserved effects on the energy budget of the testorganism are discussed in the context of the effectsemerging at the population and community level.