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

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 (r_m) 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 r_m and the total brood size(18 µg/l). For lindane the CEA based LOAEC was twotimes lower than the LOAEC based on r_m(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 andr_m 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 r_m-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.     

Dynamic energy budget theory and population ecology: lessons from Daphnia

Dynamic energy budget (DEB) theory offers a perspective on population ecology whose starting point is energy utilization by, and homeostasis within, individual organisms. It is natural to ask what it adds to the existing large body of individual-based ecological theory. We approach this question pragmatically--through detailed study of the individual physiology and population dynamics of the zooplankter Daphnia and its algal food. Standard DEB theory uses several state variables to characterize the state of an individual organism, thereby making the transition to population dynamics technically challenging, while ecologists demand maximally simple models that can be used in multi-scale modelling. We demonstrate that simpler representations of individual bioenergetics with a single state variable (size), and two life stages (juveniles and adults), contain sufficient detail on mass and energy budgets to yield good fits to data on growth, maturation and reproduction of individual Daphnia in response to food availability. The same simple representations of bioenergetics describe some features of Daphnia mortality, including enhanced mortality at low food that is not explicitly incorporated in the standard DEB model. Size-structured, population models incorporating this additional mortality component resolve some long-standing questions on stability and population cycles in Daphnia. We conclude that a bioenergetic model serving solely as a 'regression' connecting organismal performance to the history of its environment can rest on simpler representations than those of standard DEB. But there are associated costs with such pragmatism, notably loss of connection to theory describing interspecific variation in physiological rates. The latter is an important issue, as the type of detailed study reported here can only be performed for a handful of species.