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TP3 Metals in the Environment: Dietary Concerns in Aquatic Systems
() Dietary zinc causes reproductive toxicity in Daphnia magna: differences with waterborne zinc and implications for the chronic biotic ligand model.
De Schamphelaere, KAC1, Canli, M1, Van Lierde, V2, Muyssen, BTA1, Heijerick, DG1, 3, Vanhaecke, F2, Janssen, CR1, 1 Ghent University - Laboratory of Environmental Toxicology and Aquatic Ecology, Gent, Belgium2 Ghent University - Institute of Nuclear Sciences - Laboratory of Analytical Chemistry, Gent, Belgium3 EURAS, Gent, Belgium
ABSTRACT- In this study the effect of dietary zinc on Daphnia magna was investigated and compared to the effect of waterborne zinc. The green alga Pseudokirchneriella subcapitata was exposed for three days to four dissolved zinc concentrations, i.e. a control, 20, 30 and 60 ug/L, resulting in internal zinc burdens in the algae of 130, 200, 320 and 490 ug/g dry weight, respectively. These algae were used as a food source in chronic, 21-day bioassays with D. magna in a test medium to which no dissolved zinc was added. No effects were observed on feeding rates or somatic growth in any treatment. Contrary, a significant 40% decrease of total reproduction (number of juveniles / adult) was observed in the 30 and 60 ug/L treatments, although zinc body burdens in D. magna were only elevated in the 60 ug/L treatment. These data suggest that total body burdens are no good measure of dietary zinc toxicity, and that dietary zinc specifically targets reproduction in D. magna through accumulation in particular target sites. Based on earlier reported dietary metal toxicity in copepods (Hook S and Fisher N, 2002, Marine Environ Res, 53:161-174), those sites are most likely cells or tissues where vitellogenin synthesis or processing occurs. The likeliness of some candidate target sites will be discussed with special emphasis to the cladoceran reproductive physiology. Furthermore, parallel experiments with waterborne exposures indicated large differences between mechanisms of dietary vs. waterborne zinc toxicity, with the latter resulting in a combination of impaired Ca balance, reduced feeding, reduced growth, and reduced reproduction. Currently, the recently developed chronic biotic ligand model (BLM) for D. magna (Heijerick D, De Schamphelaere K, Janssen C, submitted to Can J Fish Aquat Sci) predicts zinc bioavailability and toxicity assuming that toxicity only occurs via the waterborne exposure route, without considering the dietary exposure route or the different toxicity mechanisms for waterborne and dietary zinc exposure. Yet, the BLM is able to rather accurately predict zinc toxicity in natural surface waters, even in the presence of a food source (i.e. a potential dietary source). Potential mechanisms underlying this puzzling observation will be hypothesized, future research needs will be addressed, and the implications for the use of the chronic Zn-BLM in ecological risk assessment will be adressed.
Key words: dietary exposure, Daphnia magna, zinc, BLM
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