Dietary assimilation and elimination of Cd, Se, and Zn by Daphnia magna at different metal concentrations

This study examines the influences of Cd, Se, and Zn concentrations in ambient water on the assimilation, elimination, and release budget of metals, and the clearance rate of Daphnia magna through ingestion of phytoplankton diets. With increasing ambient metal concentration over two to three orders of magnitude, the assimilation efficiencies (AEs) of Cd and Se from two green algal species, Scenedesmus obliquus and Chlamydomonas reinhardtii, decreased significantly. In contrast, the Zn AE from ingested food, which typically was low (<7%), was not impacted by the difference in Zn or Cd concentrations in ingested cells. The Cd AE also was reduced significantly by a high Zn concentration in ingested algae, perhaps due to the competitive assimilation between the two metals. Metal elimination was determined after the daphnids were exposed to the radiolabeled diets for 3 d. The elimination rate constant was not dependent on the metal concentration in ingested food. Similarly, the routes of metal elimination (excretion, egestion, molting, and neonate production) were less influenced by the metal concentration. The rapid elimination of Se and Zn may be attributed to the transfer of these metals from mother to offspring. Except at the highest Cd (917 nM) and Se (1,333 nM) concentrations, there was no significant effect of metal concentration on the clearance rate of the animals. Consequently, the metal assimilation in D. magna may play an important role in the regulation of Cd and Se influx into the animals, whereas the regulation of the metal tissue burden in the animals is unlikely to be achieved through the metal elimination. The trophic transfer factors (TTF) of Cd and Se were related inversely to the metal concentration in the water, whereas the TTF of Zn was less variable at different Zn concentrations. The change of TTF with metal concentration will have important implication for ecotoxicity testing using D. magna, especially when the dietary exposure is incorporated.