Abstract
| - The uptake kinetics of Cd and Zn in a freshwater cladoceranDaphnia magna after exposure to different concentrationsof Cd for various durations was quantified. The accumulatedCd concentrations increased with ambient Cd concentrationand exposure duration. As a detoxification mechanism,metallothioneins (MTs) were induced when the Cdpreexposure condition was beyond the noneffect threshold.The MT induction was dependent on both Cd concentrationand duration of preexposure. Increasing the Cd exposureconcentration to 20 μg L-1 for 3 d caused a 44% reductionin Cd assimilation efficiency (AE, the fraction assimilatedby the animals after digestion) by the daphnids from thedietary phase, but a 2.4-fold increase in Zn AE. Generally,the dissolved metal uptake rate was not significantly affectedby the different Cd preexposure regimes, except at amuch higher Cd concentration (20 μg L-1) when the Zninflux was enhanced. Significant effects from Cd exposureon the ingestion rate of the daphnids were also observed.When the MT synthesis was not coupled with theaccumulated Cd tissue burden (e.g., a delay in MT synthesis),apparent Cd toxicity on the feeding behavior and the CdAE was observed, thus highlighting the importance of MTsin modifying the metal uptake kinetics of D. magna.Overall, daphnids responded to acute Cd exposure byreducing their Cd AE and ingestion, whereas they developeda tolerance to Cd following chronic exposure. Thebioavailability of Zn was enhanced as a result of Cdpreexposure. This study highlights the important influencesof Cd preexposure history on the biokinetics and potentialtoxicity of Cd and Zn to D. magna.
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