Influence of water hardness on zinc toxicity in Daphnia magna

Abstract

Zinc is an essential trace metal required for the maintenance of multiple physiological functions. Due to this, organisms can experience both zinc deficiency and toxicity. Hardness is recognized as one of the main modifying physiochemical factors regulating zinc bioavailability. Therefore, the present study analyzed the effect of hardness on zinc toxicity using Daphnia magna. Endpoint parameters were acutetoxicity, development, reproduction, and expression data for genes involved in metal regulation and oxidative stress. In addition, the temporal expression profiles of genes during the initiation of reproduction and molting were investigated. Water hardness influenced the survival in response to exposures to zinc. A zinc concentration of 50 μg/l in soft water (50 mg CaCO 3 /L) caused 73% mortality after 96 h exposure, whereas the same zinc concentration in the hardest water did not cause any significant mortality. Moreover, increasing water hardness from 100 to 200 mg CaCO 3 /L resulted in a reduced number of offspring. Fecundity was higher at first brood for groups exposed to higher Zn concentrations. The survival data were used to assess the precision of the bioavailability models (Bio-met) and the geochemical model (Visual MINTEQ). As the Bio-met risk predictions overestimated the Zn toxicity, a competition-based model to describe the effects of hardness on zinc toxicity is proposed. This approach can be used to minimize differences in setting environmental quality standards. Moreover, gene expression data showed that using the toxicogenomic approach was more sensitive than the physiological endpoints.Therefore, data presented in the study can be used to improve risk assessment for zinc toxicity.

Utilization of Computer Classification Methods for Exposure Prediction and Gene Selection in Daphnia magna Toxicogenomics

“…Consistent with the previous results obtained by comparing error bars, the gst gene has the highest ranking of all three methods, whereas vtg1 also ranked second for χ 2 -test and third for the other two methods. The results were also similar to previously published data, where effect of the exposure on oxidative stress and reproduction was shown on physiological endpoint levels [44]. The advantage of using Shapley values is that the gene importance can be divided per class and per sample.…”

“…The dataset contained fold change values for 128 samples with 22 genes obtained by using ∆∆Ct method [45]. These genes were selected based on their involvement in key biological pathways that respond to Zn and hardness exposure such as metal regulation, reproduction, and oxidative stress [44]. The data is normalized by removing the mean and scaling to unit variance.…”

“…The study was conducted using Daphnia magna; a freshwater Cladocera commonly used in toxicity testing. The gene expression data used in this study was obtained from our previous study conducted by following OECD guidelines with media modifications to achieve desired Zn and hardness concentrations [43,44]. Exposure groups consisted of daphnids that were subjected to six varying concentrations of Zn (3.1, 5, 10, 25, 50, and 100 µg/L) across three different levels of water hardness: soft (50 mg CaCO 3 /L), medium (100 mg CaCO 3 /L), and hard (200 mg CaCO 3 /L).…”

“…Exposure groups consisted of daphnids that were subjected to six varying concentrations of Zn (3.1, 5, 10, 25, 50, and 100 µg/L) across three different levels of water hardness: soft (50 mg CaCO 3 /L), medium (100 mg CaCO 3 /L), and hard (200 mg CaCO 3 /L). For details on experimental setup, RNA isolation, and gene expression analysis, see [44].…”