Date of Award

2016

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Biology

Abstract

This study characterizes binary-metal mixture effects of cadmium (Cd2+) + zinc (Zn2+) and Cd2+ + nickel (Ni2+) on Daphnia magna. Although acute studies have shown protective Ni2+ (Traudt et al. 2016) and Zn2+ effects (Meyer et al. 2015) against Cd2+ toxicity, no study has fully characterized a protective effect on D. magna at several endpoints (survival, reproduction, growth, and accumulation) on a chronic study. In this study, the titration design was selected to characterize the 21 day (21-d) chronic effects of the binary-metal mixtures on survival, growth, reproduction, and metal accumulation in D. magna. Using this design, increasing concentrations of Zn2+ (10, 20, 40, 80, 120, 160 and 200µg/L) and Ni2+ (20, 40, 80, 100, 120, 140, and 160µg/L) were titrated against a constant concentration of 1.5µg/L Cd2+. A single-metal assay (Zn2+/Ni2+ alone), a Cd2+ alone treatment, and a control treatment were concurrently conducted for comparison. The results in this thesis demonstrate that Cd2+ alone was highly toxic to D. magna. In a mixture with Cd2+ and Zn2+, sublethal concentrations of 10 and 20µg/L Zn2+ were consistently insufficient to protect D. magna from chronic Cd2+ toxicity since Cd2+ toxic effects were observed on the survival, reproductive, and growth endpoints (Figs. 1, 2, 9), whereas mixtures containing 40, 80, and 120µg/L Zn2+ provided strong protective effects to D. magna at all the endpoints examined. Higher Zn2+ concentrations of 160 and 200µg/L exceeded the necessary concentrations needed to protect D. magna, and no protective effects were observed. In contrast, sublethal and moderate Ni2+ concentrations

of 20, 40, and 80µg/L were found to strongly protect D. magna from chronic Cd2+ toxicity, whereas higher Ni2+ concentrations (≥100µg/L) exceeded the necessary concentration needed to protect D. magna, and Ni2+ protective effects were absent. Interestingly, 1.5µg/L Cd2+ was found to be protective in the Cd2+-Ni2+ mixture containing 100µg/L Ni2+. Additionally, no concentration of Ni2+ or Zn2+ was found to provide complete protective effects to D. magna (i.e. 100% protective effects). Therefore, suggesting that no concentration of Zn2+ or Ni2+ used in this study, entirely outcompeted the binding of Cd2+ ions to the biotic ligand. Embryos analyzed for morphological alterations in both the Cd2+-Zn2+ and Cd2+-Ni2+ mixtures demonstrate severe developmental defects. The results of the present study are useful for development of environmental quality guidelines for metal mixtures.

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