An Analysis of the Toxic Effects of Mercury Cyanide Complexes on Zebrafish

Abstract

Mercury (Hg) is a unique heavy metal toxicant that is found in numerous environmental and occupational settings. A major source of environmental Hg is from artisanal and small-scale gold mining (ASGM), whereby metallic mercury (Hg0) is used to amalgamate gold from mined ore. Hg extraction of gold is inefficient and thus, a significant amount of gold remains in the ore. The leftover Hg-contaminated tailings are often subjected to cyanidation to extract the remaining gold. During this process, mercury and cyanide form mercuric cyanide complexes that are held together with a strong, covalent Hg-carbon bond. These toxic complexes are proposed to be prevalent in terrestrial and aquatic environments around mining sites. The purpose of the current study was to determine how mercuric cyanide complexes, particularly Hg(CN)2, affect the health of aquatic organisms in contaminated environments. We used adult and larval zebrafish (Danio rerio) as model organisms for this study. Zebrafish, in various life stages, were exposed to several different concentrations of Hg(CN)2 to determine the physical and behavioral effects of this exposure. Embryos were exposed to varying concentrations of Hg(CN)2 at one-hour post fertilization (4th cell cycle) until 72 hours post fertilization when hatching begins. Concentrations ranged from 0 to 0.1 mg/L Hg(CN)2. Embryos exposed to concentrations of Hg(CN)2 above 0.08 mg/L had a lower hatching rate and survival rate. Exposure of adult fish to various concentrations of Hg(CN)2 led to significant alterations in behavior and mercury content of muscle. The current study is the first to report the way in which Hg(CN)2 affects aquatic organisms in various stages of life.