Impact of chronic mercury cyanide exposure on the health of the zebra-fish (Danio rerio).

Mercury cyanide (II) (Hg(CN)₂) is a substance formed through cyanidation and amalgamation techniques involving mercury, commonly used in gold mining activities at artisanal and small-scale sites. In sites such as the Puyango river, Ecuador, 6000 tons of cyanide-laden wastes are dumped in the river annually, including mercury cyanides. A few toxicological data on sodium cyanide are available in the literature, but data on the toxicity of mercury cyanides are not. This study investigated the chronic toxic effects of Hg(CN)₂ on the zebra-fish Danio rerio, a widely used model in environmental toxicology. The fish were divided into three experimental groups, exposed to concentrations of 0, 0.08, and 0.12 mg/L of Hg(CN)₂ for 8 weeks, followed by an 8-week recovery period without the contaminant. During the exposure phase, total mercury (THg) concentrations were quantified in the fish tissues, revealing a preferential accumulation in the kidneys, followed by the liver and gills. Histopathological analysis showed significant alterations, particularly in the gills and kidneys. Antioxidant enzymes, such as superoxide dismutase, catalase, and glutathione peroxidase, exhibited changes in activity, indicating a stress response induced by Hg(CN)₂ exposure. Hematological alterations were also observed, including a decrease in lymphocytes and an increase in the formation of micronuclei in erythrocytes, suggesting potential genetic damage. Although the fish demonstrated a gradual capacity to eliminate Hg(CN)₂ over time, the persistent accumulation of mercury in critical organs, even with reduced THg concentrations during the recovery phase, suggests a slow excretion process. The effects observed here are probably underestimated, since the biota of contaminated watersheds is constantly exposed, experiencing no recovery phases. These findings highlight not only the immediate risks but also the long-term effects of Hg(CN)₂ exposure on aquatic organisms, with potentially serious implications for aquatic biota and ecological balance.