Acute ecotoxicological effects of Hg(CN)2 in Danio rerio (zebrafish)

Artisanal and small-scale gold mining (ASGM) is the largest source of anthropogenic Hg emissions on the planet. In addition, Hg-contaminated tailings are often reprocessed with sodium cyanide (NaCN) to extract the residual gold remaining in the material. This leads to the formation of mercury cyanide (Hg(CN)(2)) complexes, which are often discharged in untreated form into local drainages, leading to large amounts of free cyanide being released. However, data on mercury-cyanide interactions are scarce. In this study, we investigated the impact of cyanide and Hg bioavailability in zebrafish when added as Hg(CN)(2). Different concentrations of Hg(CN)(2) and NaCN were used, leading to an LC50 of 0.53 mg.L-1 for NaCN and 0.16 mg.L-1 for Hg(CN)(2). Analyzing free cyanide concentrations in aquarium water, >40% dissociation was observed for NaCN and about 5% for Hg(CN)(2). The accumulation of total Hg (THg) in the brain, gills, muscle and kidney was quantified. All fish exposed to Hg(CN)(2) had higher THg levels than their controls and kidney was the tissue with higher Hg(CN)(2) accumulation. Histological effects on the kidney and gills of both cyanides in D. rerio tissues were investigated, suggesting renal alterations in fish exposed to Hg(CN)(2) and showing hyperplasia in the gills of animals exposed to NaCN and Hg(CN)(2). The results alert to the risks of the presence of these complexes in aquatic environments.