Is Cl − protection against silver toxicity due to chemical speciation?

Abstract

In freshwater teleosts, the primary mechanism of acute silver toxicity is inhibition of Na +/K + ATPase and carbonic anhydrase at the gill, leading to net Na + and Cl − loss due to the continued diffusion of these ions into the hypoosmotic external environment. External Cl − has been shown to protect rainbow trout ( Oncorhychus mykiss) against silver toxicity presumably by complexation to form AgCl. However, Cl − does not appear to greatly influence silver toxicity to at least two other species, the European eel ( Anguilla Anguilla) and the fathead minnow ( Pimephales promelas). We hypothesized that differences in protective effects of Cl − at the gill were due to differing requirements or mechanisms for Cl − uptake among fish species. To test this hypothesis, we exposed Fundulus heteroclitus, which does not take up Cl − across the gills, and Danio rerio and P. promelas, which do rely on Cl − uptake across the gills, to Ag + in waters of varying Cl − concentration. The 96-h LC50s of F. heteroclitus exposed to Ag + in soft water with 10 μM Cl −, 1 mM KCl, and 0.5 mM MgCl 2 were 3.88, 1.20, and 3.20 μg/L, respectively, and not significantly different. The 96-h LC50s for D. rerio exposed to Ag + in soft water with 10 μM Cl − and 1 mM KCl were 10.3 and 11.3 μg/L, respectively and P. promelas exposed under the same conditions were 2.32 and 2.67 μg/L, respectively. Based on these results, increasing external Cl − concentration by as much as 1 mM (35.5 mg/L) did not offer protection against Ag + toxicity to any fish species tested. Although previous results in our laboratory have demonstrated that P. promelas do take up Cl − at the gill, a mechanism of uptake has not been identified. Additional experiments, investigating the mechanisms of Na + and Cl − influx at the gill of P. promelas and the influence of silver, demonstrated that Cl − uptake in P. promelas acclimated to soft water occurs through both a Na +:K +:2Cl − co-transporter and a Cl −/HCO 3 − exchanger, but is not dependent on carbonic anhydrase. Further, acclimation water chemistry was found to greatly influence subsequent branchial silver accumulation, but Cl − uptake was not sensitive to 10 μg/L Ag +.