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

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 MgCl2 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−/HCO3− 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+.