Biochemistry and physiology of silver toxicity to rainbow trout: Effects on ionoregulation, smoltification, and seawater survival

Silver (Ag) toxicity in rainbow trout (Oncorhychus mykiss) has been shown to be due to ionoregulatory and osmoregulatory failure precipitated by decreases in plasma Na and Cl concentrations. These events appear to be diagnostic of blockage of the branchial sodium, potassium-adenosinetriphophatase (Na{dollar}{bsol}sp+{dollar}/K{dollar}{bsol}sp+{dollar}-ATPase). Past research has shown Ag to be a potent inhibitor of this enzyme in mammalian tissue. It seems plausible that Ag blockage of the branchial Na{dollar}{bsol}sp+{dollar}/K{dollar}{bsol}sp+{dollar}-ATPase causes the decrease in plasma Na and Cl and therefore is the key mechanism of Ag toxicity in freshwater fish. This study provides a detailed investigation of the enzyme kinetics of Na{dollar}{bsol}sp+{dollar}/K{dollar}{bsol}sp+{dollar}-ATPase in the presence of Ag. Results from these experiments suggest that Ag binds to the Mg site on the enzyme. The inhibition coefficient of Ag was determined to be 72 pM. This strong K{dollar}{bsol}sb{lcub}{bsol}rm i{rcub}{dollar} can be overcome, however, by the binding of Ag to the cysteine rich protein, metallothionein, which can provide complete protection to the Na{dollar}{bsol}sp+{dollar}/K{dollar}{bsol}sp+{dollar}ATPase from Ag inhibition.; Sub-lethal physiological effects of Ag on iono and osmoregulation due to partially inhibited Na{dollar}{bsol}sp+{dollar}/K{dollar}{bsol}sp+{dollar}-ATPase are difficult to discern using typical measurements of enzyme activity and plasma ion parameters. This work uses a simple osmotic stress test, the seawater challenge test, to the show subtle differences in the ionoregulatory abilities of rainbow trout chronically exposed to environmentally relevant Ag concentrations versus control trout. The results of this test indicate an inability of Ag exposed trout (0.93 nM and 3.2 nM) to regulate Na when placed directly into seawater from freshwater during the usual period of increased hypoosmoregulatory ability (i.e. smoltification). Conversely, the control fish show the ability to regulate Na during the spring smoltification cycle.; The free silver ion (Ag{dollar}{bsol}sp+{dollar}) is considered to be the metal species directly responsible for Ag toxicity in freshwater. Based on geochemical speciation modeling presented here, Ag{dollar}{bsol}sp+{dollar} is virtually non-existent in seawater; therefore it might be expected that Ag toxicity would be ameliorated in waters of increasing salinity. 96-h acute toxicity tests were performed on seawater acclimated rainbow trout, which resulted in an LC{dollar}{bsol}sb{lcub}50{rcub}{dollar} of 3.75 {dollar}{bsol}mu{dollar}M Ag in water of 25 ppt salinity.