Adding magnesium to the silver-gill binding model for rainbow trout (Oncorhynchus mykiss)

Rainbow trout (Oncorhynchus mykiss; 2-17 g) were exposed to approximately 0 .1 muM silver as AgNO3 for 3 to 4 h in synthetic, ion-poor water (20 muM Ca , 100 muM Na, 150 muM Cl, pH 7) to which was added Mg, Ca, or thiosulfate ( S2O3). Gills were extracted and assayed for Ag using graphite furnace atomi c absorption spectrophotometry. Up to 210 mM Mg (fourfold the concentration of Mg in seawater) did not reduce accumulation of Ag by trout gills. The c onditional equilibrium stability constant (K) for Mg at silver-binding site s on the gills was calculated to be log KMg-gillAg = 3.0, or approximately half-as-strong binding as for Ca at these sites. The inclusion of the Mg-gi ll stability constant into the original Ag-gill binding model increases the flexibility of the model, although the competitive effects of Mg are only important in sodium-poor systems.