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 microM silver as AgNO3 for 3 to 4 h in synthetic, ion-poor water (20 microM Ca, 100 microM Na, 150 microM Cl, pH 7) to which was added Mg, Ca, or thiosulfate (S2O3). Gills were extracted and assayed for Ag using graphite furnace atomic absorption spectrophotometry. Up to 210 mM Mg (fourfold the concentration of Mg in seawater) did not reduce accumulation of Ag by trout gills. The conditional equilibrium stability constant (K) for Mg at silver-binding sites on the gills was calculated to be log K(Mg-gillAg) = 3.0, or approximately half-as-strong binding as for Ca at these sites. The inclusion of the Mg-gill 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.