Complexation and time-dependent accumulation of copper by larval fathead minnows (Pimephales promelas): Implications for modeling toxicity

Mechanistic models predicting copper (Cu) toxicity to aquatic biota in natural waters require organic and inorganic water chemistry, and quantified values for Cu binding by sensitive biological receptors. In bioaccumulation experiments using larval fathead minnows (FHM; Pimephales promelas), we investigated time to asymptotic accumulation of Cu and quantified the conditional stability constants (binding affinity; log KCu–FHM) and binding-site densities of Cu–FHM complexation. Cu bioaccumulation increased rapidly, approaching an asymptote in exposures longer than 12 h, indicating that Cu loading at 24 h is an appropriate exposure duration for modeling Cu complexation by larval FHM. Results of Langmuir and Scatchard analyses of other bioaccumulation experiments produced log KCu–FHM values of 6.52, and binding-site densities of 0.39 μmol g−1 dry weight. These whole-body log KCu–FHM values are approximately an order of magnitude lower than those reported for adult FHM gills. However, binding-site densities for larval and adult FHM are similar. Under similar test conditions, comparable concentrations of aqueous Cu cause 50% mortality in adult and larval FHM suggesting that binding site densities determine comparable metal–tissue loadings and have greater influence on Cu bioavailability than binding affinity.