Effects of pH on cadmium and zinc uptake by the midge larvae Chironomus riparius

We studied the effect of pH on the uptake of cadmium and zinc by fourth ins tar larvae of the midge Chironomus riparius within the pH range 5.5-10.0, u sing chemically defined solutions. The effect of prior acclimation on metal uptake was examined for four pH levels, i.e. pH 5.5, 7.0, 8.0 and 9.5. At least three factors were important in determining the effect of pH on the c admium and zinc uptake by midge larvae. The effect of pH on metal uptake is the combined result of changes in free metal ion activity, changes in pH o f exposure and changes in pH of acclimation, the latter representing a phys iological effect. Within each acclimation group metal uptake in larvae incr eased with increasing pH of exposure in the range 5.5-9.0 but decreased bet ween pH 9.0 and 10.0. Taking into account the decreased free metal ion acti vity, metal uptake was still high at pH 10.0. A possible explanation for th is is that an increase in pH alters the metal uptake process by decreasing the protonation of the binding sites. That is, the biological availability of the free metal ion increases with increasing pH. Among the different pH exposure groups, acclimation had a positive effect up to pH 9.0 but a negat ive effect between 9.0 and 10.0. Two different uptake models were applied t o describe the observed Variation in metal uptake. With a non-linear, semi- empirical model, the integration of the different pH effects for the pooled data described no more than 38% of the total variation in cadmium uptake a nd 36% of the total variation in zinc uptake by midge larvae. When the mode l was fitted to the uptake data of larvae acclimated to the exposure condit ions, 78 and 69% respectively of the variation was described. The second mo del, a biological ligand model, was not able to discriminate between effect s of pH in acclimated and non-acclimated exposure groups. Only for the data of larvae acclimated to the exposure conditions the model could describe a significant amount of the observed variation in metal uptake, R-2 values b eing comparable to those of the first model. The remaining high undescribed variation could be ascribed to the high natural variation in metal uptake by midge larvae. (C) 2000 Elsevier Science B.V. All rights reserved.