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 instar larvae of the midge Chironomus riparius within the pH range 5.5-10.0, using 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 cadmium 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 of exposure and changes in pH of acclimation, the latter representing a physiological effect. Within each acclimation group metal uptake in larvae increased with increasing pH of exposure in the range 5.5-9.0 but decreased between pH 9.0 and 10.0. Taking into account the decreased free metal ion activity, metal uptake was still high at pH 10.0. A possible explanation for this 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 negative effect between 9.0 and 10.0. Two different uptake models were applied to 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 and 36% of the total variation in zinc uptake by midge larvae. When the model was fitted to the uptake data of larvae acclimated to the exposure conditions, 78 and 69% respectively of the variation was described. The second model, a biological ligand model, was not able to discriminate between effects 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 being 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.