Effects of Mg2+ and H+ on the toxicity of Ni2+ to the unicellular green alga Pseudokirchneriella subcapitata: Model development and validation with surface waters

In this study, increasing Mg concentrations and decreasing pH were observed to decrease Ni toxicity to the green alga Pseudokirchneriella subcapitata. To investigate to what extent the original biotic ligand model (BLM) concept could explain Ni toxicity as a function of water chemistry, the protective effects of Mg2+ and H+ were modeled as BLM-type single-site competition effects. The model parameters representing these effects were log KMgBL = 3.3 and log KHBL = 6.5. The BLM was capable of predicting Ni toxicity by an error of less than a factor of 2 in most synthetic and natural waters used in this study. However, since the relationship between 72-h ErC50Ni2+ (i.e. the 72-h ErC50 expressed as Ni2+ activity) and H+ activity was not linear over the entire tested pH range, only the ‘linear part’ between pH 6.45 and 7.92 was used for derivation of log KHBL. This nonlinearity indicates that the effect of pH can probably not be attributed to H+ competition with Ni2+ for a single site alone. When modeling the effect of pH as a linear relation between 72-h ErC50pNi2+⁎ (= − log (72-h ErC50Ni2+ corrected for the presence of Mg)) and pH, the applicability of the model was successfully extended to pH levels as low as 6.01. This type of empirical model has also been used in our previous studies on the development of a chronic Ni bioavailability model for Daphnia magna and a long-term Ni bioavailability model for rainbow trout. Finally, we could not detect a statistically significant interactive effect of pH and Mg on the toxicity of Ni2+ to P. subcapitata and this is in line with the formulation of our empirical model.