Development of the terrestrial biotic ligand model for predicting nickel toxicity to barley (Hordeum vulgare): ion effects at low pH
2009
Antunes, Paula M.C. | Kreager, Nancy J.
The focus of the present study was to investigate the potential for Al3+, Mg2+, and H+ to influence Ni2+ toxicity for barley seedlings grown in acidic aqueous solutions and to assess the capacity of a two-site terrestrial biotic ligand model (tBLM) to accurately predict 50% effect activities (EA50s). To accomplish these objectives, 48-h EA50(Ni2+) values were obtained for three sets of exposures in which the pH and activity of Al3+ and Mg2+ were varied. Exposures contained both Al alone and in combination with Mg so that compound ion effects could be investigated. A tBLM was then constructed to predict EA50Ni2+ values from the exposure solution chemistry. The results show a slight protective effect of H+ against Ni2+ toxicity and a strong protective effect of Mg2+, as indicated by a 4.6- and 8.0-fold increase in the measured EA50(Ni2+) values corresponding to changes in pH from 6.0 to 4.5 and {Mg2+} from 0 to 1.40 mM, respectively. Increasing solution {Al3+} from 0 to 0.5 μM had no effect on Ni2+ toxicity, although Al itself negatively affected root elongation. Comparison of EA50 values calculated as both Ni2+ and measured concentration of total Ni in the root ([Root-Ni](T)) showed [Root-Ni](T) to be a more normalized measure of Ni bioavailability. The strong correlation between root growth inhibition and tBLM-predicted root-Ni accumulation suggests that toxicity was influenced by Ni2+ binding to low-affinity ligands within the cell wall, in addition to Ni2+ uptake through Mg2+ transporters. Predicted EA50(Ni2+) values generated with the model were all within a factor of ±1.5 from measured values--a result that emphasizes the advantage of using the tBLM for risk assessment.
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