The effect of EDTA on the assessment of Cu toxicity in the submerged aquatic macrophyte, Lemna trisulca L

Lemna trisulca was grown in axenic cultures with regular replacement of a filter-sterilized medium. The toxicity of Cu was examined at 8 Cu concentrations at each of three EDTA concentrations. The data was used to estimate Cu toxicity on the basis of external Cu concentration, pCu (−log[Cu 2+]) and internal Cu content. The chelator EDTA antagonized Cu toxicity. Fifty percent effect concentration (EC 50) values based on external Cu increased from 3.6 to 55 μM as the EDTA concentration increased from 9 to 81μM. The estimated pCu at the EC 50 also varied, from 8.1 to 10.9 at the various EDTA concentrations. EC 50 values based on the internal Cu content, however, did not vary with the external EDTA concentration. They ranged from 1310 to 1640 μg Cu/g dry wt. and had overlapping 95% confidence intervals. The data indicate that EC 50 values based on total external Cu or pCu do not yield reliable estimates of toxicity. In contrast, EC 50 values based on internal Cu content are independent of the chelating properties of the medium and are therefore superior for the estimation of metal toxicity in aquatic organisms.