The effects of organic ligands on the survival of Daphnia in Zn solution.

Bioassays involving zinc alone and in combinations with two organic ligands, namely histidine and EDTA were conducted on Daphnia longispina over a period of 96 h. The percentage of mortality was recorded and the Median Lethal Concentration (LC50) at 96 h estimated. The 96 h LC50 of D. longispina in zinc solution was 91 micrograms/l whereas no values of LC50 were revealed when D. longispina were exposed to histidine or EDTA alone although the organic ligands seemed to exert harmful effects (e.g. The mortality at 96 h was 18% at a concentration of 10 x 10(-5) M histidine and 16% at a concentration of 6 x 10(-6) M EDTA). However, addition of histidine and EDTA effectively reduced the toxicity of zinc to the test animals. The detoxification effect was the most obvious at the concentration of 100 micrograms/l Zn added with 5 x 10(-5) M histidine and 3 x 10(-6) M EDTA where 7-fold and 5-fold reduction in terms of mortality were noted.