Toxicity and mode of action studies of copper in species of daphnia

Abstract

The object of the project was to study the toxic effects of copper on Daphnia species by a series of experiments starting with a determination of the acute toxicity followed by sequential generation exposure to sub-lethal doses of copper to induce a tolerant strain for radiochemical, histochemical and pathological studies in order to characterise tolerance and elucidate its mechanism. The 24 hour LC50 of copper against 1st instar D. magna was 15.2ug Cu++/1. To overcome the analytical problems which this low level posed, a solution of 0.009 m M EDTA added to the culture medium gave a 24 hour LC50 value of 169.1 ug Cu++/1 which was amenable to analysis by the technique of ion concentration and solvent extraction. A continuous flow system was constructed for the administration of copper to daphnia. It incorporated the mass culture of the feeding organism Chlorella vulgaris and the cells and copper solutions were simultaneously relayed to daphnia. Background levels of copper in the system were consistently high. This was unfortunate and since the minimum background corresponded with the maximum safe dosage of copper for administration to daphnia this part of the study had to be discontinued and a modified programme was devised. Studies on the influence of copper on the respiratory rate of daphnia showed that concentrations of 5.0, 0.5 or 0.25mg Cu++/l caused significant increases in oxygen uptake compared with the non-copper control. At a concentration of 0.125 mg Cu++/l the rate of oxygen uptake did not differ significantly from the control. The elevated rates of respiration would deplete the energy reserves and it is postulated that this effect represented one of the main toxicological routes of copper. Experiments with 64Cu++ showed that after 1 hour's exposure to a concentration of 3.80-1 μ Ci 64Cu++/m l. adioactive copper accumulated to 0.72-3μg. 64Cu++ per single daphnid. After six to nine hours' exposure daphnia excreted a high proportion of the absorbed radioactive copper. A single experiment tentatively indicated the absence of a metallothionein binding system in daphnia.