Protein and nucleic acid content in Daphnia magna during chronic exposure to cadmium

Daphnia magna Straus were exposed to cadmium concentrations of 0, 0.4, 0.8, 2.1, 4.3, and 7.2 micrograms/liter for 21 days. Protein, RNA, DNA, glycogen and lipid content were measured after 4, 7, and 21 days of exposure, and the results were related to survival and reproduction. The no observed effect concentration (NOEC) for survival, protein growth, and reproduction at Day 21 was 2.1, 0.8, and 0.8 microgram/liter cadmium, respectively. Protein growth was most sensitive to cadmium exposure following the rapid growth phase which occurred between Days 4 and 6 of growth and development. All concentrations of cadmium produced significant reductions in protein growth at Day 7 indicating that the NOEC was less than 0.4 micrograms/liter. RNA:protein and protein:RNA:DNA ratios, which are related to the growth rate of daphnids, were most appropriately monitored at Day 4 which was prior to the rapid growth phase. Protein:RNA:DNA ratio at Day 4 identified the same NOEC as identified by protein growth and reproduction after 21 days of exposure.