Characterization of Organic Copper-Complexing Agents Released by Daphnia magna

Abstract

Zooplankton release a variety of organic and inorganic compounds into their aqueous surroundings. To test the effects of this on trace metal speciation, we raised Daphnia magna in a defined, synthetic medium at various population densities. A series of Cu(II) and pH titrations using an ion-selective electrode apparatus confirmed the excretion of moderately strong metal-binding organic compounds. Control experiments established that bacteria do not produce the complexing agents. The mean hourly excretion rate in the cultures was 40 ± 8 pmol/organism. The complexing material is characterized by two model ligands with conditional stability constants (at pH 6.30) log K1 = 8.6 and log K2 = 6.4. These stability constants are similar to those obtained for humic and fulvic acids and suggest a role for this material in the trace metal speciation of natural waters. The release of complexing agents may also affect the results of Daphnia toxicity tests by reducing metal activities during incubation.