Poor elemental food quality reduces the toxicity of fluoxetine on Daphnia magna

Most chemical contaminants released into the aquatic environment have been tested for their toxicity to aquatic organisms using standardized test protocols. The extent that results from such toxicity tests vary with the quality of food provided to the test organism remains largely unknown. Here, we determined whether the elemental food quality consumed by a common test organism, Daphnia magna, affects the toxicity of a pharmaceutical, fluoxetine. We found strong interactive effects of food quality and fluoxetine on daphnid survival, growth, and reproduction. Specifically, we found that D. magna fed phosphorus (P)- and nitrogen (N)-rich algal food experienced greater toxicity due to fluoxetine. For example, the 6-day LC(50) values for fluoxetine decreased from 0.33 to 0.15 mg L(-1) when food C:P ratios were increased from 100 to 800. One explanation for this result is that fluoxetine, as a selective serotonin reuptake inhibitor, increases the activity of serotonin, whose production is linked to growth metabolism in D. magna. A regulatory role of serotonin for D. magna's growth and reproduction would be consistent with its mitogenic roles in other organisms, although this possibility would require further study. Nevertheless, these results demonstrate the need to consider elemental food quality in ecotoxicological testing protocols that are aimed at evaluating the risks of exposure to chemicals in the aquatic environment.