Food Reduction Adds Stress to Chronic Silver Nanoparticle Toxicity for Daphnia magna

Abstract

Chronic toxicity of silver nanoparticles (AgNP) to Daphnia magna has received increasing attention in research, also with regard to influencing factors such as coating or feeding regime. With regard to the later, only the effect of increasing food quantities has been tested up to now. In the present study, we investigated whether a decrease in food quantity causes increasing toxicity; as suggested by the results in the above-mentioned study. For this purpose, a chronic test using the reference material NM-300K at increasing concentrations up to 6.6 µg Ag L-1 and three different food levels was conducted. The moderate decrease in food quantity (75% of standard food level) caused comparable toxicity results compared to the standard food level, while the intense decrease (50% of standard food) caused a clear increase in toxicity. However, the extent of this increase varied with the investigated endpoint of the chronic test and some endpoints did not vary between food levels at all. Number of clutches and onset of reproduction showed the clearest response in terms of decreasing Lowest-Observed-Effect-Concentrations (from > 6.6 to 4.6 µg Ag L-1, and from 6.6 to 5.6 µg Ag L-1, respectively). Mortality showed the highest change in terms of relative increase: at the highest AgNP concentration, mortality increased from 40% in the standard food treatment to 90% at 50% food level. Even though this may raise concern about risk assessment of AgNP using standard food conditions, the high similarity in concentration dependence between most endpoints indicates that the chronic Daphnia test is protective despite the use of unnaturally high food supply.