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Response of biochemical biomarkers in the aquatic crustacean Daphnia magna exposed to silver nanoparticles

The proliferation of silver nanoparticles (AgNPs) production and use owing to their antimicrobial properties justifies the need to examine the resulting environmental impacts. The discharge of biocidal nanoparticles to water bodies may pose a threat to aquatic species. This study evaluated the effects of citrate-coated AgNPs on the standardized test organism Daphnia magna Straus clone MBP996 by means of biochemical biomarker response. AgNPs toxicity was compared against the toxic effect of Ag+. The toxicity endpoints were caluculated based upon measured Ag concentrations in exposure media. For AgNPs, the NOAEC and LOAEC values at 48 h were 5 and 7 microg Ag/L, respectively, while these values were 0.5 and 1 microg Ag/L, respectively, for Ag+. The EC50 at 48 h was computed to be 12.4 ± 0.6 and 2.6 ± 0.1 microg Ag/L for AgNPs and Ag+, respectively, with 95% confidence intervals of 12.1 – 12.8 and 2.3 – 2.8 microg Ag/L, respectively. These results indicateing significantly less toxicity of AgNP compared to free Ag+ ions. Five biomarkers were evaluated in Daphnia magna neonates after acute exposure to Ag+ or AgNPs, including glutathione (GSH) level, reactive oxygen species (ROS) content, and catalase (CAT), acetylcholinesterase (AChE) and superoxide dismutase (SOD) activity. AgNPs induced toxicity and oxidative stress responses in D. magna neonates at 10-fold lower concentrations than Ag. Biochemical methods revealed a clear increase in AChE activity, decreased ROS level, increased GSH level and CAT activity, but no significant changes in SOD activity. As Ag+ may dissolve from AgNPs, these two types of Ag could act synergistically and produce a greater toxic response. The observed remarkably high toxicity of AgNPs (in the parts-per-billion range) to crustaceans indicates that these organisms are a vulnerable link in the aquatic food chain with regard to contamination by nanosilver.

silver nanoparticles; Daphnia magna; toxicity; biomarkers; oxidative stress

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