Effects of silver nanoparticles in early life-stage zebrafish are associated with particle dissolution and the toxicity of soluble silver

The aim of the present study was to investigate and compare the acute and sub-lethal toxicity of silver nano-particles (Ag NPs) with soluble Ag (as AgNO3) in early life-stage zebrafish using an experimental design suited to separating the nanoscale specific effects of NPs from the toxicity of released metal ions. Early life-stage zebrafish were exposed to AgNO3 and Ag NPs in four separate acute toxicity exposures, each of 24 h duration, in increments from 24 to 120 hours post fertilisation (hpf). The data showed that the sensitivity of zebrafish to both materials increased from 24 to 120 hpf. Overall, the Ag NPs were much less toxic than AgNO3 but the relative magnitudes of change in median lethal concentrations (LC50 ) across each increment of development were similar. The physiological targets of both materials were therefore likely to be the same and a radiotracer (Na-22(+)) flux technique confirmed the main acute toxic mechanism of both materials to be inhibition of Na(+ )uptake. Sub-lethal exposures at one quarter of the LC50 (i.e. the same level of effect) showed that concentrations of total Ag in zebrafish were greater from Ag NP exposure than AgNO3. But, the NPs were unlikely to have been accumulated intracellularly or have contributed to the toxicity observed; expression of the metal ion chaperone metallothionein (mt2) and genes involved in oxidative stress pathways were the same at equal relative concentrations of Ag NPs and AgNO3. In conclusion, these data indicated that toxicity of Ag NPs in early life-stage zebrafish was caused by NP dissolution and the release of soluble Ag.