The effects of nanomaterials in the physiology and ecology of the freshwater crustacean Daphnia magna
Paul, Kai Benjamin
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This research studied 3 differently coated silver nanomaterials (Ag NMs) and their ionic counterpart, dissolved Ag, focussing on their fate and toxicity in the aquatic environment. Silver is one of the most used nanomaterials with Ag NMs degradation potentially resulting in ionic silver release, which is well known to be highly toxic to aquatic organisms. The organism studied here was the aquatic model species, Daphnia magna, chosen for their non-selective particle feeding behaviour, sensitivity and importance in the aquatic food chain (Chapter 1). In Chapter 3 the Ag NMs physicochemical properties were characterised within the appropriate aquatic matrices (i.e. Mill-Q water and environmental medium) using DLS, TEM-EDX, hyperspectral imaging, UV/Vis spectroscopy, centrifugal filtration and ICP-MS. Hyperspectral imaging was a novel technique but, highlighted is its potential usefulness within this discipline. Chapter 4 explores the acute and chronic toxicity of Ag NMs and dissolved Ag to D. magna. The research here is one of the first to demonstrate that direct conversions of acute data for chronic predictions of hazard in D. magna may not be possible and that Ag NM toxicity cannot be fully explained by Ag+ release. Critical modelling values were derived (biotic ligand modelling, BLM and biodynamic model, BDM) and the best predictors of toxicity established for use in silico models (Chapter 5). Using biochemical techniques (Chapter 6) it was shown that Ag+ and Ag NM toxicity differed in their modes of action (MOA). The study is the first to derive possible schematic Ag NM MOA using multiple biochemical endpoints, with Ag+ affecting body cations and all Ag NMs causing perturbations to mitochondrial function and oxidative stress levels. Within this thesis there are the first SEM images of Ag NMs trapped within the organism filtering apparatus. The study is the first to take an in depth look at ligand binding characteristics as they concern to Ag NMs and highlight any differences with Ag+. In conclusion; Ag NMs are currently still one of the most toxic of all studied NMs current levels in the environment may not cause immediate impact on D. magna populations. However sub-lethal and long term effects under continuous exposure may still be of concern.