Abstract
Nanomaterials (NMs) are being widely incorporated into a variety of fields such as medicine, cosmetics and electronics, due to the exceptional qualities provided by their small size and high surface area to volume ratio. Increased use of NMs leads to their deposition into environmental waters where they interact with organisms such as the fresh water zooplankton Daphnia magna (D. magna). D. magna is an ideal candidate for fresh water toxicity testing and a central study species used by the Organization for Economic Cooperation and Development (OECD) which sets the gold standard for regulatory testing protocols. The ecotoxicity protocols using D. magna were originally designed for bulk chemicals though have been deemed acceptable for NM testing, despite NMs existing as suspensions rather than dissolved chemicals. These protocols fail to account for key exposure features of NMs such as the fact that the natural clearance processes in these organisms require food to push out previously accumulated matter, and that under realistic exposure scenarios, NMs will have acquired a biomolecule corona that changes their identity, stability, uptake and excretion. Thus, the lack of biomolecules added to the medium and lack of feeding can lead to significant over or underestimation of the amount of NMs taken up by, or retained within, D. magna leading to uncertainty of dose and ultimately miscalculation of NMs toxicity and the risks posed by these materials. Herein we present evidence to support the call for revised guidelines for D. Magna acute and chronic toxicity tests for hazard and risk assessment of NMs.
Highlights
Nanomaterials (NMs) are widely incorporated into a vast range of consumer products already on the market such as zinc oxide (ZnO) NMs incorporated into sunscreen due to their Ultra-violet deflection properties (Kokura et al, 2010), cerium dioxide (CeO2) NMs in catalytic converters in cars (Montini et al, 2016), titanium dioxide (TiO2) NMs as a food colouring agent (Periasamy et al, 2015) and gold (Au) NMs for cancer therapy due to their high photo-thermal capacity (Kennedy et al, 2011)
D. magna have been used as a model organism long before NMs emerged as an environmental pollution issue, which is the crux and focus of this review; while D. magna is an ideal species to be used for chemical toxicity testing, it is not clear that the current protocols are suitable for NMs and we suggest how they can be adapted to correctly represent NMs for robust risk assessment
The focus of our analysis of the impacts of NM-specific factors on the applicability of the Organization for Economic Cooperation and Development (OECD) test guidelines for NMs, includes: (1) the lack of feeding at different time-points during the feeding cycle of D. magna which may lead to an over or underestimation of the residency time of NMs within the gut and to altered toxicities; and (2) the lack of natural biomolecules in the standard test media, despite the ubiquity of biomolecules in environmental waters which inevitably bind to the NMs causing changes to the NM surface and creating an ‘eco-corona’ leading to altered stability/instability of NMs and affecting NM uptake and toxicity
Summary
Nanomaterials (NMs) are widely incorporated into a vast range of consumer products already on the market such as zinc oxide (ZnO) NMs incorporated into sunscreen due to their Ultra-violet deflection properties (Kokura et al, 2010), cerium dioxide (CeO2) NMs in catalytic converters in cars (Montini et al, 2016), titanium dioxide (TiO2) NMs as a food colouring agent (Periasamy et al, 2015) and gold (Au) NMs for cancer therapy due to their high photo-thermal capacity (Kennedy et al, 2011). The focus of our analysis of the impacts of NM-specific factors on the applicability of the OECD test guidelines for NMs, includes: (1) the lack of feeding at different time-points during the feeding cycle of D. magna which may lead to an over or underestimation of the residency time of NMs within the gut and to altered toxicities; and (2) the lack of natural biomolecules in the standard test media, despite the ubiquity of biomolecules in environmental waters which inevitably bind to the NMs causing changes to the NM surface and creating an ‘eco-corona’ leading to altered stability/instability of NMs and affecting NM uptake and toxicity.
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