Bioaccumulation and trophic transfer of engineered nanoparticles in aquatic organisms

Use of engineered nanoparticles (ENPs) (particles with a diameter of 1 to 100nm) is increasing. Engineered NPs are used in a wide variety of consumer product, industrial uses and remediation of pollutants. The increasing use is due to novel physical and chemical properties varying from that of their bulk forms.
With release of ENPs to the environment a need for evaluation of the potential risk of ENPs is necessary. Potential risks are assessed through a chemical safety assessment. Test guidelines (TGs) to evaluate the risk of compounds for the chemical safety assessment were developed for soluble chemicals. However, with fundamentally different chemical and physical properties of ENPs compared to soluble chemicals current TGs could be inadequate and possibly lead to wrong interpretation of results obtained.
One of the key issues is the dual action of ENPs consisting both of a chemical identity and a physical identity. For soluble chemicals the chemical identity has been the parameter controlling ecotoxicological endpoints (e.g. toxicity and bioaccumulation). However, with ENPs consisting of a wide range of particle sizes, coatings and functionalizations influencing the performance and result of test carried out the intrinsic properties of the ENP becomes critical in relation to endpoints assessed. Consequently, a central theme in this thesis is to increase the understanding of the intrinsic properties of ENP and how it influence bioaccumulation. Different particle sizes, coatings and functionalizations with different aquatic organisms were investigated. Furthermore, multiple microscopy methods were used to assess internationalization in the aquatic organisms. Finally, different exposure routes were used to determine if it could affect localization in the aquatic organisms.
The influence of different particle sizes, coatings and functionalizations were investigated using model ENPs (Au ENPs) with two different sizes (10 and 30nm) and coatings (citrate and mercaptoundecanoic acid (MUDA)) and a standardized test setup with a standardized test organism (Daphnia magna). It was found that while MUDA coated ENPs showed a clear trend of smaller ENPs taken up faster than larger ENPs contradictory findings was observed for the citrate coated ENPs showing similar uptake for both sizes. Consequently, both coating and size was found to affect bioaccumulation. Using differently functionalized ZnO ENPs (-OH and -Octyl functionalization) it was found that large micron sized aggregates was also available for uptake in D. magna showing high uptake, possibly also associated with the carapace of the test organism. Functionalization with -Octyl increased the uptake compared to pristine ZnO ENPs while ZnO-OH ENPs showed no significant uptake compared to control. These results showed that larger size aggregates and functionalization could influence bioaccumulation potential. It should be highlighted that this type of interactions associated with the physical properties of ENPs and their influence on bioaccumulation is not accounted for in TGs.
Internalization of ENPs in the tested aquatic organisms was not identified through any of the microscopy techniques used. However, it was highlighted for proper interpretation of results multiple methods have to be used, and especially the need for element analysis was highlighted to identify artefacts and avoid misinterpretation of results. Furthermore, a general lack of understanding of internalization processes of ENPs after in vivo exposure was identified in the literature in regards to intrinsic properties of ENPs (e.g. particle sizes, coatings and functionalizations).
Exposure pathways were found to influence the localization of ENPs using light sheet microscopy. In zebrafish (Danio rerio) aqueous exposure to ENPs showed ENPs associated with gill, head region and gut whereas after dietary exposure ENPs were only found associated with the gut region. Consequently, ecotoxicological tests should be carried out for different exposure routes so possible effects are not overlooked due to the exposure route employed. However, it is not clear which pathway would be most relevant for testing with ENPs or if different pathways should be employed for different physical and chemical properties of ENPs.
Finally, it should be stressed that successful interpretation of all ecotoxicological tests with ENPs will ultimately rely on comprehensive characterization of the ENPs used, especially in relevant test media. This also underlines the immediate need for implementation of some level of physical characterization in TGs when testing ENPs.