Ecotoxicity and sub-lethal effects of accidentally dispersed and purposely produced nanoparticles through a multi-trophic approach

The use of nanotechnology in everyday life received a boost in the last twenty years thanks to improved performances and adjustable properties of engineered nanomaterials (ENMs) and nanoparticles (NPs) compared to traditional materials. ENMs and NPs are purposely produced for a wide variety of applications and, as recently discovered, are also accidentally formed during some industrial processes. Being so extensively employed they end up into the natural environment and their occurrence, especially in the aquatic environment, has been recently confirmed, although only for selected NPs. In fact, due to technical limitations in the detection of NPs in natural matrices, the actual concentrations are mostly established based on predicted environmental concentrations (PECs), in the range of ng-mg L-1. The consequences of environmental exposure for aquatic organisms, especially in the long-term, are not well-understood and, due to the large variety of features contributing to NP diversity, common guidelines for NP risk assessment are in their infancy. The aim of this thesis was to investigate the effects of two different type of NPs, polystyrene nanoparticles (PS NPs) as proxy for nanoplastics and silver nanoparticles (AgNPs), toward aquatic microorganisms at different levels of ecological organization. Selected organisms belonging to the bacterial, microalgal, and microcrustacean communities, from the freshwater and the marine environment, have been investigated. Both NPs were thoroughly characterized for their properties and their behaviour in exposure media by means of dynamic light scattering (DLS), transmission electron microscopy (TEM) and, in the case of AgNP, inductively coupled plasma mass spectrometry (ICP-MS) for the assessment of ion release. When possible, all the elements possibly interfering with the observed effects were determined and reported, in order to help in the correct assessment of NP mode of action in terms of ecotoxicity. AgNP synthesis process was reported in detail, while additives in PS NP suspension were listed and their final concentration in exposure media was calculated. Moreover, for a more correct interpretation of NP behaviour data, the composition of all exposure media and their physico-chemical parameters were reported and considered in data analysis. All biological models were tested for acute toxicity and, when allowed by the organism ecology, chronic toxicity was assessed as well. Particular attention was given to sub-lethal effects occurring upon exposure to NPs and how these could be linked to more detrimental effects in the long-term and furthermore having implications at the ecological level. Results enriched the data pool on NP ecotoxicity at various levels of aquatic food chains, while also highlighting the importance of long-term exposure and sub-lethal effects. Marine diatom exposed to PS NPs showed no effect on growth but resulted in PS NP adhesion to the algal surface and reduced chain length of algal cells, with possible implications for algal buoyancy and bloom formation. The first evaluation of novel AgNPs designed for water remediation successfully resulted in low dissolution and no toxicity for two freshwater and marine microalgae. A more thorough investigation, however, showed effects in the long-term especially for marine microcrustaceans and suggested a toxicity linked to the AgNP nano-size rather than to dissolved Ag values. Effect comparison of two differently coated AgNP to bacteria and microalgae revealed a different mode of action based on coatings: one confirmed to be influenced by the nano-size while for the other a dissolution-based toxicity was observed. Overall results showed that the consequences of NP exposure more likely originate from the physical interaction with the organism membrane/cell wall or body surface in long-term exposure scenarios, and with, often, unexpected outcomes compared to traditional toxicity endpoints (i.e., cell growth and mortality). New ecotoxicological outcomes should be considered in order to provide a more realistic assessment of the risk associated to NP use and discharge in the aquatic environment, while a more ecologically-based design for new generations of ENMs should be promoted.