Toxic effect of different metal bearing nanoparticles (ZnO NPs, TiO2 NPs, SiO2 NPs, Ag NPs) toward marine phytoplankton
The advent of nanotechnology and the commercialization of several nanoparticle-containing-products call to a thorough assessment of the environmental risks derived from the exposure to these new materials. The most important criticisms of new nano-structured materials are represented by the emerging properties, the absence of a dedicate regulation, the increasing world-market, the implementation of the application fields. At “nano” size, materials show different physicochemical properties compared to the same material of larger size (bulk material), particularly with respect to conductivity, density, hardness, surface area and surface layer composition. At the same time, these novel properties of nanoparticles (NPs) generate special concerns about their potential hazards to humans and other organisms when released into the environment.
In this context, studies on the potential toxicity of NPs in different biological systems are urgently needed in order to define adequate guidelines for toxicity studies and to harmonize the production of new and safe materials. Since marine environment represents the ultimate sink for any materials discharged into the environment, the effect on marine organisms should be considered a critical point in the definition of NP toxicity.
In coastal ecosystems, microalgae play a key role as primary producers and, being at the base of the aquatic food web, any modification of their growth could affect higher trophic levels additionally, phytoplankton represents an excellent aquatic model for the study of the effects of pollutant exposure at population level due to a short generation time and high sensitivities. For all these reasons, they could be considered as key targets for NPs toxicity.
In this PhD thesis marine phytoplankton have been used in order to assess the potential toxicity and the mode of action of different metal bearing NPs: ZnO, SiO2, TiO2, and Ag. Several endpoint such as population growth inhibition, microscopy observations, cytotoxicity and evaluation of DNA damage are evaluated in the aim of understand the different interaction among algae/NPs and how this interaction could be related to the toxic mechanisms.
The comparison among the tested nanomaterial toxicity pattern highlighted that the algae population growth inhibition occurred through specific pathways related to different physicochemical NP behavior in seawater.
ZnO seems to exert its toxic action upon algae by a punctual and continuous ion release from aggregates in proximity of algae cell wall. In addition, in the case of Ag NPs, the toxicity is related to the ion release but to a greater extend respect to ZnO NP. For SiO2 a cascade of effects (ROS production-DNA damages-growth inhibition) are evidenced suggesting a toxicity starting from oxidative stress generation. TiO2, instead, firstly acts on DNA structure and then, being not soluble in seawater, after internalization during cell division or cell wall destruction, gives place to activation of cellular signals destabilizing DNA structure. These results underline the importance and the necessity of further long-term toxicological experiments. In addition, more attention should be paid on how the toxic effects induced by NPs has impact on the food chain.