Fate of engineered cerium oxide nanoparticles in an aquatic environment and their toxicity toward 14 ciliated protist species

Highlights

• Acute toxicity of CeO₂ NPs toward 14 ciliated protist species was first assessed.

• 48 h aggregation kinetics in Dryl's solution showed CeO₂ NPs were relatively stable.

• CeO₂ NPs generally elicited greater toxicity in a particle state than a bulk state.

• LC₅₀ values were negatively correlated with surface-to-volume ratio of the protists.

• LC₅₀ values of 14 protist species were not significantly phylogenetically conserved.

Abstract

The potential environmental impacts of engineered cerium oxide nanoparticles (CeO₂ NPs) on aquatic organisms have remained largely unknown. Therefore, the laboratory study featured herein was performed to determine the fate of CeO₂ NPs in an aquatic environment and their toxicity towards 14 different ciliated protist species at a specified population level. An investigation of 48 h aggregation kinetics in the Dryl's solution showed the CeO₂ NPs to be relatively stable. The pH values in three test medium were too far away from PZC, which explained the stability of CeO₂ NPs. CeO₂ NPs generally elicited more toxicity with increasing NP concentration, following certain dose-response relationships. Nano-CeO₂ resulted in greater toxicity in a particle state than when added as bulk material. LC₅₀ values showed a negative correlation with the surface-to-volume ratio for these protists, suggesting that surface adsorption of CeO₂ NPs might contribute to the observed toxicity. Additionally, acute toxic responses of 14 ciliated protist species to CeO₂ NPs were not significantly phylogenetically conserved. The results of these observations provide a better insight into the potential risks of CeO₂ NPs in an aquatic environment.

Introduction

Nanotechnology utilizes nanoparticles (NPs) that are unique not only due to their minute size (smaller than 100 nm) but also because of their size-dependent characteristics. NPs are being spotlighted as a center stage phenomenon for future industry (Arnaldi, 2014). The production and number of applications of engineered NPs are increasing rapidly worldwide. Current applications include the use of NPs in consumer products, construction materials, medical and pharmaceutical industries, agriculture, and information technology (Karen et al., 2009). Metal oxide NPs are an important category of manufactured NPs, accounting for about one-third of the consumer products nanotechnology market. For instance, cerium oxide (CeO₂) NPs are increasingly used as a catalyst in the automotive industry (Zhao et al., 2012, Zhang et al., 2014). Consequently, CeO₂ NPs are expected to enter environmental water samples via waste streams from industries that synthesize or use CeO₂ (Hu et al., 2012a, Hu et al., 2012b, Auffan et al., 2013). CeO₂ NPs are on the Organization for Economic Co-operation and Development (OECD) list of priority nanomaterials for immediate testing and it is imperative to perform the risk assessment of their potential eco-toxicological effects (OECD, 2010a, OECD, 2010b). To date, the studies on the CeO₂ NPs toxicity conducted with aquatic organisms are rather limited and their environmental fate and impacts have remained largely unknown.

NP dispersion in aqueous media for toxicity studies remains a challenge because of the tendency for NPs to aggregate. Aggregation will cause the decrease of exposed surface area (Dhawan et al., 2009), changing oxidative damage induced by the self-quench of reactive oxygen species (Hotze et al., 2010). Previous studies have shown that the problems are exacerbated in culture media due to the presence of salts and other compounds (Fubini et al., 2010). Solution pH is an essential factor governing NP aggregation. Aggregation can be prevented by adjusting the solution pH away from the NP point of zero charge (PZC), thereby conferring a net negative or positive charge to cerium oxide surfaces. This results in an increased interparticle repulsion, decreasing the rate of aggregation (Hotze et al., 2010). The extent to which aggregation of CeO₂ NPs in different types of solutions influences NP dispersion needs to be addressed before conducting the toxicity study. Additionally, CeO₂ NPs dissolution could play an important role in toxicity evaluation. During the incubation the core metal Ce could be released by oxidative and photolytic conditions from the NPs, which might be a key factor in determining its toxicity to aquatic organisms (Dahle et al., 2015, Zhao et al., 2012).

Previous tests on environmental toxicity of NPs in freshwater usually focused on conventional model organisms such as bacteria, algae (single-celled with cell walls), Daphnia (multicellular organisms), and zebrafish (Zhang et al., 2012a, Zhang et al., 2012b). Additionally, the factors (and related underlying mechanism) that influence environmental toxicity among different species remain largely unknown. A variety of other organisms in the aquatic environment are important to maintain the balance of ecological systems, and the toxicity of NPs against these organisms has not been extensively investigated.

Freshwater protist species provide a good model for study on how environmental toxicity differs among species because of their ubiquitous global distribution and special sensitivity to environmental contaminants. Protozoa are single cell organisms that do not possess a protective cell wall. As a consequence, NPs could enter protozoan cells more easily than bacterial and algal cells and interact directly with the cellular structures and organelles (Mortimer et al., 2014, Zou et al., 2013). The ciliated protists have been widely studied by eco-toxicologists not only because of their role in the regulation of microbial populations through the ingestion and digestion of bacteria but also because of their high sensitivity to chemical materials; hence, they are often used as indicator species of environmental pollution (Bick, 1972).

Species phylogenetic histories often place a constraint on their trait evolution, such that more closely related species tend to exhibit more similar traits, a phenomenon termed phylogenetic trait conservatism (Harvey and Pagel, 1991, Bello et al., 2009). On the other hand, convergent evolution could result in distantly related species that are similar in their traits; this could serve to decouple the linkage between species phylogenetic relatedness and trait similarity (Losos et al., 2003).

The objectives of this research were 1) to assess the environmental fate of CeO₂ NPs and their impacts on 14 freshwater ciliated protist species by conducting acute toxicity tests 2) to further evaluate whether acute toxicological responses of these ciliated protist species to CeO₂ NPs are phylogenetically conserved. Towards these goals, the fate of CeO₂ NPs in test media was first determined via the kinetic study of hydrodynamic size and zeta potentials. LC₅₀ values of CeO₂ NPs derived from acute toxicity tests for 14 different protist species were then estimated, and the phylogenetic signal of LC₅₀ for each tested protist species was analyzed. Our findings establish a useful scientific basis for CeO₂ NPs ecological risk assessment.