Nanotoxicology in the environment

Kristin Schirmer

Kristin Schirmer is head of Department of Environmental Toxicology at the Swiss Federal Institute of Aquatic Science and Technology, Eawag, and holds a Professorship at the ETH Lausanne, lecturing Environmental Toxicology at both the ETH in Lausanne and in Zürich, Switzerland. Her specific areas of interest include elucidating how nanomaterials are taken up and distributed in organisms of the aquatic environment, accumulated and transferred along the food chain. As well, she aims to understand the molecular mechanisms underlying the adaptive or toxicological responses of organisms upon uptake and/or interaction with nanomaterials.

Melanie Auffan

Melanie Auffan is a CNRS research scientist at the CEREGE (European Geosciences Center) in Aix en Provence. She is one of the co-directors of iCEINT international consortium for the Environmental Implications of Nanotechnology and member of the CEINT steering committee. Her research addresses the physico-chemical properties and surface reactivity of nanoparticles in contact with living organisms. Since 2012 she is part of the safer by design project called Labex Serenade (Laboratory of Excellence for Safe(r) Ecodesign Research and Education applied to NAnomaterial DEvelopment).

Over the past decade, nanoecotoxicology has emerged as an important area of toxicological research. Its development was sparked by the uncertainties arising from the rapid evolution of nanotechnology and associated widespread use of nanomaterials, in particular nanoparticles, in numerous applications, such as personal care and household products, textiles and fuel additives. Considering such applications, it is evident that nanomaterials can enter the environment. Moreover, nanomaterials might be purposely introduced into the environment, for example, in order to trap or degrade chemical contaminants. Each of these current or potential future uses calls for knowledge that enables proper risk assessment. Nanoecotoxicology therefore aims to identify and predict effects elicited by nano-sized materials on ecosystems. To achieve this aim, nanoecotoxicology considers entry routes and fate of nanomaterials in the environment (exposure) and identifies alterations in the functioning of organisms, from cells to complex communities (effects). Together, this information forms the base to evaluate the risk that nanomaterials may have in a given environment.

While the principal aims and elements of nanoecotoxicology are well-defined, it is faced with tremendous challenges. Characterization of the behavior of nanomaterials, taking into consideration the respective exposure media and relevant test concentrations, is now a standard requirement. The thereby gained knowledge is of utmost importance to properly account for bioavailability. Yet, it cannot by itself account for the dynamic nature of nanomaterial properties. For example, the properties can change once in contact with organisms, which in turn may influence bioavailability. Once taken up by organisms, tissue-internal distribution is an important consideration because it can help identify sensitive target tissues. Such analysis is, however, hampered by the difficulties to quantify and/or visualize nanomaterials in complex samples. Another challenge is to derive a mechanistic understanding of the interaction of nanomaterials with biomolecules, from molecular initiating events in cells to organism outcomes, such as reduced survival and growth. By linking internal exposure and ensuing mechanisms of organism response, nanoecotoxicology can build transferable knowledge to understand and potentially predict, for example, consequences of long-term fluctuating or continuous exposure or species-sensitivity differences. Given the tremendous diversity of nanomaterials in terms of material type, size, shape and coating, it is simply impossible to test all materials to the same extent. Therefore, nanoecotoxicology also needs to build tiered testing strategies in which nanomaterials with the highest potential risk are flagged for further analysis while high-throughput screens are used as the first tier. Finally, it is essential to link the knowledge obtained with single organisms or populations and well-defined conditions to more complex scenarios, such as simultaneous exposures to multiple stressors or impact on community composition and function upon both short and long-term exposure. To showcase examples that address each of these challenges is the aim of this themed issue.

A review and perspective of nano–bio–eco-interactions with regard to the toxicity of engineered metal oxide nanomaterials is provided by He et al. (DOI: 10.1039/C5EN00094G). They present a comprehensive review of the recent experimental and theoretical studies on the toxicity of engineered metal oxide nanomaterials. Indeed, nanomaterials interact with their surrounding environments, biotic and abiotic, immediately upon introduction into the environment; and their behavior and fate are influenced by the dynamics of the environment. Using appropriate examples, they highlight that a thorough investigation of the potential nanotoxicity of engineered nanomaterials at the nano–bio–eco interface is urgently needed to select and design nanomaterials with minimum adverse impacts. An interesting discussion regards how the careful characterization of nanomaterials and the choosing of methodologies that promote further risk assessment generate more reliable and accurate data output. In the perspective, they also call for more open collaborations between industry, academia, and research labs to facilitate nano-toxicological studies focused specifically on interactions at the nano–bio–eco interface, leading to safe and effective nanotechnology for commercial, environmental, and medicinal use.

The oral bioavailability and sex-specific tissue distribution of quantum dots in fathead minnow were investigated by Lavelle et al. (DOI: 10.1039/C5EN00122F). They exposed fish orally to a single dose or to sequential doses of differently surface-functionalized quantum dots over a period of two weeks upon which the tissue content of quantum dots was quantified by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The ratio of cadmium and tellurium in each sample was used as a means to distinguish the presence of quantum dots rather than simply ions released from the quantum dots. Analysis not only proved that the quantum dots were able to cross the fish intestinal barrier, reach the blood stream and from there various tissues, but also that the quantum dots were retained in the respective tissues and accumulated upon sequential dosage. Carboxyl- and amino-surface functionalized quantum dots were more readily taken up than PEG-coated quantum dots. A particularly interesting finding was that the carboxyl-quantum dots accumulated significantly more in the gonads of the female than of the male fish. The authors hypothesize that the binding of proteins, such as vitellogenin, onto the carboxyl-coated quantum dots may lead to such a sex-specific tissue accumulation.

Two studies focus on the distribution and relation to toxicity of silver nanoparticles in zebrafish embryos and alga. Li et al. (DOI: 10.1039/C5EN00093A) thoroughly explored the interactions of Ag nanoparticles and AgNO₃ with the green alga Euglena gracilis, an interesting biological model with no cell wall but a pellicle. They observed that photosynthetic yield decreased in a concentration-dependent manner with Ag nanoparticles being less toxic than AgNO₃ based on the total silver added. Interestingly, damaging effects of nanoparticles on the photosynthesis and morphology were completely prevented by cysteine, suggesting that dissolved Ag mediated the toxicity of these nanoparticles. Moreover, by applying Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), they demonstrated that the difference was not caused by the cellular uptake of Ag nanoparticles, but by their strong sorption onto the pellicle. Böhme et al. (DOI: 10.1039/C5EN00118H) recorded dose–response curves for different zebrafish embryo developmental stages exposed to Ag nanoparticles and AgNO₃. They observed that the internal silver dose per organism differs around 0.5–1 ng Ag per organism irrespective of the developmental stage. However, for earlier developmental stages, the chorion of the embryo was found to effectively adsorb silver, leading to an up to 20-fold increase in total silver concentrations compared to the later chorion-free stages. Finally, they found a correlation between increased internal silver (particulate and ionic) concentrations and the occurrence of sub-lethal and lethal effects. These two studies highlight that beyond the assessment of the effects, information on real exposure concentrations, particle uptake and distribution patterns as well as the determination of internal effect concentrations contribute to a deeper understanding of nanoparticles–organism interactions.

Gene expression as an indicator of the molecular response and ensuing outcomes on the organism level upon exposure to gold nanoparticles were investigated by Qiu et al. (DOI: 10.1039/C5EN00037H). The bacterium, Shewanella oneidensis, and the water flea, Daphnia magna, served as model organisms and the gold nanoparticles were either positively or negatively surface-functionalized. Gene expression was quantified as abundance of mRNA transcripts. As to be expected, gene expression was dynamic in nature, i.e., it changed over time, demonstrating that the organisms mount a molecular response to combat particle exposure. Moreover, most of the regulation of gene expression in the bacterium was explained by the ligand alone, while a partly particle-specific response was seen in the water flea. The authors hypothesize that the cell wall of the bacterium might explain this difference as it protects the cells from particle uptake in contrast to the wall-free cells of the water flea. As well, the bacteria only passively interact with the particles while the water flea can actively internalize them. The positively charged particles were overall significantly more toxic than the negatively charged particles, which might be explained by a stronger interaction of the positively charged particles with the organisms due to electrostatic interaction because the organism surface is overall negatively charged. Along these lines, the gene actin, which encodes the respective cytoskeleton protein, was specific to the long-term exposure of the water flea toward the positively charged nanoparticle.

The toxicity of 12 metal-based nanoparticles to algae, bacteria and protozoa was investigated by Aruoja et al. (DOI: 10.1039/C5EN00057B). Indeed, consistent biological-effect data of nanoparticles for environmentally relevant test species, which are accompanied by thorough characterization of nanoparticles, are scarce but indispensable for understanding their possible risks. In this study, they have composed and tested a library of 12 metal-based nanoparticles (Al₂O₃, Co₃O₄, CuO, Fe₃O₄, MgO, Mn₃O₄, Sb₂O₃, SiO₂, ZnO, TiO₂, WO₃ and Pd) using the alga Pseudokirchneriella subcapitata, three bacterial species (Vibrio fischeri, Escherichia coli, Staphylococcus aureus) and the protozoa Tetrahymena thermophila. One of their interesting findings is that algal toxicity correlated with abiotic reactive oxygen species production of nanoparticles, and the majority of the nanoparticles formed agglomerates that entrapped algal cells. Despite the different sensitivities, they also found a common trend in the toxicity of the nanoparticles across different species and test formats (with the highest toxicities for CuO and ZnO due to their dissolution).

Two studies in this themed issue deal with more complex exposure scenarios. Falconer et al. (DOI: 10.1039/C5EN00111K) report on the ability of carbon nanomaterials to protect zebrafish embryos from phenanthrene toxicity. The carbon materials were multi-walled carbon nanotubes and carbon black. Both types of materials had a similar concentration-dependent protective effect toward phenanthrene embryo toxicity when added during phenanthrene exposure. When the materials were added at the same concentration after an initial exposure to phenanthrene, they again both protected the embryo from ensuing toxicity but the protection by carbon black was significantly greater than that by the multi-walled carbon nanotubes. These differences were explained by the greater adsorption capacity of the carbon black due to a larger surface area. Taken together, these results highlight the importance of taking multiple-stressor exposures into account. Carbon-based nanomaterials might in fact protect organisms in the environment by lowering exposure to chemicals via adsorption. On the other hand, the strong adsorption capacity of carbon-based materials may lead to a precarious reduction of bioavailable essential molecules, such as lipids and proteins, once internalized by organisms.

An even more complex scenario is considered in the study by Tella et al. (DOI: 10.1039/C5EN00092K). It assesses the fate and transport of CeO₂ nanoparticles upon chronic exposure of a simulated pond ecosystem in indoor aquatic mesocosms. Two types of CeO₂ nanoparticles, namely bare or citrate-coated, were added to the mesocosm via a continuous point-source discharge. In order to properly characterize the nanoparticle behavior and distribution in this complex environment, geochemical modeling was combined with chemical analysis, time-resolved laser diffraction, and high-energy resolution fluorescence-detected X-ray absorption spectroscopy. The bare CeO₂ particles largely homo-agglomerated and settled out from solution. The citrate-coated CeO₂ particles were more stable in suspension but the citrate coating was degraded over time. Moreover, Ce^III organic complexation led to greater release of Ce ions into the medium. These different behaviors were reflected in the exposure of a planktonic filter feeder and a benthic grazer. For example, significant amounts of Ce were associated with the planktonic filter feeder but the amounts were much larger for the citrate-coated CeO₂. On the other hand, much larger amounts of Ce from bare CeO₂ exposure were detected in the digestive gland of the benthic grazer. This study therefore highlights the importance of taking the kinetics of different particle characteristics under realistic exposure scenarios into account, as these will influence the extent of exposure of organisms of different ecological niches and traits.

We hope that you find this issue inspiring. Many of the aspects addressed here are at their early stage of development but will be essential for nanoecotoxicology to live up to its full potential and aims.

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