Biological Surface Coating and Molting Inhibition as Mechanisms of TiO2 Nanoparticle Toxicity in Daphnia magna

Joel M. Schnur,Lars Duester,André Dabrunz,Gabriele E. Schaumann,Frank Seitz,Ralf Schulz,Carsten Prasse,Ricki Rosenfeldt,Carsten Schilde

doi:10.1371/journal.pone.0020112

Joel M. Schnur, Lars Duester + Show 7 more

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https://doi.org/10.1371/journal.pone.0020112

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Abstract

The production and use of nanoparticles (NP) has steadily increased within the last decade; however, knowledge about risks of NP to human health and ecosystems is still scarce. Common knowledge concerning NP effects on freshwater organisms is largely limited to standard short-term (≤48 h) toxicity tests, which lack both NP fate characterization and an understanding of the mechanisms underlying toxicity. Employing slightly longer exposure times (72 to 96 h), we found that suspensions of nanosized (∼100 nm initial mean diameter) titanium dioxide (nTiO2) led to toxicity in Daphnia magna at nominal concentrations of 3.8 (72-h EC50) and 0.73 mg/L (96-h EC50). However, nTiO2 disappeared quickly from the ISO-medium water phase, resulting in toxicity levels as low as 0.24 mg/L (96-h EC50) based on measured concentrations. Moreover, we showed that nTiO2 (∼100 nm) is significantly more toxic than non-nanosized TiO2 (∼200 nm) prepared from the same stock suspension. Most importantly, we hypothesized a mechanistic chain of events for nTiO2 toxicity in D. magna that involves the coating of the organism surface with nTiO2 combined with a molting disruption. Neonate D. magna (≤6 h) exposed to 2 mg/L nTiO2 exhibited a “biological surface coating” that disappeared within 36 h, during which the first molting was successfully managed by 100% of the exposed organisms. Continued exposure up to 96 h led to a renewed formation of the surface coating and significantly reduced the molting rate to 10%, resulting in 90% mortality. Because coating of aquatic organisms by manmade NP might be ubiquitous in nature, this form of physical NP toxicity might result in widespread negative impacts on environmental health.

Highlights

The steady increase in the production and use of synthetic nanoparticles (NP) in consumer products [1], such as cosmetics or paint, is likely to result in an unintentional release of NP into the environment, causing unknown risks to human and wildlife health [2,3]
The majority of NP toxicity studies so far have been conducted with nTiO2, its agglomerates, and the water flea Daphnia magna Straus, an important standard test organism in aquatic ecotoxicity testing [6]
Almost no data from systematical investigations concerning particle size effects are available to date [14]

Summary

Introduction

The steady increase in the production and use of synthetic nanoparticles (NP) in consumer products [1], such as cosmetics or paint, is likely to result in an unintentional release of NP into the environment, causing unknown risks to human and wildlife health [2,3]. The majority of NP toxicity studies so far have been conducted with nTiO2, its agglomerates, and the water flea Daphnia magna Straus, an important standard test organism in aquatic ecotoxicity testing [6]. Most of these studies found acute effect levels to be .100 mg/L [7]. Available nTiO2 toxicity studies often lack information on NP characteristics and data regarding the verification of the results, e.g., particle size distribution and concentration in the test media. The only study using nTiO2 and D. magna that provided both particle size distribution and concentration measurements of nTiO2 found a 72-h EC50 of 1.62 mg/L [9]. The control effects in this study exceeded the maximum level of the OECD acute toxicity testing guideline 202 of 10% [10]

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