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

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.

Behavior and chronic toxicity of two differently stabilized silver nanoparticles to Daphnia magna

“…Attachment to the carapax has been reported for several NPs with different core materials and coatings (Asghari et al, 2012;Baumann et al, 2014a;Bozich et al, 2014;Dabrunz et al, 2011;Zhao and Wang, 2012). In most cases, it caused reduction or even inhibition of molting (Baumann et al, 2014a;Bozich et al, 2014;Dabrunz et al, 2011) which was not the case in the present study indicating low levels of attachment of the AgNPs to the exoskeleton here.…”

Freshwater ecotoxicity characterisation factor for metal oxide nanoparticles: A case study on titanium dioxide nanoparticle

“…suspension preparation method with use of solvent, sonication, filtration, (Clement et al, 2013;Arouja et al, 2009; and iii) mode of exposure to organism i.e. the time of exposure, UV exposure (Zhu et al, 2010;Dabrunz et al, 2011;Seitz et al, 2013;Ma et al, 2012). In order to increase the reliability of the estimated EF criteria have been applied to select the toxic value involved into the EF calculation.…”

“…EC50) for nanoparticles is influenced by several aspects (Li et al, 2014;Seitz et al, 2013;Dabrunz et al, 2011;Gottschalk et al, 2013) as: the crystalline structure of nano-TiO2, the treatments of the sample to test (i.e. filtered or unfiltered, use of solvent), the com-bination of different materials with varying testing condition, the expo-sure time and the exposure under UV radiation.…”

“…Indeed, the ecotoxicity studies are mainly performed in lab conditions and in absence of UV exposure (Li et al, 2014). Even if the influence of these factors to the toxicity test results are reported and discussed in literature (Zhu et al, 2009;Dabrunz et al, 2011;Seitz et al, 2013Seitz et al, , 2014Li et al, 2014), a systematic assessment of their im-portance is still missing. This leads to a difficult comparison of the toxic-ity values among the studies and a scarce understanding of the magnitude of the influence of different factors on the toxicity results.…”

Significance of particle size and charge capacity in TiO2 nanoparticle-lipid interactions

“…TiO 2 is refractory with a very low water solubility as well as showing a strong tendency to aggregate [2][3][4][5][6]. In spite of this, its biological activity remains uncertain with conflicting reports as to its hazard to environmental and human health [7][8][9][10]. A specific feature of TiO 2 is its semiconductor properties which means that the material can absorb UV which enables its photoactivity [11] and photocatalytic activity [12] and may enhance its incipient toxicity [13].…”