Abstract
Nanoparticulate titanium dioxide (TiO2) is highly photoactive, and its function as a photocatalyst drives much of the application demand for TiO2. Because TiO2 generates reactive oxygen species (ROS) when exposed to ultraviolet radiation (UVR), nanoparticulate TiO2 has been used in antibacterial coatings and wastewater disinfection, and has been investigated as an anti-cancer agent. Oxidative stress mediated by photoactive TiO2 is the likely mechanism of its toxicity, and experiments demonstrating cytotoxicity of TiO2 have used exposure to strong artificial sources of ultraviolet radiation (UVR). In vivo tests of TiO2 toxicity with aquatic organisms have typically shown low toxicity, and results across studies have been variable. No work has demonstrated that photoactivity causes environmental toxicity of TiO2 under natural levels of UVR. Here we show that relatively low levels of ultraviolet light, consistent with those found in nature, can induce toxicity of TiO2 nanoparticles to marine phytoplankton, the most important primary producers on Earth. No effect of TiO2 on phytoplankton was found in treatments where UV light was blocked. Under low intensity UVR, ROS in seawater increased with increasing nano-TiO2 concentration. These increases may lead to increased overall oxidative stress in seawater contaminated by TiO2, and cause decreased resiliency of marine ecosystems. Phototoxicity must be considered when evaluating environmental impacts of nanomaterials, many of which are photoactive.
Highlights
Phytoplankton are the dominant primary producers in marine ecosystems [1], where they are the base of oceanic food webs and a dominant component of the global carbon cycle, as well as other biogeochemical cycles
Isochrysis galbana, toxicity was evident at the lowest concentration tested, 1 mg L21 (Dunnett’s method, d = 2.65, p = 0.02), indicating a no-effect concentration (NOEC),1 mg L21
In the other two species affected, Thalassiosira pseudonana, and Dunaliella tertiolecta, significant toxicity was evident at 3 mg L21, a slight depression of growth rates was seen for D. tertiolecta at 1 mg L21 (Fig. 1)
Summary
Phytoplankton are the dominant primary producers in marine ecosystems [1], where they are the base of oceanic food webs and a dominant component of the global carbon cycle, as well as other biogeochemical cycles. As abundant small (0.2–200 mm) single or clustered cells with high surface-to-volume ratios suspended in water, phytoplankton have high probability of encountering suspended particles, including pollutants, especially in coastal zones where contaminants are found in highest concentrations. Information on the impact of emerging contaminants on phytoplankton, and the potential interaction of contaminants with environmental variables such as irradiance is necessary to predict potential impacts on coastal marine food webs and the ecosystems that they support. Estimated environmental concentrations indicate that among the most commonly used nanomaterials, TiO2 may reach highest concentrations in surface waters and pose a significant threat to aquatic ecosystems. Estimated environmental concentrations indicate that among the most commonly used nanomaterials, TiO2 may reach highest concentrations in surface waters and pose a significant threat to aquatic ecosystems. [8,9] Nanoparticulate TiO2 is often phototoxic to cells in vitro and has been used for wastewater disinfection [10,11] and investigated as an anti-cancer agent. [12] Oxidative stress mediated by photoactive TiO2 is the likely mechanism of its toxicity [13,14], and experiments demonstrating cytotoxicity of
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