Article pubs.acs.org/est
Cellular Partitioning of Nanoparticulate versus Dissolved Metals in Marine Phytoplankton Gretchen K. Bielmyer-Fraser,*,† Tayler A. Jarvis,† Hunter S. Lenihan,‡ and Robert J. Miller§ †
Valdosta State University, 1500 North Patterson Street, Valdosta, Georgia 31698, United States Bren School of Environmental Science and Management, University of California, Santa Barbara, California 93106, United States § Marine Science Institute, University of California, Santa Barbara, California 93106, United States ‡
ABSTRACT: Discharges of metal oxide nanoparticles into aquatic environments are increasing with their use in society, thereby increasing exposure risk for aquatic organisms. Separating the impacts of nanoparticle from dissolved metal pollution is critical for assessing the environmental risks of the rapidly growing nanomaterial industry, especially in terms of ecosystem effects. Metal oxides negatively affect several species of marine phytoplankton, which are responsible for most marine primary production. Whether such toxicity is generally due to nanoparticles or exposure to dissolved metals liberated from particles is uncertain. The type and severity of toxicity depends in part on whether phytoplankton cells take up and accumulate primarily nanoparticles or dissolved metal ions. We compared the responses of the marine diatom, Thalassiosira weissf logii, exposed to ZnO, AgO, and CuO nanoparticles with the responses of T. weissf logii cells exposed to the dissolved metals ZnCl2, AgNO3, and CuCl2 for 7 d. Cellular metal accumulation, metal distribution, and algal population growth were measured to elucidate differences in exposure to the different forms of metal. Concentration-dependent metal accumulation and reduced population growth were observed in T. weissf logii exposed to nanometal oxides, as well as dissolved metals. Significant effects on population growth were observed at the lowest concentrations tested for all metals, with similar toxicity for both dissolved and nanoparticulate metals. Cellular metal distribution, however, markedly differed between T. weissf logii exposed to nanometal oxides versus those exposed to dissolved metals. Metal concentrations were highest in the algal cell wall when cells were exposed to metal oxide nanoparticles, whereas algae exposed to dissolved metals had higher proportions of metal in the organelle and endoplasmic reticulum fractions. These results have implications for marine plankton communities as well as higher trophic levels, since metal may be transferred from phytoplankton through food webs vis à vis grazing by zooplankton or other pathways.
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applications.10 Exposure of aquatic organisms to nanoparticles is difficult to quantify; however, some modeling efforts have reported the presence of metal oxides in aquatic systems at levels that may cause toxicity.11 Phytoplankton require the essential metals zinc (Zn) and copper (Cu) for enzyme functioning, but at elevated concentrations, these metals may exert toxicity.12−14 Most ecotoxicity work on phytoplankton has been done using dissolved metals, with a few recent studies on nanoparticles. For example, ZnO nanoparticles are known to reduce population growth rates in the diatoms Skeletonema marinoi, Thalassiosira pseudonana, and Thalassiosira weissf logii.13,14 Metal oxide nanoparticles dissolve in seawater to varying degrees, and release free metal ions,15 which may be the predominant cause of toxicity to aquatic organisms. However, the dynamics of dissolution and release of metal ions, and how these processes vary with concentration and material type is poorly understood.
INTRODUCTION Diatoms are the dominant primary producers in the ocean, and because they are small and have a high surface-to-volume ratio, they can take up and accumulate substantial amounts of contaminants.1−4 This uptake may cause multiple impacts to marine food webs: reductions in population growth rate and possibly nutritional content of phytoplankton cells could reduce resources available for consumers,5 and accumulation of contaminants in phytoplankton can lead to trophic transfer and resulting toxic effects on consumers.6 Metals are an important class of such contaminants, and now are being discharged into coastal ecosystems as nanomaterials in addition to traditional bulk and dissolved forms. Nanomaterials are now widely utilized for their enhanced mechanical and optical properties, as well as their efficient electrical conductivity, relative to larger forms of similar materials.7 Because of their growing application in an array of fields including electronics, chemical, cosmetics, and biomedicine,7,8 nanomaterials are emerging as a new class of contaminants, with unknown environmental consequences.9 Metal oxide nanomaterials in particular are commonly used because they are synthesized relatively easily and have myriad industrial and consumer © 2014 American Chemical Society
Received: Revised: Accepted: Published: 13443
March 25, 2014 October 13, 2014 October 22, 2014 October 22, 2014 dx.doi.org/10.1021/es501187g | Environ. Sci. Technol. 2014, 48, 13443−13450
Environmental Science & Technology
Article
prior to inoculation. Synthetic seawater was made by mixing Instant Ocean salt with 18 mΩ Milli-Q water and aerating at least 24 h before use.32 The sterilized media was inoculated with 2.5 × 105 algal cells and the algae were cultured for 7 d with continuous aeration. Algae were incubated under cool white fluorescent lights (12 h light: 12 h dark) at a light intensity of 36 μmol photons m−2 s−1 and a temperature of 20 °C. Algal densities were measured using a hemocytometer (Hausser Scientific, Horsham, PA) and a compound microscope. Nanoparticles. ZnO nanoparticles were obtained from Meliorum Technologies (Rochester, NY, USA) and characterized for size, morphology and chemical composition.33,34 ZnO nanoparticles were spheroid, 100% zincite, and 20−30 nm in diameter. CuO nanoparticles were obtained from SigmaAldrich (St. Louis, MO, USA) and described as
