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Environmental context. It cannot be assumed that nanomaterials entering aquatic environments will have the same impacts on aquatic biota as their macroscopic particle equivalents. If their toxicities are different, this will have implications for the way in which nanomaterial usage is regulated. Algae, at the bottom of the food chain, are likely to be a sensitive indicator of toxic effects. Understanding the physical and chemical factors controlling nanoparticle toxicity to algae will assist in evaluating their ecological risk. Abstract. In assessing the risks posed by nanomaterials in the environment, the overriding research challenges are to determine if nanomaterials are more toxic than the bulk forms of the same material, and the extent to which toxicity is governed by particle size and reactivity. In this study, the toxicity of nanoparticulate CeO2 (nominally 10–20 nm) to the freshwater alga Pseudokirchneriella subcapitata was compared to the same material at the micron size (nominally <5 μm). Growth inhibition experiments revealed inhibitory concentration values, giving 50% reduction in algal growth rate after 72 h (IC50), of 10.3 ± 1.7 and 66 ± 22 mg L–1 for the nanoparticles and bulk materials respectively. Cells exposed to CeO2 particles were permeable to the DNA-binding dye SYTOX® Green in a concentration-dependent manner indicating damage to the cell membrane. Screening assays to assess the oxidative activity of the particles showed that the light illumination conditions used during standard algal bioassays are sufficient to stimulate photocatalytic activity of CeO2 particles, causing the generation of hydroxyl radicals and peroxidation of a model plant fatty acid. No oxidative activity or lipid peroxidation was observed in the dark. These findings indicate that inhibitory mode of action of CeO2 to P. subcapitata is mediated by a cell-particle interaction causing membrane damage. The effect is most likely photochemically induced and is enhanced for the nanoparticulate form of the CeO2.

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Although it has been well established that different species of marine algae have different sensitivities to metals, our understanding of the physiological and biochemical basis for these differences is limited. This study investigated copper adsorption and internalisation in three algal species with differing sensitivities to copper. The diatom Phaeodactylum tricornutum was particularly sensitive to copper, with a 72-h IC50 (concentration of copper to inhibit growth rate by 50%) of 8.0 microg Cu L(-1), compared to the green algae Tetraselmis sp. (72-h IC50 47 microg Cu L(-1)) and Dunaliella tertiolecta (72-h IC50 530 microg Cu L(-1)). At these IC50 concentrations, Tetraselmis sp. had much higher intracellular copper (1.97+/-0.01 x 10(-13)g Cu cell(-1)) than P. tricornutum (0.23+/-0.19 x 10(-13)g Cu cell(-1)) and D. tertiolecta (0.59+/-0.05 x 10(-13)g Cu cell(-1)), suggesting that Tetraselmis sp. effectively detoxifies copper within the cell. By contrast, at the same external copper concentration (50 microg L(-1)), D. tertiolecta appears to better exclude copper than Tetraselmis sp. by having a slower copper internalisation rate and lower internal copper concentrations at equivalent extracellular concentrations. The results suggest that the use of internal copper concentrations and net uptake rates alone cannot explain differences in species-sensitivity for different algal species. Model prediction of copper toxicity to marine biota and understanding fundamental differences in species-sensitivity will require, not just an understanding of water quality parameters and copper-cell binding, but also further knowledge of cellular detoxification mechanisms.