Dissolved organic carbon reduces uranium toxicity to the unicellular eukaryote Euglena gracilis Melanie A. Trenfield • Jack C. Ng • Barry Noller • Scott J. Markich • Rick A. van Dam Accepted: 9 January 2012 / Published online: 26 January 2012 Ó Springer Science+Business Media, LLC 2012 Abstract The influence of dissolved organic carbon (DOC), in the form of Suwannee River fulvic acid (SRFA), on uranium (U) toxicity to the unicellular eukaryote, Euglena gracilis (Z strain), was investigated at pH 6. In a background medium without SRFA, exposure of E. grac- ilis to 57 lgL -1 U resulted in a 50% reduction in growth (IC 50 ). The addition of 20 mg L -1 DOC (as SRFA), reduced U toxicity 4 to 5-fold (IC 50 increased to 254 lgL -1 U). This reduction in toxicity was also evident at more sensitive effect levels with a 10% reduction in growth (IC 10 ) occurring at 5 lgL -1 U in the background medium and at 17 lgL -1 U in the SRFA medium, respectively. This amelioration of toxicity with the addition of SRFA was linked to a decrease in the bioavailability of U, with geochemical speciation modelling predicting 84% of U would be complexed by SRFA. The decrease in bioavailability of U in the presence of SRFA was also evident from the 11–14 fold reduction in the cellular con- centration of U compared to that of E. gracilis in the background medium. Stepwise multiple linear regression analyses indicated that UO 2 2? alone explained 51% of the variation in measured U toxicity to E. gracilis. Preliminary U exposures to E. gracilis in the presence of a reactive oxygen species probe, suggest exposure to C60 lgL -1 U may induce oxidative stress, but this endpoint was not considered to be a sensitive biological indicator. Keywords Euglena Á Fulvic acid Á Uranium Á Toxicity Á Freshwater Á Dissolved organic carbon Introduction The growing demand for uranium (U, IAEA 2008) could potentially lead to an increased risk of U exposure to humans and the general environment. Anthropogenic activities such as uranium mining operations have the potential to release U in the environment at concentrations higher than natural background levels (Hynes et al. 1987; Pyle et al. 2002; Antunes et al. 2007). Concentrations of 600–1,200 lgL -1 U have been measured in creekwater downstream from a U mine, and 5,500 lgL -1 U in groundwater influenced by a U processing mill (Landa and Gray 1995). Uranium toxicity studies conducted with freshwater organisms at pH 6–7 have shown that U concentrations as low as 50 lgL -1 can cause 50% effects on species, including Hydra viridissima and Chlorella sp. (population growth; Franklin et al. 2000; Trenfield et al. 2011a), Moinodaphnia macleayi (reproduction; Semaan et al. 2001) and Daphnia magna (growth and reproduction; Zeman et al. 2008). Although quite a few studies have Electronic supplementary material The online version of this article (doi:10.1007/s10646-012-0855-x) contains supplementary material, which is available to authorized users. M. A. Trenfield (&) Á R. A. van Dam Ecotoxicology Program, Environmental Research Institute of the Supervising Scientist, GPO Box 461, Darwin, NT 0801, Australia e-mail: Melanie.Trenfield@environment.gov.au M. A. Trenfield Á J. C. Ng National Research Centre for Environmental Toxicology, The University of Queensland, Coopers Plains, QLD 4108, Australia B. Noller Centre for Mined Land Rehabilitation, The University of Queensland, St. Lucia, QLD 4067, Australia S. J. Markich Aquatic Solutions International, Dundas, NSW 2117, Australia 123 Ecotoxicology (2012) 21:1013–1023 DOI 10.1007/s10646-012-0855-x