The influence of phosphate on the toxicity of uranium to Lemna gibba G3 was tested in semicontinuous culture with synthetic mine water developed as an analogue of surface water of two abandoned uranium mining and ore processing sites in Saxony, Germany. Six concentrations of uranium were investigated under five different supply regimes of PO 4 3À at constant pH (7.0 6 0.5) and alkalinity (7.0 6 1.6 mg L À1 total CO 3 2À). The results showed significant inhibition of specific growth rates in cultures exposed to the highest uranium concentrations (3500 and 7000 g U L À1 ) at lowest PO 4 3À supply of 0.01 mg LÀ1 . An increase of phosphate concentration from 0.01 to 8.0 mg L À1 resulted in an increase of EC 50 from 0.9 6 0.2 to 7.4 6 1.9 mg L À1 (significant with Student's t test, P > 0.05). The accumulation of uranium in L. gibba increased exponentially with the increase in uranium concentration in cultures with 0.01 and 0.14 mg PO 4 3À L À1. Accumulation also increased significantly when PO 4 3À supply was increased from 0.14 to 1.36 mg PO 4 3À L À1 for all uranium concentrations. However, as the supply of PO 4 3À gradually increased from 1.36 to 8.0 mg PO 4 3À L À1 , uranium bioaccumulation increased slightly but insignificantly before leveling off. Uranium speciation modeling with PhreeqC geochemical code predicted increases in the proportions of uranyl phosphate species when PO 4 3À concentrations increase in the media. Most of these uranyl phosphate species have a high probability of precipitation [saturation indices (SI) > 0.93]. Therefore, the alleviation of uranium toxicity to L. gibba with phosphates is due to interactions among components of the media, mainly uranyl and phosphate which results in precipitation. Consequently, bioavailable fractions of uranium to L. gibba are reduced. This might explain lack of consistent EC 50 values for uranium to most aquatic organisms. # 2007 Wiley Periodicals, Inc. Environ Toxicol 22: 9-16, 2007.

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