Chronic toxicity of uranium to a tropical green alga (Chlorella sp.) in natural waters and the influence of dissolved organic carbon.

The chronic toxicity (72-h cell division rate) of uranium (U) to the unicellular alga, Chlorella sp., was assessed in natural Magela Creek water (NMCW) to provide data for the derivation of a site-specific water quality trigger value for U in Magela Creek, NT, Australia. In addition, the data were compared to those for Chlorella sp. when tested for U toxicity using synthetic Magela Creek water (SMCW), which simulates the inorganic composition of Magela Creek water and contains no organic component. Based on one rangefinder and four definitive toxicity tests, concentrations causing a 50% inhibition of algal growth after 72 h exposure (72 h IC50s) ranged between 137 and 238 microg/LU, no-observed-effect concentrations (NOECs) from 72 to 157 microg/LU and lowest-observed-effect concentrations (LOECs) from 120 to 187 microg/LU. Based on these data, Chlorella sp. was the second most sensitive organism to U of five local species that have been assessed using NMCW. The U toxicity data for Chlorella sp. were incorporated with existing data for the four other species to derive a site-specific guideline value for Magela Creek that is protective of 99% of species of 6 microg/L. The toxicity of U to Chlorella sp. in NMCW was approximately two to four times lower than in SMCW. Based on geochemical speciation modelling, this difference corresponded to a four-fold decrease in the proportion of free uranyl ion (UO2(2+)) in NMCW compared to SMCW, most likely due to the presence of dissolved organic carbon (DOC) in NMCW. Relatively, large variability in U toxicity across the tests conducted in NMCW was found to be inversely related to DOC concentration (r2 = 0.996, n = 4, P = 0.002). Speciation modelling indicated that the increase in DOC was associated with an increase in the proportion of U complexed with DOC (r2 = 0.986, n = 4, P < 0.001) and a decrease in the proportion of the UO2(2+) (r2 = 0.989, n = 4, P = 0.006). When the proportion of UO2(2+) was regressed against U toxicity, a very strong, positive relationship was observed (r2 = 1, n = 4, P < 0.001). The results indicate that the bioavailability and toxicity of U is highly influenced by dissolved organic matter and that the relationship should be further quantified.