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

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 (UO22+) 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 UO22+ (r2 = 0.989, n = 4, P = 0.006). When the proportion of UO22+ 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.