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Toxicity to aquatic algae and cyanobacteria

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Description of key information

Using a study conducted with Scenedesmus quadricauda and H4EDTA, a read across approach was used to predict the NOEC and EC50 values for DTPA.   The predicted NOEC and EC50 values for DTPA (free acid) are 310 mg/L and >310 mg/L, respectively.

Key value for chemical safety assessment

EC50 for freshwater algae:
310 mg/L
EC10 or NOEC for freshwater algae:
310 mg/L

Additional information

The algal study available for DTPA was deemed not reliable (Klimish = 3) mainly due to the fact that the observed effects were due to nutrient limitation and not inherent toxicity of the test substance. OECD 23 (Guidance Document on Aquatic Toxicity Testing of Difficult Substances and Mixtures) states that “Data from tests in which complexation has been judged to have had a significant bearing on the result are likely to be of questionable value for classifying substances and for extrapolating to a predicted no effect concentration for risk assessment” (OECD, 2000). In OECD 23, compensatory adjustment of the algal media with essential ions is the suggested method for obtaining a valid toxicity study with chelating agents. Compensatory adjustment of the algal media was performed in an algal test with Na2H2EDTA 2*H2O (Dufkova, 1984). Dufková (1984) demonstrated with Scenedesmus quadricauda that it was not the absolute EDTA concentration, but rather the ratio of the EDTA to the bivalent cations that is crucial to algae growth. Higher concentrations (400 mg/l Na2H2EDTA 2*H2O =310 mg/l H4EDTA), when in surplus over trace elements in the nutrient solution, inhibited cell division, chlorophyll synthesis and the production of algal biomass, especially in the earlier phase of algae growth. No negative influence was observed when the concentration of trace elements in the nutrient solution was increased corresponding to the increased EDTA concentration. Thus, the NOEC and EC50 values based on growth inhibition ofScenedesmus quadricauda were greater than 310 mg/L H4EDTA.  The aquatic toxicity of DTPA (free acid) and H4EDTA are similar among fish and daphnid species. For example, the EC50 values for daphnids ranged from 480 – 790 mg/L and from 245 – 600 mg/L for H4EDTA and DTPA (free acid), respectively (EU, 2004).  Due to the similarity in chemical structure, mechanism of action, and reported aquatic toxicity of H4EDTA and DTPA (free acid), it is therefore assumed that the NOEC and LC50 values for a valid algal study with DTPA (free acid) would be in the 300 mg/L range, similar to that of H4EDTA.

In addition, a QSAR to determine the toxicity of DTPA to algae was attempted, however the results were not in line with reported toxicity data, and the low Log Kow for DTPA (-4.91 as estimated by KowWin) was likely out of the range of the data set used to generate the structure activity relationship.