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Toxicity to aquatic algae and cyanobacteria

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Description of key information

72 hour NOEC > 100 mg/L in  freshwater Scenedesmus subspicatus, OECD 201, EU Method C3, Thomas 1996

Key value for chemical safety assessment

Additional information

The toxicity potential of the test material was determined in the key study by Thomas (1996), in an algal inhibition test conducted using Scenedesmus subspicatus. The study was conducted under GLP conditions and in accordance with the standardised guidelines OECD 201 and EU Method C.3. Based on the principles for assessing data quality defined by Klimisch et al. (1997), the study was assigned a reliability score of 1.

A preliminary test was conducted at nominal concentrations of 0, 0.1, 1, 10 and 100 mg/L. Cultures were exposed for 72 hours under static conditions. The number of cell per culture was determined at 0, 24, 48 and 72 hours using a Mallassez cell counter.

As the test material solution was yellow-orange in colour at 100 mg/L and absorbed daylight, the test design was modified in order to assess the effects of reduced light falling on the algae cultures. Shading controls were incorporated at each test concentration. For the shading controls, the test solutions were placed in a mini aquarium above the flasks containing the algal cultures. The algae flasks remained untreated. Culture flasks were wrapped with an opaque material so that light could penetrate from above only, and passed through the mini-aquarium directly above containing the corresponding test solution. Results were then corrected for shading, allowing any effects on growth inhibition caused by shading to be accounted for. The coefficient of shading was calculated as the growth inhibition of shaded solution divided by the growth inhibition of test solutions; a coefficient of ≥ 0.9 is considered proof of shading rather than toxic effect.

Results before correction for shading clearly demonstrate inhibition of cell growth from 10 mg/L. After correction for the effect of shading by the test material, no reduction in cell growth was noted with respect to the control in any of the test solutions. The No Observed Effect Concentration (NOEC) was determined to be > 100 mg/L (at p = 0.05) based on the specific growth rate and > 100 mg/L (at p = 0.05) based on the biomass data. Using the coefficient of shading, none of the solutions with positive inhibition resulted in a coefficient of less than 0.9.

As the test material was found not to be toxic at the nominal limit concentration of 100 mg/L, a definitive study was deemed unnecessary and samples of the control and 100 mg/L solutions were analysed to verify the concentration of the test material at 0 and 72 hours during the test. As measured concentrations were >80 and < 120 % of the nominal concentrations, all results were based on nominal values.

Under the conditions of this study the NOEC was determined to be > 100 mg/L (maximum dose level tested) for both growth rate and biomass using a test method which compensated for the absorption of light by the test material.