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Toxicity to aquatic algae and cyanobacteria

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Description of key information

The analysis of the growth rate of the algal population within the 72 h exposure period showed an ErC50 of >100 mg/L, a NOEC[r] of 3.4 mg/L and a LOEC[r] of 20.4 mg/L for the test substance.

Key value for chemical safety assessment

EC50 for freshwater algae:
100 mg/L
EC10 or NOEC for freshwater algae:
3.4 mg/L

Additional information

The reported ErC50 value was >100 mg/L.

In order to test acute toxicity to algae (Desmodesmus subspicatus) of the substance was exposed to the test solution of nine nominal concentrations of the test substance (0.01, 0.032, 0.1, 0.32, 1.0, 3.2, 10, 32 and 100 mg/L) and blank control solution for a period of 72 hours under static conditions. The applied method was the EU Method C.3 (Algal Inhibition test). The cell densities were measured at 24 hour intervals. Inhibition of the algal population was measured as reduction in growth rate (index r), relative to control cultures grown under identical conditions. The substance is dark coloured and reduce the light emission for the algae cells because the shading effects. Enough light is essential for the growth of the algae, so for a reduction of shading effects, the test was carried out with strong illumination (110 -140µE x m-2 x s-1) and with a reduced test volume of 25 mL in 300 mL Erlenmeyer flasks.

The measured concentrations confirmed that deviation from the nominal concentrations were higher than 30 % ( Effective concentrations ranged from 74.9 % to 90.8 % of nominal values at 0 hours, from 36.3 % to 62.7 % at 10 mg/L and 32 mg/L of nominal values at 24 hours, and from to 26.8 % to 55.4 % at 10 mg/L and 32 mg/L of nominal values at 48 hours and from 18.5 % to 84.7 % of nominal values at 72 hours. After 72 hours the test item concentrations 1.0 mg/L and 3.2 mg/L concentrations were below the quantification limit (0.392 mg/L)).

The analysis of the growth rate of the algal population within the 72 h exposure period showed an ErC50 at 89.8 mg/L, a NOEC[r] of 3.4 mg/L and a LOEC[r] at 20.4 mg/L. NOEC/LOEC (72 h) determinations for growth rate could not be performed due to mathematical reasons. In a regulatory context EC 10 values are generally regarded as surrogates for the NOEC. In the present case, 10 % effect on growth rate is effectuated by a concentration > 3.431 mg/L and < 20.412 mg/L and therefore NOEC and LOEC were read directly from the test results (NOEC = 3.431 mg/L and LOEC = 20.412 mg/L).

The results are expressed in terms of geometric mean measured concentrations.

The substance has not a definite or unique structure and consists of several components. No information about the correlation between molecular weight and the structural formula of the test item are available.

Investigations on stability in aqueous solutions shows that the substance is photoinstable, (especially at concentrations < 100 mg/L) so the strong illumination results in a rapid degradation. Furthermore, if the strong illumination for the reduction of the shading effect facilitates the degradation of the substance, it is technically not possible to test just the chemical toxicity of the substance on algae growth according to OECD 201. So it is conceivable that the result of this alga, growth inhibition test is not due to the chemical toxic effect of the substance but is rather caused by shading effects. Therefore the results should be handled with care.

In an alternative approach to evaluate the EC50 of growth rate without the NOEC/LOEC calculation, only the highest concentration have to be considered. Exponentially growing algal cells were exposed for a period of 72 hours to a concentration of nominally 100 mg/l of the substance dissolved in dilution water. The measured concentrations ranged from 90.79% of the nominal value of 100 mg/l at 0 hours to 84.68% of the nominal value at 72 hours. The guideline 201 postulates, if there is evidence that the concentration of the substance being tested has been satisfactorily maintained within +/- 20% of the nominal concentration throughout the test, analysis of the results can be based on nominal values. According to the analytically results an evaluation based on the nominal concentration for the concentration of 100 mg/l substance is satisfactorily because the exposure concentrations vary less than 20% from the nominal values during the test.With this alternative approach these values result in an EC50 value higher than 100 mg/L: 72h-ErC50 >100 mg/L.