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Toxicity to aquatic algae and cyanobacteria

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Description of key information

The read-across substance L-Cystine had no acute toxic effects on Pseudokirchneriella subcapitata up to its solubility limit in the test water under the present test conditions. 

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Additional information

The read-across is based on an oxidation study of L-Cysteine in water.

It is known that aqueous solutions of L-Cysteine oxidize readily in air to give L-Cystine at neutral or basic pH (data for Biochemical Research, 3rd ed., Dawson,R. M. C., et al., Oxford University Press (New York, NY: 1986), pp. 12-13). Within this analysis it could also be demonstrated that the initial concentrations of two similar samples of the same batch taken on January 11, 2013 show L-Cysteine levels of 94.69 and 97.60 area % when dissolved in the mobile phase. L-Cystine was present at a range of 0.238 to 0.277 area percent at this starting point. Taken together it can be stated that after 4 h of stirring at room temperature the L-Cysteine has been oxidized nearly completely to L-Cystine.

L-Cystine had no statistically significant inhibitory effect on the growth of the algae during the test period of 72 hours at the loading rate of 100 mg/L (results of Student t-tests, one-sided smaller, α = 0.05). The loading rate of 100 mg/L was, therefore, determined to be the 72 -hour NOEC. This value might even be higher, but concentrations of the test item exceeding the loading rate of 100 mg/L were not tested, in accordance with the test guidelines. The 72 -hour LOEC was above the loading rate of 100 mg/L. In conclusion, the test item L-Cystine had no acute toxic effects on Pseudokirchneriella subcapitata up to its solubility limit in the test water under the present test conditions (OECD 201). At the loading rate of 100 mg/L, the algal growth (average growth rate and yield) was not inhibited (but enhanced) compared to the control after 72 hours of test duration.