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Toxicity to aquatic algae and cyanobacteria

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Description of key information

ErC50 (72 h) > 130.2 mg/L; NOEC (72 h) = 7.8 mg/L for Pseudokirchneriella subcapitata (OECD 201), recalculated values

Key value for chemical safety assessment

EC50 for freshwater algae:
130.2 mg/L
EC10 or NOEC for freshwater algae:
7.8 mg/L

Additional information

For toxicity to aquatic algae, one study with 1,3,5-triazine-2,4,6(1H,3H,5H)-trithione, trisodium salt (TMT, CAS No. 17766-26-6) and one GLP-study with 1,3,5-triazine-2,4,6(1H,3H,5H)-trithione (TMT-H3, CAS No. 638-16-4) as read across of a surrogate substance are available.

This read across approach is in accordance with Regulation (EC) No 1907/2006, Annex XI, 1.5. "Grouping of substance and read across approach" and is explained within the read across justification in IUCLID section 13.

TMT-H3 is the acidic form of 1,3,5-triazine-2,4,6(1H,3H,5H)-trithione. Except for different cations (hydrogen vs sodium), the anion of both the source and target substance have an identical structure. Since dissociation of the sodium salt under environmental conditions (pH 4-9) yields the same ionic species as the acidic form of the substance, this read across can be considered as justified.

The first study was carried out according to OECD guideline 201 with TMT 15 as test substance (93-0074-DKO). The exposure of Scenedesmus subspicatus resulted in an ErC50 (72 h) > 100 mg/L nominal TMT concentration based on the growth rate from 24 h to 72 h. The NOEC is stated as 1 mg/L nominal TMT concentration, although no reduction of growth occurred up to 5 mg/L nominal TMT concentration.

 

This study is not considered valid due to a number of deficiencies. The validity criteria of the guideline concerning the coefficients of variation for growth rates are not met. Furthermore, the EC50 was not reached within the concentration range tested and effect concentrations were derived graphically without statistical evaluation. In addition, the derivation of the NOEC value is statistically not possible due to the low number of samples and both the derivation and basis of the NOEC value remain unclear. Finally, the test was split up in two parts with the second part being carried out one week after the first.

 

Therefore a valid study with TMT-H3, which is the acidic form of TMT, is taken into account for the chemical safety assessment of TMT (2007-0308-FGO). This read across study was carried out similar to OECD guideline 201 with analytics. Pseudokirchneriella subcapitata was used as test organism and the pH of the test solutions ranged from 6.3 to 7.7 at test start and from 6.0 to 8.6 at test end. The reported ErC50 (72 h) exceeded the maximum nominal test concentration of 100 mg/L (94.9 mg/L geom. mean measured concentration) and the NOEC was determined to be nominal 7.3 mg/L (5.7 mg/L geom. mean measured concentration).

 

As afore mentioned, with respect to the study with TMT itself being invalid, the result for TMT-H3 (ErC50 (72 h) > 94.9 mg/L, NOEC (72 h) = 5.7 mg/L) as a part of a read across approach are taken into account for further assessment. For this purpose the ErC50 (72 h) and NOEC (72 h) values were recalculated to be > 130.2 mg TMT/L and 7.8 mg TMT/L, respectively