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Ecotoxicological information

Toxicity to aquatic algae and cyanobacteria

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Description of key information

OECD 201/EU Method C.3 (Pseudokirchneriella subcapitata): no toxicity observed even at highest concentration level tested; ErC10(72h) > 100 mg/L WAF, ErC20(72h) > 100 mg/L WAF, ErC50(72h) > 100 mg/L WAF, EbC10(72h) > 100 mg/L WAF, EbC20(72h) > 100 mg/L, EbC50(72h) > 100 mg/L, NOELr(72h): 100 mg/L WAF, NOELb(72h): 100 mg/L WAF [read-across from supporting substance ‘1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiol’]

Key value for chemical safety assessment

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

Additional information

The acute toxicity of the supporting substance ‘1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiol’ (CAS#91648-65-6) towards Pseudokirchneriella subcapitata was investigated according to OECD 201 / EU Method C.3 under certificated GLP compliance.Pseudokrichneriella subcapitata is a freshwater unicellular alga, thus a representative of primary producers found in natural waters and can therefore be considered as an important non-target organism in freshwater ecosystems. Based on the poor water solubility of the test substance (< 0.1 mg/L at 20°C), a modification of the standard method for the preparation of aqueous media was performed. Using the approach of Water Accommodated Fractions (WAFs), based upon an approach by several important regulatory authorities in the EU and elsewhere, aqueous media are prepared by mixing the test substance with water for a prolonged period. Pre-study work showed that a preparation period of 24 hours was sufficient to ensure equilibration between the test substance and the water phase. After a 1-hour settlement period, the test substance is phase is separated by siphon and the test organisms exposed to the aqeous phase or WAF. Based on the results of a range-finding test (no effects at 10 and 100 mg/L WAF), the definitive test was conducted as "limit test" with a single loading rate of 100 mg/L to confirm that no effect on algal growth was observed. Six substance concentration and control replicates were prepared. Samples were taken at 0, 24, 48 and 72 h and the cell densities were determined using a Coulter(R) Multisizer Particle Counter. The pH was measured at the start and end of the test, the temperature was recorded daily. Potassium dichromate was used as reference substance, revealing an ErC50(72h) of 1.4 mg/L with 95 % CL of 1.2 - 1.7 mg/L and an EbC50(72h) of 0.59 mg/L of 0.53 - 0.65 mg/L. The NOEC was 0.25 mg/L and the LOEC was determined as 0.50 mg/L. At the start of the experiment all cultures were observed to be clear colorless solutions. After 72 h, the cultures were recorded as pale green dispersions. In the microscopic examination, neither globules nor micro-dispersions were present. The temperature was maintained at 24 +/- 1 °C throughout the test. The pH in the control cultures increased from pH 7.1 (0 h) to 7.9 (72 h). Differences between test and control groups were determined by Student´s t-test incorporating Bartlett´s test for homogeneity of variance (Sokal and Rohlf, 1981). Concerning growth rate, the ErC10(72h), ErC20(72h) and ErC50(72h) are reported as > 100 mg/L WAF in each case. The NOELr is reported as 100 mg/L WAF. Identical results are reported regarding yield inhibition, whereat all obtained results in this study should be also applicable for the closely related substance ‘1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-dodecanethiol’.

It is considered adequate for assessment to use the afore-mentioned (reliable) study on the closely related substance ‘1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-nonanethiol’ as a stand alone study. The further two studies on the target substance‘1,3,4-Thiadiazolidine-2,5-dithione, reaction products with hydrogen peroxide and tert-dodecanethiol’ that have been provided were disregarded mainly due to their insufficient documentation.