2,2'-dithiobisethanol

Toxicity to aquatic algae and cyanobacteria

Description of key information

The test item reduced the growth rate of the Pseudokirchneriella subcapitata significantly at a nominal concentration of 18 mg/L. The NOEC for growth inhibition was set at 10 mg/L based on statistical significance but <10% effects observed at concentrations up to 32 mg/L, so the NOEC was set at 32 mg/L based on biological significance. Growth rate inhibition at the end of the exposure increased with the applied dose but the highest concentration tested growth was inhibited by 38% so the EC50 was set as >100 mg/L.

Key value for chemical safety assessment

EC50 for freshwater algae:

100 mg/L

EC10 or NOEC for freshwater algae:

32 mg/L

Additional information

Charles River Laboratories (2017) is a GLP-compliant study following OECD guideline 201 and is therefore reliable without restrictions (Klimisch 1). A range finding test was conducted with the concentrations of 0.10, 1.0, 10 and 100 mg/L. The expected EC50 for growth rate inhibition exceeded the nominal concentration of 100 mg/L. For the definitive study, 3 replicates of algae were exposed to the nominal concentrations of 10, 18, 32, 56 and 100 mg/L.

The validity criteria for the study was fulfilled as the cell density of the control increased exponentially by a factor of at least 16 within the exposure period of 72 hours. The mean coefficient of variation for section-by-section specific growth rates in the control cultures did not exceed 35% and the coefficient of variation of average specific growth rates in the replicate control cultures did not exceed 7%.

A reference test with Potassium dichromate was conducted to check the sensitivity of the test system. The EC50 for growth rate inhibition was determined to be 1.1 mg/L, which is within the range of historical data.

The analytical method UPLC-RID was used to measure test concentrations at 0 hrs, 24 hrs and 72 hrs. The initially measured concentrations were at the level of 77% to 96% nominal. Measured concentrations remained stable during the exposure (97-107% initially measured at the end of the exposure). The concentration measured in nominally 10 mg/L was below the lowest calibration standard therefore could not be determined. The concentrations measured in 32, 56 and 100 mg/L were at the level of nominal for the entire exposure period (84-98%). The concentration measured in 18 mg/L was at the level of 75-81% of nominal during the exposure, which was similar to the 15 mg/L procedural recovery samples (71 -81%) and confirms that the test system was stable throughout the 72-hr exposure study.

In conclusion, based on biological relevance the 72h-NOEC for growth rate inhibition was 32 mg/L and the EC50 was >100 mg/L, the highest concentration tested.