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Toxicity to aquatic algae and cyanobacteria

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Description of key information

The 72-h ErC50 and 72-h NOErC values are 0.22 mg/L and 0.074 mg/L, respectively in freshwater algae (P. subcapitata).

Key value for chemical safety assessment

Additional information

No data on aquatic toxicity is available for sodium 2 -mercaptoethanolate (target chemical). Read across can however be made to 2- mecaptoethanol (CAS 60 -24 -2). The (sodium) 2-mercapthoethanolate ion is a conjugated base, the read across substance 2-mercaptoethanol its undissociated acid. With a pKa value of ca. 9.5 for both substances, in an aqueous environment with pH 7, they will be present almost exclusively as 2-mercaptoethanol.Further, no effects on aqueous toxicity from the sodium cation is to be expected.

Reliable data on the toxicity of 2 -mercaptoethanol to aquatic algae is available from two different sources.

The first is a report of the Japanese authorities (National Institute of Technology and Evaluation (NITE; former MITI)) of 2010. Although not many details on study design are available, all tests from the Japanese Authorities are performed according to OECD guidelines (including analytics) and GLP and are generally considered to be reliable. In this study the toxicity of 2- mercaptoethanol to freshwater algae is examined in a 72 -h static test according to OECD 201. Although the used test organisms is not specified, normally in algal toxicity studies performed by the Japanese Authorities the freshwater green alga S. capricornutum is used. The 72 -h ErC50 and NOErC are reported to be 0.17 mg/L and 0.058 mg/L, respectively. After correction for molecular weight difference (for read-across) the 72 -h ErC50 and NOErC are 0.22 and 0.074 mg/L, respectively.

In the second study the toxicity towards freshwater algae was determined in a study according to OECD guideline No 201 and in compliance with GLP criteria (BASF, 2004). In this study, exponentially growing freshwater alga (D. subspicatus) were exposed to nominal test substance concentrations of 0 (control), 0.412, 1.23, 3.70, 11.1, 33.3 and 100 mg/L for 72 hours under static conditions. The test was performed in 4 replicates per test concentration. Test concentrations were analytically verified and mean effective concentrations were n.d., 0.19, 0.58, 1.74, 5.22, 15.65 and 47.0 mg/L (based on a mean effective test substance recovery of 47%). At the start of the test and after 24, 48 and 72 hours exposure duration, the algal cell densities were determined and based thereon the inhibitory effects on biomass and growth rate calculated. No effects were seen up to a test substance concentration of 1.74 mg/L. At a concentration of 5.22 mg/L a statistically significant inhibition was first reported for both biomass (39.3% inhibition) and growth rate (15.2% inhibition). The effects increased concentration-dependently up to 92.5 % inhibition of biomass and 76.1% inhibition of growth rate at the highest test concentration of 47.0 mg/L. Based on these findings the 72 -h ErC50 (growth rate) and EbC50 (biomass) are determined at 19.0 mg/L and 7.0 mg/L, respectively. The 72 -h ErC10 is calculated to be 3.4 mg/L, the 72 -h EbC10 at 2.2 mg/L. After correction for molecular weight difference (for read-across) the 72 -h ErC50 and ErC10 are 24.3 and 4.4 mg/L, respectively.

The NITE study (2010) is considered as key study as it provides the lowest effect concentration for aquatic invertebrates.