2,3-bis((2-mercaptoethyl)thio)-1-propanethiol

Long-term toxicity to aquatic invertebrates

Description of key information

long-term Daphnia EC10=0.059 mg/L based on parental mortality

Key value for chemical safety assessment

Fresh water invertebrates

Fresh water invertebrates

Effect concentration:

0.059 mg/L

Additional information

In the study from Burlingham (2011) the effect of DMPT (Name in report GST; chemical name: 4-mercaptomethyl-3,6-dithia-1,8-octanedithiol, purity 93.9%) on the reproduction of Daphnia magna was assessed under semi-static exposure conditions (daily renewal of the test solutions) over a period of 21 days using the methods detailed in the OECD Guideline for Testing of Chemicals, No. 211 ¿Daphnia magna, reproduction test¿ (1998).

 

Due to the hydrolytic nature of the test substance, aspects of the study were designed following guidance outlined in OECD ¿Guidance Document on Aquatic Toxicity Testing of Difficult Substances and Mixtures¿ Number 23 (ENV/JM/MONO(2000)6), to ensure the test organism was exposed to both parent and degradation products.

 

Test set up and analytical measurements:

Groups of ten individually-housed Daphnia were exposed for 21 days to the test substance prepared at nominal concentrations of 0.001, 0.0032, 0.01, 0.032 and 0.1 mg GST/L. The test media were prepared in Elendt M4 medium using a series of acetone stock solutions. The test media were renewed daily during the definitive test to ensure the Daphnia were exposed to maintained concentrations of the parent substance. As an auxiliary solvent was used to prepare the test media, a solvent control group (0.1 mL acetone/L) comprising twenty Daphnia and a diluent (Elendt M4) control group, comprising ten Daphnia, were employed in the study.

 

The test was initiated with first instar Daphnia that were less than 24-hours old. Daily records were maintained for mortality, floating and physical appearance, number of gravid animals, live and dead neonates, and the presence of aborted eggs and moulted carapaces. Daphnia were fed each day with a suspension of the unicellular green algae Pseudokirchneriella subcapitata.

 

It was not possible to develop an analytical method with sufficient sensitivity to detect the required levels of GST in Elendt M4 medium. Therefore, an analytical method was developed to measure the levels of GST in solvent stock solutions. The measured levels of GST in samples of the solvent stock solutions ranged between 85 and 109% of their nominal values. Using the levels measured in the stock solutions and taking into account the dilution factor for preparation of the test media (1:10,000), the levels of GST in the aqueous media were calculated to be 0.000893, 0.00317, 0.00930, 0.0321 and0.101 mg/L.

 

Findings:

The NOEC, EC10 and EC20 for parental mortality after 21 days were calculated as 0.0321, 0.0590 and 0.0699 mg/L respectively. No significant effects on either the physical appearance or mobility of the surviving adult Daphnia were observed.

 

The NOEC for parental growth after 21 days was calculated to be >= 0.101 mg/L. Statistical analysis of the lengths of surviving adults after 21 days of exposure to GST (=DMPT) indicated no adverse effects on growth.

 

The NOEC, EC10 and EC20 for reproduction within 21 days were calculated as 0.0321, 0.0696 and 0.0966 mg/L respectively. Statistical analysis of the time taken to produce the first brood of neonates indicated no significant effect. Statistical analysis of the numbers of dead neonates or aborted eggs produced by each surviving adult in the test groups compared to the solvent control group indicated that neonate survival was adversely affected at concentrations of 0.0321 and 0.101 mg/L.

 

Conclusion for the risk assessment:

The parental mortality was the most sensitive endpoint with an EC10 of 0.059 mg/L. This value will be used for the risk assessment. Due to technical problems related to the low concentrations no analytical measurements were performed for the test solutions. Instead, the stock solutions were analysed and provide evidence that the test solutions were correctly dosed. The study was designed to expose the test organisms to both parent and degradate products since the test substance hydrolyses quickly (halflifetime at 50°C < 3 hours). Since this is considered to happen also in nature (if exposure should occure), no additional factor was considered for the risk assessment. Based on the good documentation of procedures, the study is still considered to be a Klimisch 1 score study and is therefore considered as fully reliable and relevant for the risk assessment.