Sodium mercaptoacetate

Toxicity to aquatic algae and cyanobacteria

Link to relevant study record(s)

Reference

Endpoint:

toxicity to aquatic algae and cyanobacteria

Type of information:

read-across based on grouping of substances (category approach)

Adequacy of study:

key study

Justification for type of information:

REPORTING FORMAT FOR THE CATEGORY APPROACH

1. HYPOTHESIS FOR THE CATEGORY APPROACH
This scenario covers the category approach for which the hypothesis is based on transformation to a common compound. For the REACH information requirement under consideration, the effects obtained in studies conducted with different source substances are used to predict the effects that would be observed in a study with the target substance if it were to be conducted. The same type of effect is observed for the different source substances; this may include absence of effects for every member of the category. No relevant differences in strengths of effect are observed for several source substances.
There are no relevant variations in properties among source substances and the same potency is predicted for all target substances. This corresponds to Scenario 5 of the RAAF (ECHA, 2017). The substances ATG, MEATG, KTG, CaTG, and NaTG are different inorganic salts of a common acid, thioglycolic acid (TGA; synonym: 2-mercaptoacetic acid). They dissociate rapidly in aqueous media to the common thioglycolate anion and to their different counter ions. The water solubility of all category members is high.
This approach serves to use existing data on aquatic toxicity endpoints for substances in this category.
It can be predicted with high confidence that the target substances will display the same mode of action and lead to the same type and strength of effects as observed with the source substances.


2. CATEGORY APPROACH JUSTIFICATION
For details, refer to Justification for read-across attached to Iuclid section 13

Reason / purpose for cross-reference:

read-across: supporting information

Key result

Duration:

72 h

Dose descriptor:

EC50

Effect conc.:

5.07 mg/L

Nominal / measured:

meas. (geom. mean)

Conc. based on:

other: read-across, molecular weigth correction

Basis for effect:

growth rate

Key result

Duration:

72 h

Dose descriptor:

NOEC

Effect conc.:

0.54 mg/L

Nominal / measured:

meas. (geom. mean)

Conc. based on:

other: read-across, molecular weigth correction

Basis for effect:

growth rate

Conclusions:

The results of studies conducted with TGA, MeaTG, and KTG were combined and the geometic mean was calculated. After correction for molecular weight this resulted in a 72 h ErC50 = 5.07 mg/L, 72 h NOECr = 0.54 mg/L.

Description of key information

The overall 72 h EC50 and NOEC for NaTG are 5.07 mg/L and 0.54 mg/L, respectively. (OECD TG 201, Pseudokirchneriella subcapitata, RL1, GLP; read-across from TGA, MeaTG, and KTG; molecular weight correction)

Key value for chemical safety assessment

EC50 for freshwater algae:

5.07 mg/L

EC10 or NOEC for freshwater algae:

0.54 mg/L

Additional information

Experimental data on toxicity to algae are available for TGA, KTG and MeaTG.

 

TGA

The determination of the growth inhibition of the freshwater algae Pseudokirchneriella subcapitata exposed to the test item Thioglycolic acid for a duration of 72 hours was performed according to OECD Guideline 201.

Algae were exposed to the test item at nominal concentrations of 0 (control), 4.0, 7.7, 14.6, 27.2, 52.6 and 100 mg/L over a time period of 72 hours.

The study was performed using 100 ml glass erlenmeyer flasks stoppered with cotton wool wrapped in sterilised, containing 50 ml of test solution inoculated with an algal suspension so that the initial cell concentration was equal to 1E04 cells/ml. Tests flasks were incubated at 23 ± 1 °C continuously shaken and constantly illuminated. The  algal concentration was measured daily. Analytical chemistry and physico-chemical measurements were carried out at the beginning and the end of the test.

The appearance of the test solutions was visually checked at the beginning and at the end of the test. Solutions were found to be clear, colourless over the period of the test. No precipitation was observed at the end of the test. Microscopic observation confirmed that the algae appeared normal at the end of the test excepted at the highest concentration where algae appeared a little smaller in comparison to the control.

During the test, the control pH varied by 0.23 units. The quality criterion related to the algal growth was respected since the increase in cell density (R), measured in the control solution between the end and the beginning of the test, was greater than a factor of 16 (R = 88). The validity criterion specific to C3 92/69EEC method and related to the test item stability during the test was not respected: the final concentrations of the test were not maintained within the designated limit of 80 % of the initial concentrations. Therefore, effect levels are given as geom. mean measured concentrations.

The 72 h ErC50 was 27 mg/L, the 72 h ErC10 was 12 mg/L, the 72 h NOECr was 15.2 mg/L.

The 72 h EbC50 was 13 mg/L, the 72 h EbC10 was 4.8 mg/L, the 72 h NOECb was 7.6 mg/L.

 

Supporting data are available for the oxidation product Diammonium dithioglycolate. Diammonium dithioglycolate is the main degradation product of thioglycolic acid when released in the environment occurring few hours after the release (Devaux, 2003 - Smolin and Fölsing, 2009).

 

Diammonium dithioglycolate toxicity to Scenedesmus subspicatus has been investigated through a test in accordance with OECD Guideline 201 and GLP requirements.

Scenedesmus subspicatus were exposed for 72 hours to an aqueous solution of Diammonium Dithiodiglycolate at a nominal concentration of 100 mg/L.

EC 50 - 72h determined was higher than 100 mg/L according to either growth rate and biomass and NOEC was of 100 mg/L.

 

KTG

The purpose of this test was to determine the inhibitory effect of KTG on the growth of the freshwater green algae Pseudokirchneriella subcapitata. Exponentially growing cultures of this unicellular green algal species were exposed to nominal concentrations of 10, 32, 1.0, 0.3 and 0.1 mg test item/L (spacing factor 3.16) and a control (corresponding to following geometric mean measured concentrations of the test item: 8540, 2480, 81.5, 7.70 and 0.0143 µg test item/L and a control).  The inhibition of growth in relation to control cultures was determined over a test period of 72 hours, and thus over several algal generations. Th study was conducted in accordance with OECD Guideline 201.

Cell density in the control: Achieved: 48.3-fold increase (criterion: ≥ 16) within 72 hours;

Coefficient of Variation (CV) of sectional (daily) growth rate of the control: Achieved: 11.5 % (criterion: ≤ 35%);

CV of average growth of control replicates: Achieved: 6.4 % (criterion: ≤ 7%);

and thus, the validity criteria were met.

The 72-hour EyC50 was calculated to be 26.4 µg test item/L, and the 72-hour ErC50 value was calculated to be 1582 µg test item/L. The 72-hour NOEyC was determined to be 0.0143 µg test item/L and the associated 72-hour LOEyC was 7.70 µg test item/L. The 72-hour NOErC was determined to be 7.70 µg test item/L and the associated 72-hour LOErC was 81.5 µg test item/L. All reported results refer to geometric mean concentrations, since the test item concentrations were not within ± 20 % of the nominal initial concentrations during the test.

 

MeaTG

A study according to OECD TG 201 was performed to assess the effect of the test material on the growth of the green alga Scenedesmus subspicatus. Following a preliminary range-finding test, Scenedesmus subspicatus was exposed to an aqueous solution of the test material at concentrations of 6.25, 12.5, 25, 50 and 100 mg active ingredient (ai)/l (three replicate flasks per concentration) for 72 hours, under constant illumination and shaking at a temperature of 24 ± 1°C.
Samples of the algal populations were removed daily and cell concentrations determined for each control and treatment group, using a Coulter® Multisizer Particle Counter.

Exposure of Scenedesmus subspicatus to the test material based on nominal test concentrations gave an EbC50 (72 h) value of 39 mg ai/l; 95% confidence limits 34 - 45 mg ai/l and an ErC50 (0 - 72 h) value of 65 mg ai/l; 95% confidence limits 58 - 74 mg ai/l. The NOEC was 12.5 mg ai/l.  The EbC50 (72 h) based on the geometric mean measured test concentrations was 2.6 mg ai/l; 95% confidence limits 2.4 – 2.8 mg ai/l and the ErC50 (0 - 72 h) was 3.2 mg ai/l; 95% confidence limits 3.0 – 3.5 mg ai/l. The NOEC was 1.45 mg ai/l.

 

Conclusion

There were differences in effect levels in the studies conducted with TGA, MeaTG and KTG. Since no obvious cause could be identified, all three results are combined in a geometric mean value after adjustment to thioglycolate.

The overall EC50 and NOEC for NaTG are 5.07 mg/L and 0.54 mg/L, respectively.