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Toxicity to aquatic algae and cyanobacteria

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toxicity to aquatic algae and cyanobacteria
Type of information:
experimental study
Adequacy of study:
key study
Study period:
2015-11-30 to 2016-22-01
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
according to guideline
OECD Guideline 201 (Alga, Growth Inhibition Test)
GLP compliance:
Analytical monitoring:
Details on sampling:
- Concentrations: The concentration of the test item in each sample for GC analysis was determined on the basis of a comparison of the peak area on the chromatogram of the sample solution with that of a standard solution.
- Sampling method: The calibration curve was drawn by using four standard solutions of 1.01, 5.03, 10.1 and 20.1 mg/L prepared according the following way: the test sample (60µL) was taken out by microsyringe and dissolved in hexane to obtain 1010 mg/L solution of the test item, then this solution was diluted to prepare 10.1 mg/L standard solution.
not specified
Test organisms (species):
Raphidocelis subcapitata (previous names: Pseudokirchneriella subcapitata, Selenastrum capricornutum)
Details on test organisms:
- Species/Strain number: Pseudokirchneriella subcapitata, ATCC 22662
- Source: American Type Culture Collection (supplied on June 30, 1995)
- Initial cell concentration (at test initiation): 0.5×10⁴ cells/mL
- Method of cultivation: Incubation with rotary shaking, closed system

- Acclimation conditions and period (same as test or not): Same as test, 3 days
- Any deformed or abnormal cells observed: no

Test type:
not specified
Water media type:
Limit test:
Total exposure duration:
72 h
Not specified
Test temperature:
22.5°C - 22.9°C
Nominal and measured concentrations:
Nominal concentrations: 6.25, 12.5, 25.0, 50.0, 100 mg/L (a geometric series with a factor of 2.0) and a control

Measured concentrations of test item at start: 5.52-95.6 mg/L (88.4-96.4%) of nominal concentrations

Measured concentrations of test item at end: 5.35-90.7 mg/L (85.6-90.7%) of nominal concentrations

The measured concentrations of test item were kept within +/- 20% of the nominal concentration.
Details on test conditions:

- Test vessel: Sterilized 500 mL Erlenmeyer flask (closed vessel) containing 50 mL of the test solution
- Type (delete if not applicable): Incubation with rotary shaking and artificial illumination, closed system
- Aeration: No
- Initial cells density: 0.5×10⁴ cells/mL
- Control end cells density: 42×10⁴ cells/mL
- No. of vessels per concentration (replicates): 3
- No. of vessels per control (replicates): 6

- Standard medium used: yes


- Source/preparation of dilution water: 300 mL of test solution per test level (600 mL per control level), 100 mL per test vessel.
- Purified water


- Sterile test conditions: yes
- Adjustment of pH: No
- Photoperiod: Continuous illumination provided by fluorescent lights with wavelength range of 400-700 nm
- Light intensity and quality: 89-93 µmol.m-². s-¹

EFFECT PARAMETERS MEASURED (with observation intervals if applicable) :
- Determination of cell concentrations: Cell growth (biomass) on the test organisms were observed every 24 hours after the start of exposure with a particle counter instrument, Model COULTER Z1.
- Observations of cell condition was done for one vessel in each test level at the end of exposure.
- Appearance of test solution was observed at the start and end of exposure.

- Spacing factor for test concentrations: 2
- Range finding study: Yes with two replicates
- Test concentrations for the range-finding study: 0.100, 1.00, 10.0, 100 mg/L (nominal concentrations)
- Results used to determine the conditions for the definitive study: 50% growth inhibition rate at 100 mg/L

Reference substance (positive control):
Potassium dichromate
Key result
72 h
Dose descriptor:
Effect conc.:
84 mg/L
Nominal / measured:
Conc. based on:
test mat.
Basis for effect:
growth rate
Key result
72 h
Dose descriptor:
Effect conc.:
12.5 mg/L
Nominal / measured:
Conc. based on:
test mat.
Basis for effect:
growth rate

Table 1: Measured concentrations of test item in test solutions in mg/L and (Percentage of measured concentration versus nominal concentration %)

Nominal concentration (mg/L)

At the start

At the end

Geometric mean






5.52 (88.4%)

5.35 (85.6%)

5.44 (87.0%)


11.3 (90.0%)

11.0 (88.0%)

11.1 (89.0%)


22.2 (88.9%)

21.4 (85.6%)

21.8 (87.2%)


48.2 (96.4%)

44.7 (89.4%)

46.4 (92.8%)


95.6 (95.6%)

90.7 (90.7%)

93.1 (93.1%)

n.d.: <0.971 mg/L

Validity criteria fulfilled:
A 72-hour EC50 value of 84.0 mg/L has been determined for the effects of the test substance on Algae (Pseudokirchneriella subcapitata) growth rate. The NOEC value based on growth rate was 12.5 mg/L

Description of key information

Toxicity to aquatic algae: 72-h ErC50 84.0 mg/l and NOEC 12.5 mg/l (nominal, tert-butyl(dimethyl)silanol), growth rate of  Pseudokirchneriella subcapitata.

Key value for chemical safety assessment

EC50 for freshwater algae:
84 mg/L
EC10 or NOEC for freshwater algae:
12.5 mg/L

Additional information

No reliable studies for toxicity to aquatic algae and cyanobacteria are available for tert-butyl(chloro)dimethylsilane.

The registration substance, tert-butyl(chloro)dimethylsilane (CAS 18162-48-6) hydrolyses very rapidly in contact with water to form tert-butyl(dimethyl)silanol (CAS 18173-64-3) and hydrochloric acid.

As per the REACH guidance (ECHA 2016, R.16) which states that "for substances where hydrolytic DT50 is less than 12 hours, environmental effects are likely to be attributed to the hydrolysis products rather than to the parent substance itself", the aquatic toxicity assessment including aquatic algae, is based on the properties of the silanol hydrolysis product tert-butyl(dimethyl)silanol (CAS 18173-64-3).

Toxicity to aquatic algae tests are available with a structural analogue of the parent substance and the silanol hydrolysis product of the registration substance. Data with the silanol hydrolysis product are used as key since the tests with the structural analogue, butyl(chloro)dimethylsilane (CAS 1000-50-6), may have been affected by acidification of the test media.

A 72-h ErC50 value of 84.0 mg/l and NOEC of 12.5 mg/l (nominal) have been determined for the effects of tert-butyldimethylsilanol (CAS 18173-64-3) on growth rate of  Pseudokirchneriella subcapitata, in accordance with OECD Test Guideline 201 (CERI-Kurume, 2016). The nominal test concentrations are supported by GC analysis. The measured concentrations ranged between 85.6 -96.4% of the nominal.

A study of toxicity to algae is available for the analogous substance butyl(chloro)dimethylsilane (CAS 1000-50-6). A 72-h EC50 value of 10 mg/l an ErC10 of 3.8 mg/l (nominal) have been determined for the effects of tert-butyl(dimethyl)silanol (CAS 18173-64-3) on growth rate of  Desmodesmus subspicatus, in accordance with Test Guideline EU method C.3 (Huls, 1996). It is likely that the test organisms were exposed to the hydrolysis products.

The results may be therefore be expressed in terms of the silanol hydrolysis product, by adjusting for the respective molecular weights: MW butyl(dimethyl)silanol/MW butyl(chloro)dimethylsilane * EC50, EC10 or NOEC value, thus 132.28/150.73 * ErC50 of 10 mg/l, ErC10 of 3.8 mg/l and NOEC of 1.9 mg/l = ErC50 8.8 mg/l, ErC10 = 3.3 mg/l and NOEC 1.7 mg/l as butyl(dimethyl)silanol.


During this test carried out with aquatic algae exposure will have been to the hydrolysis products, butyl(dimethyl)silanol and hydrochloric acid. In this algae test, the test media was not buffered and acidification of the test media occurred with the pH dropping as low as pH 2.2. Effects on algal growth rate are likely to be a result of pH due to the presence of hydrogen chloride. This test may not, therefore, show the toxicity of the silanol hydrolysis product, but the toxicity of hydrogen chloride. However, a toxicity to aquatic algae QSAR prediction EC50 result of 24 mg/l for butyl(dimethyl)silanol (the hydrolysis product of the registered substance) supports the algal toxicity study result carried out with butyl(chloro)dimethylsilane. It is possible that conducting a new study and adjusting the pH to meet current guidelines would result in less toxicity being observed, therefore the results of the study should be treated with caution. However, the study is considered to be valid as supporting data for the purposes of this assessment on the basis that this is a worst case scenario approach.


Effects of hydrogen chloride on aquatic organisms are limited to those that result from changes to pH in unbuffered media and are discussed in the endpoint summary: ecotoxicological discussion, consideration of the non-silanol hydrolysis product.