Dichlorocyclohexylmethylsilane

Toxicity to aquatic algae and cyanobacteria

Link to relevant study record(s)

Reference

Endpoint:

toxicity to aquatic algae and cyanobacteria

Type of information:

experimental study

Adequacy of study:

key study

Study period:

1995-05-03 to 1995-10-12

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 201 (Alga, Growth Inhibition Test)

GLP compliance:

yes (incl. QA statement)

Analytical monitoring:

yes

Details on sampling:

- Concentrations: 0 (Control), 6.25, 12.5, 25, 50 and 100 mg/L.

- Sampling method: Samples were taken from the Control and each treatment (replicates R1-R3 pooled) at 0 and 72 hours

- Sample storage conditions before analysis: The samples were extracted immediately.

Vehicle:

no

Details on test solutions:

PREPARATION AND APPLICATION OF TEST SOLUTION

- Method: The test media were prepared by direct dispersion of the test substance in algal growth medium.

An amount of test substance (200 mg) was dissolved in growth medium with the aid of ultrasonic disruption and the volume adjusted to 1 L to give a 200 mg/L stock solution. Dilutions were made from the stock solution to produce 100, 50, 25 and 12.5 mg/L stock solutions. Aliquots (500 mL) of each stock solution were mixed with 500 mL of growth medium to produce the required treatments.

- Controls: Algal growth medium

Test organisms (species):

Desmodesmus subspicatus (previous name: Scenedesmus subspicatus)

Details on test organisms:

TEST ORGANISM

- Strain: CCAP 276/20

- Source (laboratory, culture collection): Culture Centre for Algae and Protozoa (CCAP), Institute of Freshwater Ecology, Ferry House, Ambleside, Cumbria.

- Method of cultivation: Cultures of algal cells were maintained in the laboratory by replenishing the culture medium approximately once per week. The culture was maintained at a temperature of 21+/-1°C under continuous illumination (approximately 7000 lux) and constant aeration.

ACCLIMATION

- Culturing media and conditions: same as test

Test type:

static

Water media type:

freshwater

Limit test:

no

Total exposure duration:

72 h

Hardness:

Not reported

Test temperature:

24+/-1°C

pH:

7.8 - 8.0 at start of test

9.8 - 10.2 at end of test

Dissolved oxygen:

Not reported

Salinity:

Not applicable

Nominal and measured concentrations:

Nominal concentrations: 0 (Control), 6.25, 12.5, 25, 50 and 100 mg/L.

Measured concentration at the start of the test ranged between 75 and 99% of nominal.

Measured concentrations at the end of the test were ≤1 mg/L in the 50 and 100 mg/L nominal concentrations and
The results are interpreted with reference to the nominal concentrations.

Details on test conditions:

TEST SYSTEM
- Test vessel: Conical flasks
- Type: loosely covered with aluminium foil to reduce evaporation.
- Material, size, fill volume: glass, 250 mL with 100 mL of test medium
- Aeration: none
- Renewal rate of test solution (frequency/flow rate): static test
- Initial cells density: 10000 cells/mL
- Control end cells density: 259000 cells/mL
- No. of vessels per concentration (replicates): 3
- No. of vessels per control (replicates): 3

GROWTH MEDIUM
- Standard medium used: yes

TEST MEDIUM / WATER PARAMETERS
- Source/preparation of dilution water: The culture medium was prepared by adding salts, macro-nutrients and trace elements to reverse osmosis purified deionised water. The medium was filter-sterilised (0.2 μm) and stored in darkness before use.
- Culture medium different from test medium: no
- Intervals of water quality measurement: start and end of test

OTHER TEST CONDITIONS
- Sterile test conditions: yes
- Adjustment of pH: no
- Photoperiod: continuous
- Light intensity and quality: approximately 7000 lux

EFFECT PARAMETERS MEASURED:
- Determination of cell concentrations: absorbance measurement at 665 nm correlated by direct counting using a haemocytometer.

TEST CONCENTRATIONS
- Justification for using less concentrations than requested by guideline: limit test
- Range finding study
- Test concentrations: 0.1, 1.0, 10 and 100 mg/L
- Results used to determine the conditions for the definitive study: Growth reduced at 100 mg/L but not in any of the other treatments.

Reference substance (positive control):

no

Duration:

24 h

Dose descriptor:

EC50

Effect conc.:

45 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Remarks:

but exposure is likely to have been to a mixture of test substance and hydrolysis products

Basis for effect:

growth rate

Remarks:

determined for the period 0-24 h

Duration:

72 h

Dose descriptor:

EC50

Effect conc.:

35 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Remarks:

but exposure is likely to have been to a mixture of test substance and hydrolysis products

Basis for effect:

other: area under the growth curve

Duration:

72 h

Dose descriptor:

NOEC

Effect conc.:

12.5 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Remarks:

but exposure is likely to have been to a mixture of test substance and hydrolysis products

Basis for effect:

other: area under the growth curve

Details on results:

- Exponential growth in the control (for algal test): yes

Reported statistics and error estimates:

EC50 values were determined by graphical interpolation of plots of log concentration versus percentage inhibition of calculated growth rate and biomass data.

The NOEC was determined by one-way analysis of variance of area under the growth curve data for the test concentrations.

Table 1. Results of analysis of test media

 

Nominal concentration (mg/L)	Measured concentration at start of test (mg/L)	Percentage of nominal	Measured concentration at end of test (mg/L)	Percentage of nominal
0 (Control)	<LOQ	-	<LOQ	-
6.25	4.71	75	<LOQ	-
12.5	10.6	85	<LOQ	-
25.0	22.3	89	<LOQ	-
50.0	45.7	91	0.395	<1
100	98.9	99	1.15	1

 

Table 2. Test results

 

Nominal concentration (mg/L)	Area under growth curve at end of test (72 h)	Percentage inhibition	Growth rate (0-24 h)	Percentage of nominal
0 (Control)	11.928	-	0.021	-
6.25	11.892	0	0.020	5
12.5	11.376	5	0.020	5
25.0	4.720	60	0.020	5
50.0	4.060	66	0.008	62
100	2.108	82	-0.002	110

It was not possible to determine the geometric mean in this study since the test substance was <LOQ at the end of the 72 hours study. It is thought that the higher temperature required for this study will have accelerated the hydrolysis half-life of the substance.

It is unclear whether a significant part of the 50% of substance seen by the GC-MS is due to poor extraction efficiencies and so a good measure of the concentration in solution. The hexane would extract the non-polar parent and not the polar hydrolysis product.

Validity criteria fulfilled:

yes

Conclusions:

A 0-24 h EC50 nominal value of 45 mg/L has been determined for the effects of the test substance on growth rate of Desmodesmus subspicatus (formerly: Scenedesmus subspicatus). A 7- h EC50 value of 35 mg/L and NOEC of 12.5 mg/L have also been determined for effects on area under the growth curve. The test substance hydrolyses in water and it is likely that the test organisms were exposed to a mixture of the parent substance and the hydrolysis products of the substance, because the study was conducted under a static regime.
It was not possible to determine geometric means in this study since the test substance was

Description of key information

Toxicity to algae: 72 hour EC₅₀ 35 mg/l and NOEC 12.5 mg/l (nominal) (OECD TG 201), read-across from an analogous/structurally related substance, cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6). Measured analysis indicated loss of parent substance therefore, the observations in this study are attributed to the exposure of test organisms to a mixture of the parent and the hydrolysis products in the test system. The results expressed in terms of the silanol hydrolysis product cyclohexyl(methyl)silanediol are EC₅₀ 30 mg/l and NOEC 11 mg/l.

Key value for chemical safety assessment

EC50 for freshwater algae:

30 mg/L

EC10 or NOEC for freshwater algae:

11 mg/L

Additional information

There are no reliable algal toxicity data available for dichloro(cyclohexyl)methylsilane (CAS 5578-42-7), therefore good quality data for an appropriate structural analogue, cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6), have been read across. Both substances share the same silanol hydrolysis product, cyclohexyl(methyl)silanediol. In the test with cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6) the observations are attributed to the exposure of test organisms to a mixture of the parent substance and the silanol hydrolysis product in the test system. There is no basis to expect that the non-silanol hydrolysis product, methanol, significantly influenced the results of the test. The toxicity of methanol is discussed further in the ecotoxicological information overview endpoint summary (additional information).

 

A 72 hour EC₅₀ value of 35 mg/l and NOEC of 12.5 mg/l (nominal) have been determined for the effects of cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6), on growth rate of Desmodesmus subspicatus. The nominal test concentrations are supported by gas chromatography analysis of the parent substance. However, it was not possible to determine geometric mean measured concentrations of the parent substance in this study because the measured concentrations at the end of the 72 hours exposure period were <LOQ. It is thought that the higher temperature required for this study and the elevated pH will have reduced the hydrolysis half-life of the substance.

In view of the test media preparation method and exposure regime it is likely that the test organisms were exposed to a mixture of the test substance and its hydrolysis products. Because the relevant assessment entity is the silanol hydrolysis product cyclohexyl(methyl)silanediol, the results may be expressed in terms of concentration of this hydrolysis product by applying a molecular weight correction: (MW of silanol = 160.29 / MW of parent = 188.34) * [CONCENTRATION OF PARENT = EC₅₀ 35 and NOEC 12.5] = EC₅₀ 30 mg/l and NOEC 11 mg/l.

The pH of the test media was acceptable at the beginning of the test (pH 7.8-8). However, it rose to between 9.8 and 10.2 towards the end of the test. Although it is difficult to prevent pH drift during static algal studies, it cannot be ruled out that the elevated pH may have affected the growth of the algae. The test results should therefore be used with some caution. However, the algal data is not used for determination of PNEC, therefore it will not affect the basis of the hazard assessment.

This read-across is considered to be a worst-case scenario because the parent test substance, cyclohexyl(dimethoxy)methylsilane (CAS 17865-32-6), hydrolyses much more slowly than the registration substance, therefore organisms will have been exposed to the parent substance during the read-across tests, but would be unlikely to be exposed to parent substance if the registration substance were tested. Due to the log K_ow of cyclohexyl(dimethoxy)methylsilane being 4.3, it is likely that the parent substance is more toxic than the silanol hydrolysis product. The read-across is therefore very conservative.