Tetraethyl orthosilicate

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:

2006-08-15 to 2006-08-18

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

Analytical monitoring:

yes

Details on sampling:

Analytical monitoring:  A single sample was removed from each test solution and the control at test initiation and test termination (72 hours) and was extracted and analyzed by GC/FID for ethyl silicate concentration.  Samples analyzed on day 0 were removed from the test solutions in the volumetric flasks prior to filling the individual test flasks.  Samples analyzed at 72 hours of exposure were removed from the composite of replicate vessels for each treatment and the control.  

Vehicle:

no

Details on test solutions:

PREPARATION AND APPLICATION OF TEST SOLUTION

- Method: A 100 mg a.i./L (mg active ingredient/L) stock solution was prepared by placing 0.2025 g of ethyl silicate (0.2005 g as active ingredient) in a 2000-mL volumetric flask and bringing it to volume with AAP medium.  The resulting stock solution was observed to be clear and colourless with no visible undissolved test substance.  Each test concentration was prepared by adding the appropriate amount of the 100 mg a.i./L stock solution to an intermediate vessel and bringing it to a final volume of 1000 mL with dilution water.

Test organisms (species):

Raphidocelis subcapitata (previous names: Pseudokirchneriella subcapitata, Selenastrum capricornutum)

Details on test organisms:

TEST ORGANISM

- Strain: Pseudokirchneriella subcapitata, formerly Selenastrum capricornutum, strain 1648, Class Chlorophyceae.  

- Source: The alga was obtained from the University of Texas, Austin, Texas, and was maintained in stock culture at Springborn Smithers.  

- Culture conditions: The stock cultures were maintained within the following conditions:  a shaking rate of 100 ± 10 rpm, a temperature of 23 ± 1ºC and continuous illumination at the surface of the medium with an intensity of 7000 to 9100 lux (650 to 800 footcandles).  Lighting was supplied by fluorescent bulbs.  Culture flasks were agitated continuously on an orbital shaker.

Test type:

static

Water media type:

freshwater

Limit test:

no

Total exposure duration:

72 h

Hardness:

Not reported

Test temperature:

23-24ºC

pH:

The pH of the test and control solutions ranged from 6.7 to 6.8 at test initiation.  At 72 hours of exposure, the pH of the test and control solutions ranged from 7.1 to 8.5. 

Dissolved oxygen:

Not reported

Salinity:

Not applicable

Nominal and measured concentrations:

Nominal concentrations: 0(Control), 2.6, 6.4, 16, 40 and 100 mg a.i./L 

Measured (geometric mean) concentrations: 0(Control), 1.8, 0.92, 3.6, 8.6 and 22 mg a.i./L

Details on test conditions:

- Growth/test medium:  The culture medium used was Algal Assay Procedure (AAP) medium prepared with sterile, deionized water.  AAP medium used to prepare the exposure solutions was formulated in the same manner as the culture medium.

- Exposure vessel type: The test was conducted in sterile 250-mL Erlenmeyer flasks containing 100 mL of test solution.  All test vessels were fitted with stainless steel caps which permitted gas exchange.  

- Water chemistry in test:  TOC concentration of the AAP sample collected in August 2006 was 0.51 mg/L. 

- Conductivity of the exposure and control solutions measured at test initiation and termination ranged from 80 to 90 µmhos/cm.  

- Light levels and quality during exposure:  7000 to 9100 lux (650 to 850 footcandles).  The photosynthetically-active radiation (PAR) of the test area measured at test initiation ranged from 90 to 140 µE/m2/s.

- Test Design:  One hundred millilitres of the appropriate exposure solution was placed in each replicate flask.  A 0.23-mL inoculum of Pseudokirchneriella subcapitata cells, at a density of approximately 445 x 10E4 cells/mL, was aseptically introduced into each flask.  This inoculum provided the required initial (0-hour) cell density of approximately 1.0 x 10E4 cells/mL.  Three replicate test vessels were established for the treatment levels and six replicates were established for the algal media control.

- Method of calculating mean measured concentrations (i.e. arithmetic mean, geometric mean, etc.): Geometric mean

Reference substance (positive control):

no

Duration:

72 h

Dose descriptor:

NOEC

Effect conc.:

>= 100 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

other: growth rate and biomass

Duration:

72 h

Dose descriptor:

EC50

Effect conc.:

> 100 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

other: growth rate and biomass

Duration:

72 h

Dose descriptor:

NOEC

Effect conc.:

>= 22 mg/L

Nominal / measured:

meas. (geom. mean)

Conc. based on:

test mat.

Basis for effect:

other: growth rate and biomass

Duration:

72 h

Dose descriptor:

EC50

Effect conc.:

> 22 mg/L

Nominal / measured:

meas. (geom. mean)

Conc. based on:

test mat.

Basis for effect:

other: growth rate and biomass

Duration:

72 h

Dose descriptor:

EC50

Effect conc.:

> 5.8 mg/L

Conc. based on:

test mat.

Remarks:

measured final

Basis for effect:

other: growth rate and biomass

Duration:

72 h

Dose descriptor:

NOEC

Effect conc.:

>= 5.8 mg/L

Conc. based on:

test mat.

Remarks:

measured final

Basis for effect:

other: growth rate and biomass

Duration:

72 h

Dose descriptor:

EC50

Effect conc.:

> 63 mg/L

Nominal / measured:

nominal

Conc. based on:

other: silanol hydrolysis product

Basis for effect:

other: growth rate and biomass

Duration:

72 h

Dose descriptor:

NOEC

Effect conc.:

>= 63 mg/L

Nominal / measured:

nominal

Conc. based on:

other: silanol hydrolysis product

Basis for effect:

other: growth rate and biomass

Details on results:

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

Reported statistics and error estimates:

Statistical methods: The data were first checked for normality using Shapiro-Wilks' Test (Weber et al., 1989) and for homogeneity of variance using Bartlett's Test (Horning and Weber, 1985).  If the data sets passed the tests for homogeneity and normality, then Williams' Test (Williams, 1971, 1972) was used to determine the NOEC.  If the data did not pass the tests for homogeneity and normality, then Kruskal-Wallis' Test (Sokal and Rohlf, 1981) was used to determine the NOEC.  All statistical determinations were made at the 95% level of certainty, except in the case of Shapiro-Wilks' and Bartlett's Tests, where the 99% level of certainty was applied.  TOXSTAT® version 3.5  (Gulley et al., 1996) was used to perform these calculations.

Table 1. Test results

Nominal test substance concentration (mg/L)	Geometric mean measured concentration (mg/L)	Mean measured cell concentration at start of test (cells/mL)	Mean measured cell concentration after 24 hours (cells/mL)	Mean measured cell concentration after 48 hours (cells/mL)	Mean measured cell concentration after 72 hours (cells/mL)
0 (Control)	-	10000	34600	261300	1030100
2.6	1.8	10000	40000	288300	1236900
6.4	0.92	10000	35000	314200	930000
16	3.6	10000	45800	365000	1343600
40	8.6	10000	40800	289200	1677800
100	22	10000	48300	287500	1685600

Observations: After 72 hours of exposure, cells exposed to all treatment levels tested and the control were observed to be normal. 

Table 2. Test results

Nominal test substance concentration (mg/L)	Geometric mean measured concentration (mg/L)	Yield (0 -72 hours) x 10000 cells/ml	Growth rate (0 -72 hours)	Inhibition of yield (biomass) at end of test (%)	Inhibition of growth rate at end of test (%)
0 (Control)	-	102	1.52	-	-
2.6	1.8	123	1.59	-20	-5
6.4	0.92	92	1.49	10	2
16	3.6	133	1.61	-31	-6
40	8.6	167	1.68	-63	-11
100	22	168	1.69	-64	-11

 

Table 3. Analytical monitoring

Nominal concentration (mg/L)	Measured concentration 0 hours (mg/L)	Measured concentration 72 hours (mg/L)
Control	<0.11	<0.097
2.6	18	0.18
6.4	5.8	<0.29
40	27	2.8
100	82	5.8

The nominal concentrations of the parent substance can be converted into concentrations of the silanol HP using MW conversions.

Validity criteria fulfilled:

yes

Conclusions:

A 72-hour EC50 value of >22 mg/L and NOEC of ≥22 mg/L have been determined for the effects of the test substance on growth rate and biomass of Pseudokirchneriella subcapitata based on geometric mean measured concentrations (>100 mg/L and ≥100 mg/L respectively based on nominal concentration and >5.8 mg/L and ≥5.8 mg/L respectively based on measured final concentrations). It is likely that the test organisms were exposed to the hydrolysis products of the substance. The results of this test were selected in preference to those of a second test (72-hour EC50 value >1039.3 mg/L for Scenedesmus subspicatus (new name: Desmodesmus subspicatus)) because they were considered to be more reliable. The appropriate interpretation of these data is to use nominal concentrations, because rapid hydrolysis during test media preparation means the test organisms would have been exposed predominantly to hydrolysis products. The low recoveries based on GC analysis are due to removal of test substance by hydrolysis. The nominal results in terms of hydrolysis product are, calculated by MW conversion are EC50 value of >63 mg/L and NOEC of ≥63 mg/L.

Description of key information

Toxicity to algae: 72-hour EC₅₀ >100 mg/l (nominal) (highest concentration tested) and NOEC ≥100 mg/l (nominal) (OECD Guideline 201 (Alga, Growth Inhibition Test)). The EC₅₀ is equivalent to >46 mg/l and the NOEC as ≥46 mg/l when expressed in terms of the silanol hydrolysis product.

Additional information

A 72-hour EC₅₀ value of >100 mg/l and NOEC of ≥100 mg/l (nominal concentration) (highest concentration tested) have been determined for the effects of tetraethyl orthosilicate (CAS 78-10-4; EC No. 201-083-8) on growth rate and biomass of Pseudokirchneriella subcapitata (Springborn Smithers, 2008) in accordance with test guideline OECD 203 and in compliance with GLP. In view of the test media preparation method/exposure regime it is likely that the test organisms were exposed predominantly to the hydrolysis products of the tested substance.

Measured concentrations of the parent substance were determined at the beginning and end of the test. The geometric mean measured concentrations were 1.8, 0.92, 3.6, 8.6 and 22 mg a.i./L. The low recoveries based on GC analysis are due to removal of test substance by hydrolysis. In view of the rapid hydrolysis rate of the substance, the measured concentrations of the parent substance are not thought to be relevant. The effect concentrations reported are therefore expressed in terms of nominal concentration, corrected to concentration of the silicon hydrolysis product by means of a molecular weight correction. The results may therefore be expressed in terms of concentration of the hydrolysis product, monosilicic acid, by applying the following molecular weight correction: EC₅₀: (MW of silanol = 96.1 / MW of parent = 208.33) * >100 = >46 mg/l; NOEC: (MW of silanol = 96.1 / MW of parent = 208.33) * ≥100 = ≥46 mg/l.

A second algal toxicity test is available with tetraethyl orthosilicate (CAS 78-10-4) and is used as supporting data:

A 72-hour EC₅₀ value of >1039.3 mg/l, EC₁₀value of 289.7 mg/l and NOEC value of 115.5 mg/l (nominal concentrations) have been determined for the effects of the test substance on growth rate of Scenedesmus subspicatus (new name: Desmodesmus subspicatus) (Hüls, 1983) in accordance with test guideline EU method C.3 and in compliance with GLP.

The results of the Springborn Smithers, 2008 test are selected in preference to those of the Hüls, 1983 test because they are of higher reliability.

Refer to IUCLID Section 6, CSR Section 7.0, for further discussion of the approach to chemical safety assessment.