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Toxicity to aquatic algae and cyanobacteria

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Endpoint:
toxicity to aquatic algae and cyanobacteria
Type of information:
read-across based on grouping of substances (category approach)
Adequacy of study:
weight of evidence
Justification for type of information:
REPORTING FORMAT FOR THE CATEGORY APPROACH
Please refer also to the read-across statement attached in section 13

1. HYPOTHESIS FOR THE CATEGORY APPROACH (ENDPOINT LEVEL)
The target and the source substances are structurally similar substances that share the common organometallic core structure consisting of a central zinc metal bonded to four alkyldithiophosphate esters (ligands) by coordinate covalent bonds -Zn[(S2P(OR)2]2. Structural variations between the target and the source substances are related only to the alkyl (R) groups of the alkyldithiophosphate ligands. The substances in this category give thus rise to an (identical) common compound Phosphorodithioic acid moiety that can be released by the breakage of ester bonds and dissociation from the Zinc complex to which the organism would be exposed if the target substance was tested in the toxicity studies. Exposure to the parent compounds (non-transformed constituents) and to the counter alkyl alcohols, possibly released by hydrolysis of P-O bonds – non-common compounds – would not influence the prediction of the (eco)toxicological properties because they are considered to have the same biological targets and to cause the same type of effects through a common underlying mechanism due to the same functional groups (zinc cation, phosphorodithioic cation and aliphatic alcohol anionic moieties). The impurities of the target and the source substances are not expected to impact the prediction because they are identical or, if slightly structural different, belong to the same class of compounds with the same functional groups and their percentages are very low.

2. CATEGORY APPROACH JUSTIFICATION (ENDPOINT LEVEL)
The aquatic toxicity of the ZDDP category members is mainly influenced by the molecular weight (depends on the length of alkyl rests), and the water solubility. Numerous studies demonstrate decreasing toxicity with the increased chain length. Since the alkyl rests in the target substance are shorter (isobutyl, isopropyl and pentyl) than that of the source substance CAS 4259-15-8 (ethylhexyl) and the water solubility is higher (40.7 mg/L vs 9.1 mg/L for the target and the source substance, respectively) a higher bioavailability is expected for the target substance when compared to source substance CAS 4259-15-8. Regarding the source substance CAS 84605-29-8, the water solubility is with 2764 mg/L above the one of the target substance. Therefore, this source substance is expected to be more bioavailable than the target substance. Thus, data on CAS 84605-29-8 can serve as worst-case data for the target substance. In general, intrinsic toxicities of substances may be the same, because they have the same functional groups and thus the same mode of action. Therefore, taking into account the similar predicted environmental fate and behaviour, the target substance is considered not to possess significantly higher toxicity potential to aquatic environment then the source substance. Thus, the results of the acute toxicity studies in aquatic invertebrates and in algae available for the source substance are considered to be similar if these studies were conducted with the target substance. However, the target substance should be classified as aquatic toxic with long-lasting effects because its alkyl chains lengths are shorter (< C8) and thus aquatic toxicity potential cannot be ruled out.
Key result
Duration:
72 h
Dose descriptor:
EL50
Effect conc.:
410 mg/L
Nominal / measured:
nominal
Conc. based on:
test mat.
Basis for effect:
growth rate
Remarks on result:
other: 320 - 520 mg/L
Duration:
72 h
Dose descriptor:
EL50
Effect conc.:
240 mg/L
Nominal / measured:
nominal
Conc. based on:
test mat.
Basis for effect:
biomass
Details on results:
- Exponential growth in the control (for algal test): yes

- Observation of abnormalities (for algal test): All test and control cultures were inspected microscopically at 72 hours. After 72 hours there were no abnormalities detected in the control or test cultures at 10, 32, and 220 mg/L loading rate WAFs, however few intact cells were observed to be present in the test cultures at 320 and 1000 mg/L loading rate WAF.

Table. Inhibition of Growth Rate and Biomass

Nominal Loading Rate (mg/L)

Area Under Curve at 72 h

% Inhibition

Growth Rate

(0-72 h)

% Inhibition

Control

2.82E7

--

0.062

--

10

2.61E7

7

0.065

[5, increase compared to controls]

32

2.46E7

13

0.057

8

220

2.65E7

6

-0.064

[3, increase compared to controls]

320

7.72E5

97

-0.016

74

1000

-4.09E5

101

-0.012

119

Validity criteria fulfilled:
yes
Conclusions:
Exposure of Scenedesmus subspicatus to the test material gave an EbL50 (72h) value of 240 mg/L loading rate WAF and an ErL50 (0-72hr) value of 410 mg/L loading rate WAF. The No Observed Effect Loading rate was 220 mg/L loading rate WAF.
Executive summary:

Introduction: A study was performed to assess the effect of the test material on the growth of the green alga Scenedesmus subspicatus. The method followed that described in the OECD Guidelines for Testing of Chemicals (1984) No. 201, “Alga Growth Inhibition Test”. 

 

Methods:

Following a preliminary range finding test, the alga were exposed to Water Accommodated Fractions of test material over a range of nominal loading rates of 10, 32, 220, 320, and 1,000 mg/L (three replicate flasks per concentration) for 72 hours, under constant illumination and shaking at a temperature of 24 degrees 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.

 

Results:

Exposure of Scenedesmus subspicatus to the test material gave an EbL50 (72h) value of 240 mg/L loading rate WAF and an ErL50 (0-72 hr) value of 410 mg/L loading rate WAF. The No Observed Effect Loading rate was 220 mg/L loading rate WAF.

Endpoint:
toxicity to aquatic algae and cyanobacteria
Type of information:
read-across based on grouping of substances (category approach)
Adequacy of study:
weight of evidence
Justification for type of information:
REPORTING FORMAT FOR THE CATEGORY APPROACH
Please refer also to the read-across statement attached in section 13

1. HYPOTHESIS FOR THE CATEGORY APPROACH (ENDPOINT LEVEL)
The target and the source substances are structurally similar substances that share the common organometallic core structure consisting of a central zinc metal bonded to four alkyldithiophosphate esters (ligands) by coordinate covalent bonds -Zn[(S2P(OR)2]2. Structural variations between the target and the source substances are related only to the alkyl (R) groups of the alkyldithiophosphate ligands. The substances in this category give thus rise to an (identical) common compound Phosphorodithioic acid moiety that can be released by the breakage of ester bonds and dissociation from the Zinc complex to which the organism would be exposed if the target substance was tested in the toxicity studies. Exposure to the parent compounds (non-transformed constituents) and to the counter alkyl alcohols, possibly released by hydrolysis of P-O bonds – non-common compounds – would not influence the prediction of the (eco)toxicological properties because they are considered to have the same biological targets and to cause the same type of effects through a common underlying mechanism due to the same functional groups (zinc cation, phosphorodithioic cation and aliphatic alcohol anionic moieties). The impurities of the target and the source substances are not expected to impact the prediction because they are identical or, if slightly structural different, belong to the same class of compounds with the same functional groups and their percentages are very low.

2. CATEGORY APPROACH JUSTIFICATION (ENDPOINT LEVEL)
The aquatic toxicity of the ZDDP category members is mainly influenced by the molecular weight (depends on the length of alkyl rests), and the water solubility. Numerous studies demonstrate decreasing toxicity with the increased chain length. Since the alkyl rests in the target substance are shorter (isobutyl, isopropyl and pentyl) than that of the source substance CAS 4259-15-8 (ethylhexyl) and the water solubility is higher (40.7 mg/L vs 9.1 mg/L for the target and the source substance, respectively) a higher bioavailability is expected for the target substance when compared to source substance CAS 4259-15-8. Regarding the source substance CAS 84605-29-8, the water solubility is with 2764 mg/L above the one of the target substance. Therefore, this source substance is expected to be more bioavailable than the target substance. Thus, data on CAS 84605-29-8 can serve as worst-case data for the target substance. In general, intrinsic toxicities of substances may be the same, because they have the same functional groups and thus the same mode of action. Therefore, taking into account the similar predicted environmental fate and behaviour, the target substance is considered not to possess significantly higher toxicity potential to aquatic environment then the source substance. Thus, the results of the acute toxicity studies in aquatic invertebrates and in algae available for the source substance are considered to be similar if these studies were conducted with the target substance. However, the target substance should be classified as aquatic toxic with long-lasting effects because its alkyl chains lengths are shorter (< C8) and thus aquatic toxicity potential cannot be ruled out.
Duration:
72 h
Dose descriptor:
EL50
Effect conc.:
21 mg/L
Nominal / measured:
nominal
Conc. based on:
test mat.
Basis for effect:
biomass
Remarks on result:
other: 20 - 22 mg/L
Key result
Duration:
72 h
Dose descriptor:
EL50
Effect conc.:
24 mg/L
Nominal / measured:
nominal
Conc. based on:
test mat.
Basis for effect:
growth rate
Remarks on result:
other: 22 - 26 mg/L
Duration:
72 h
Dose descriptor:
NOELR
Effect conc.:
10 mg/L
Nominal / measured:
nominal
Conc. based on:
test mat.
Basis for effect:
biomass
Details on results:
- Exponential growth in the control (for algal test): yes

- Observation of abnormalities (for algal test): All test and control cultures were inspected microscopically at 72 hours. After 72 hours there were no abnormalities detected in the control or test cultures at 10 and 20 mg/L loading rate WAFs; however no intact cells were observed to be present in the test cultures at 40, 80, and 160 mg/L loading rate WAF.

Table. Inhibition of Growth Rate and Biomass

Nominal Loading Rate (mg/L)

Area Under Curve at 72 h

% Inhibition

Growth Rate

(0-72 h)

% Inhibition

Control

1.09E7

--

0.060

--

10

1.23E7

[13, increase vs controls]

0.056

7

20

6.84E6

37

0.047

22

40

-3.97E5

104

-0.022

137

80

-4.88E5

104

-0.028

147

160

-5.88E5

105

-0.046

177

Validity criteria fulfilled:
yes
Conclusions:
Exposure of Scenedesmus subspicatus to the test material gave an EbL50 (72h) value of 21 mg/L loading rate WAF and an ErL50 (0-72hr) value of 24 mg/L loading rate WAF. The No Observed Effect Loading rate was 10 mg/L loading rate WAF.
Executive summary:

A study was performed to assess the effect of the test material on the growth of the green alga Scenedesmus subspicatus. The method followed that described in the OECD Guidelines for Testing of Chemicals (1984) No. 201, “Alga Growth Inhibition Test”. 

Following a preliminary range finding test, the alga were exposed to Water Accommodated Fractions of test material over a range of nominal loading rates of 10, 20, 40, 80, and 160 mg/L (three replicate flasks per concentration) for 72 hours, under constant illumination and shaking at a temperature of 24 °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 gave an EbL50 (72h) value of 21 mg/L loading rate WAF and an ErL50 (0-72hr) value of 24 mg/L loading rate WAF. The No Observed Effect Loading rate was 10 mg/L loading rate WAF.

Description of key information

A study with the source substance Zinc bis[O,O-bis(2-ethylhexyl)] bis(dithiophosphate) (CAS 4259 -15 -8) was performed to assess the effect of the test material on the growth of the green alga Scenedesmus subspicatus. The method followed that described in the OECD Guidelines for Testing of Chemicals (1984) No. 201, “Alga Growth Inhibition Test” and was performed under GLP conditions. The quality of the study was assessed with Klimisch 2. Exposure of Scenedesmus subspicatus to the test material gave an EbL50 (72h) (based on biomass) value of 240 mg/L loading rate WAF and an ErL50 (72hr) (based on growth rate) value of 410 mg/L loading rate WAF. The No Observed Effect Loading rate was 220 mg/L loading rate WAF. 

A study with the source substance Phosphorodithioic acid, mixed O,O-bis(1,3 -diemtylbutyl and iso Pr) esters, zinc salts (CAS 84605 -29 -8) was performed to assess the effect of the test material on the growth of the green alga Scenedesmus subspicatus. The method followed that described in the OECD Guidelines for Testing of Chemicals (1984) No. 201, “Alga Growth Inhibition Test” and was performed under GLP conditions.The quality of the study was assessed with Klimisch 2. Exposure of Scenedesmus subspicatus to the test material gave an EbL50 (72h) (based on biomass) value of 21 mg/L loading rate WAF and an ErL50 (72hr) (based on growth rate) value of 24 mg/L loading rate WAF. The No Observed Effect Loading rate was 10 mg/L loading rate WAF.

The values are different from each other. But this is in range with the category approach, where nevertheless the members of the categories may have different values.

For the CSA the lower value is chosen as worst-case value.

Key value for chemical safety assessment

EC50 for freshwater algae:
24 mg/L
EC10 or NOEC for freshwater algae:
10 mg/L

Additional information

Zinc bis[O,O-bis(2-ethylhexyl)] bis(dithiophosphate) (CAS 4259 -15 -8)

Introduction:

A study was performed to assess the effect of the source substance Zinc bis[O,O-bis(2-ethylhexyl)] bis(dithiophosphate) (CAS 4259 -15 -8) on the growth of the green alga Scenedesmus subspicatus. The method followed that described in the OECD Guidelines for Testing of Chemicals (1984) No. 201, “Alga Growth Inhibition Test”.

 

Methods:

Following a preliminary range finding test, the alga were exposed to Water Accommodated Fractions of test material over a range of nominal loading rates of 10, 32, 220, 320, and 1,000 mg/L (three replicate flasks per concentration) for 72 hours, under constant illumination and shaking at a temperature of 24 degrees 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.

Results:

Exposure of Scenedesmus subspicatus to the test material gave an EbL50 (72h) value of 240 mg/L loading rate WAF and an ErL50 (0-72 hr) value of 410 mg/L loading rate WAF. The No Observed Effect Loading rate was 220 mg/L loading rate WAF.

Phosphorodithioic acid, mixed O,O-bis(1,3 -diemtylbutyl and iso Pr) esters, zinc salts (CAS 84605 -29 -8)

Introduction:

A study was performed to assess the effect of the test material on the growth of the green alga Scenedesmus subspicatus. The method followed that described in the OECD Guidelines for Testing of Chemicals (1984) No. 201, “Alga Growth Inhibition Test”. 

Methods:

Following a preliminary range finding test, the alga were exposed to Water Accommodated Fractions of test material over a range of nominal loading rates of 10, 20, 40, 80, and 160 mg/L (three replicate flasks per concentration) for 72 hours, under constant illumination and shaking at a temperature of 24 °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.

Results:

Exposure of Scenedesmus subspicatus to the test material gave an EbL50 (72h) (based on biomass) value of 21 mg/L loading rate WAF and an ErL50 (72hr) (based on growth rate) value of 24 mg/L loading rate WAF. The No Observed Effect Loading rate was 10 mg/L loading rate WAF.


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