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Toxicity to aquatic algae and cyanobacteria

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Reference
Endpoint:
toxicity to aquatic algae and cyanobacteria
Data waiving:
study scientifically not necessary / other information available
Justification for data waiving:
other:
Justification for type of information:
JUSTIFICATION FOR DATA WAIVING
The substance is a phosphonate, containing four P-(OH)2 groups. Phosphonate compounds, similar to this test substance, are known to be effective chelating agents that bind tightly to metal ions (Guidance document on Aquatic toxicity testing of difficult substances and mixtures (OECD, 2000)). It is well documented for many phosphonate compounds, which act as chelating agents, that algal growth inhibition results can be strongly affected by the way the substance causes complexation of essential test nutrients from the test medium used in the studies. These effects may be incorrectly interpreted as toxic effects on the algae, whereas the real effect may be due to limitation of nutrients required for algal growth. Therefore, the planned algal growth inhibition test on this substance was anticipated to potentially show effects on algal growth, which would need to be reviewed in order to determine if they were true toxic effects or related to complexation. Initially a study was performed to assess the effect of the test item on the growth of the green alga Pseudokirchneriella subcapitata. The method followed, was designed to be compatible with the OECD Guidelines for Testing of Chemicals (2006) No 201, “Freshwater Alga and Cyanobacteria, Growth Inhibition Test” referenced as Method C.3 of Commission Regulation (EC) No 440/2008. A standard culture medium, as recommended in the above testing guidelines, was used in the study. The results of this study showed that the growth rate and yield of Pseudokirchneriella subcapitata were affected by the presence of the test item over a 72-hour exposure period. However, as outlined above, as the test item was known to be a phosphonate, it was considered possible that complexation may have affected the toxicity of the test item by reducing the availability of salts such as Iron, Magnesium and Calcium and other key nutrients in the test medium, which are essential for supporting healthy algal growth. Inhibition of algal growth by metal complexing agents can potentially be eliminated by compensating for the deficit in the concentration of essential ion(s). Therefore, in order to determine whether the inhibition observed in the initial test was due to a true toxic response or possible chelation of essential cations, in accordance with the OECD Guidance Document on Aquatic Testing of Difficult Substances and Mixtures, further tests were performed using a modified algal growth medium and a double strength culture medium. The standard culture medium used in the initial test was supplemented with additional CaCl.2H20 to increase the hardness from 15 mg/l to approximately 150 mg/l as CaCO3 for the modified algal growth medium test. In the second modification, the standard culture medium was modified such that the concentration of each component was doubled for the double strength culture medium test. The results obtained from the modified culture medium test showed that the use of the modified media had no significant effect on inhibition of growth observed at the tested concentration. The results obtained from the double strength culture medium test showed that the use of the double strength media had no significant effect on the inhibition of growth observed at the tested concentrations. Although no significant differences were observed, in the additional tests using a modified culture medium or double strength (when compared to the initial test using the standard culture medium), growth inhibition by chelation of essential cations cannot be precluded as the specificity of this phosphonate to various metal ions and rate of uptake is unknown. In consequence chelation of certain essential cations and nutrient limitation may remain. Therefore, the algal growth inhibition test has been disregarded for use in classification purposes and for environmental risk characterisation purposes, as the effects observed on growth inhibition cannot be attributed to true toxicity. The industrial uses of the substance also support the argument that effects seen on algal growth inhibition are due to chelation of essential cations rather than true toxicity. The substance (contained in formulated products) is used as a processing aid in water cooling systems, in mining applications, in gas cleaning agents and in the manufacture of paper products. The substance is able to function as intended in these systems in part due to its chelating properties. This is supported by the fact that phosphonates are widely used in a wide range of industrial applications, such as water cooling systems, often as scale inhibitors Phosphonates can function in this way as they are effective chelating agents that bind tightly to metal ions, preventing them from forming insoluble precipitates (scale). They are also stable under harsh conditions. Therefore, the substance is expected to have chelating properties, in order for it to function as intended in its industrial use. This would support the effects observed on algal growth inhibition being due to the substances chelation of cations and nutrients rather than true toxic effects.

Description of key information

The substance is a phosphonate, containing four P-(OH)2 groups. Phosphonate compounds, similar to this test substance, are known to be effective chelating agents that bind tightly to metal ions (Guidance document on Aquatic toxicity testing of difficult substances and mixtures (OECD, 2000)).   It is well documented for many phosphonate compounds, which act as chelating agents, that algal growth inhibition results can be strongly affected by the way the substance causes complexation of essential test nutrients from the test medium used in the studies. These effects may be incorrectly interpreted as toxic effects on the algae, whereas the real effect may be due to limitation of nutrients required for algal growth.  Therefore, the planned algal growth inhibition test on this substance was anticipated to potentially show effects on algal growth, which would need to be reviewed in order to determine if they were true toxic effects or related to complexation.   Initially a study was performed to assess the effect of the test item on the growth of the green alga Pseudokirchneriella subcapitata. The method followed, was designed to be compatible with the OECD Guidelines for Testing of Chemicals (2006) No 201, “Freshwater Alga and Cyanobacteria, Growth Inhibition Test” referenced as Method C.3 of Commission Regulation (EC) No 440/2008. A standard culture medium, as recommended in the above testing guidelines, was used in the study.  The results of this study showed that the growth rate and yield of Pseudokirchneriella subcapitata were affected by the presence of the test item over a 72-hour exposure period.  However, as outlined above, as the test item was known to be a phosphonate, it was considered possible that complexation may have affected the toxicity of the test item by reducing the availability of salts such as Iron, Magnesium and Calcium and other key nutrients in the test medium, which are essential for supporting healthy algal growth. Inhibition of algal growth by metal complexing agents can potentially be eliminated by compensating for the deficit in the concentration of essential ion(s). Therefore, in order to determine whether the inhibition observed in the initial test was due to a true toxic response or possible chelation of essential cations, in accordance with the OECD Guidance Document on Aquatic Testing of Difficult Substances and Mixtures, further tests were performed using a modified algal growth medium  and a double strength culture medium. The standard culture medium used in the initial test was supplemented with additional CaCl.2H20 to increase the hardness from 15 mg/l to approximately 150 mg/l as CaCO3 for the modified algal growth medium test. In the second modification, the standard culture medium was modified such that the concentration of each component was doubled for the double strength culture medium test.  The results obtained from the modified culture medium test showed that the use of the modified media had no significant effect on inhibition of growth observed at the tested concentration.  The results obtained from the double strength culture medium test showed that the use of the double strength media had no significant effect on the inhibition of growth observed at the tested concentrations.  Although no significant differences were observed, in the additional tests using a modified culture medium or double strength (when compared to the initial test using the standard culture medium), growth inhibition by chelation of essential cations cannot be precluded as the specificity of this phosphonate to various metal ions and rate of uptake is unknown. In consequence chelation of certain essential cations and nutrient limitation may remain. Therefore, the algal growth inhibition test has been disregarded for use in classification purposes and for environmental risk characterisation purposes, as the effects observed on growth inhibition cannot be attributed to true toxicity.  The industrial uses of the substance also support the argument that effects seen on algal growth inhibition are due to chelation of essential cations rather than true toxicity.    The substance (contained in formulated products) is used as a processing aid in water cooling systems, in mining applications, in gas cleaning agents and in the manufacture of paper products.  The substance is able to function as intended in these systems in part due to its chelating properties. This is supported by the fact that phosphonates are widely used in a wide range of industrial applications, such as water cooling systems, often as scale inhibitors Phosphonates can function in this way as they are effective chelating agents that bind tightly to metal ions, preventing them from forming insoluble precipitates (scale). They are also stable under harsh conditions.  Therefore, the substance is expected to have chelating properties, in order for it to function as intended in its industrial use. This would support the effects observed on algal growth inhibition being due to the substances chelation of cations and nutrients rather than true toxic effects.

Key value for chemical safety assessment

Additional information

The substance is considered non-toxic to algae, no key effect value for chemical safety assessment is relevant for algae and the substance is not considered to be hazardous to this aquatic trophic level.