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Ecotoxicological information

Toxicity to aquatic algae and cyanobacteria

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Description of key information

The lowest acute value for freshwater algae was found for Selenastrum capricornutum. The EbC50 was 129 mg/l after 72 hours. The lowest NOEC for freshwater algae was found for Selenastrum capricornutum and was 62.5 mg/l.

The lowest endpoint for marine algae with a more standard nitrate concentration was found for Nitzschia closterium, with a NOEC of 100 mg ClO3-/l, which is equal to 147 mg KClO3/l.

Key value for chemical safety assessment

EC50 for freshwater algae:
129 mg/L
EC50 for marine water algae:
735 mg/L
EC10 or NOEC for freshwater algae:
62.5 mg/L
EC10 or NOEC for marine water algae:
147 mg/L

Additional information

Three studies, one on Selenastrum capricornutum and two on Scenedesmus subspicatus were found. One was valid without restrictions (Ward and Boeri, 1991e) and two were valid with restrictions because not all details on the results were provided.

The lowest acute value for freshwater algae was found for Selenastrum capricornutum. The ErC50 was 129 mg/l after 72 hours. The lowest NOEC for freshwater algae was found for Selenastrum capricornutum and was 62.5 mg/l.

One valid test with marine algae is available (Tobiesen, 2010a) and one test (valid with restriction) performed by Hutchinson (1994b) (see table 7.6). Tobiesen performed a standard test with Skeletonema costatum according to guideline and GLP. Chemical analyses were performed on the test concentrations. He found that S. Costatum was not sensitive to sodium chlorate, with a NOEC greater than or equal to 1000 mg/l. Hutchinson performed a test with Phaeodactylum tricornutum and found a NOEC of 64 mg sodium chlorate/l for biomass and a NOEC of 128 mg sodium chlorate/l for the growth rate.

Two tests (valid with restriction) were found for marine algae performed with potassium chlorate. Stauber performed the tests with potassium chlorate and expressed the endpoints in mg chlorate/l. The endpoints were recalculated to mg potassium chlorate/l for comparison with the studies on sodium chlorate and are provided in table 7.7. The original data for the EC50was for Dunaliella tertiolecta > 1000 mg chlorate/l and for Nitzschia closterium 10 mg chlorate/l and > 500 mg chlorate/l, respectively. The NOEC values were 500 mg chlorate/l for Dunaliella tertiolecta and 100 mg chlorate/l for Nitzschia closterium.

Stauber performed the test at three different nitrate levels namely <0.005, 1 and 15 mg nitrate/l. At the lower nitrate concentrations cell growth in the controls was not according to the standard criteria and these results cannot be used therefore. Only valid results are given in the table above.

Tests performed with potassium chlorate can be used in this dossier, because the toxicity of the test substance is caused by the chlorate ion. In water sodium and potassium are naturally present and the amounts added with the test substance are not considered to have an impact on the total concentration and on the test result, therefore the counter ion present is not relevant for the test result.

The lowest short-term value for marine algae species was found for Nitzschia closterium, the EC­­50was 15 mg/l at 1 mg nitrate/l.

The difference in toxicity noted between marine and freshwater algae, appears to be related more to the relative difference in concentration of nitrate in freshwater and marine compartments than to different mechanisms of toxicity between species. The concentration of nitrate in the test water influences the effect concentration of chlorate (Stauber, 1998) indicating that competitive inhibition occurs between nitrate and chlorate with excess nitrate inhibiting chlorate toxicity. This is supported by the acute studies on marine species using chlorate performed by Stauber (1998) at several nitrate concentrations. Seawater has considerably lower concentrations of nitrate than freshwater systems so competitive inhibition of chlorate by nitrate will be reduced in marine environments. This is discussed in detail in the introduction of this chapter. 

When comparing studies, comparison should be made of studies performed under the same standard conditions as much as possible. Phaeodactylum tricornutum showed the same sensitivity under standard test conditions, with a NOEC for growth rate of 128 mg/l (100.4 mg ClO3-/l). Stauber found exactly the same outcome with a more standard nitrate concentration for Nitzschia closterium, with a NOEC of 100 mg ClO3-/l.