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Toxicity to aquatic algae and cyanobacteria

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Description of key information

No valid data could be located regardng the toxicity of butyric acid to aquatic algae and cyanobacteria.
To compensate for this lack of data, information resulting from isobutyric acid as supporting substance is used as substitute.
Supporting substance isobutyric acid: in a valid growth inhibition test with green algae similar to OECD TG 201, the EC10 respectivly the EC50 of isobutyric acid was determined to be 29.4 mg/l respectivly 44.7 mg/L (BASF AG, 1990).
As molecular weights of butyric acid and isobutyric acid are equal, the EC10 and EC50 values determined for isobutyric acid can be taken unchanged for butyric acid.

Key value for chemical safety assessment

EC50 for freshwater algae:
45.1 mg/L
EC10 or NOEC for freshwater algae:
29.4 mg/L

Additional information

The toxicity of butyric acid to aquatic algae was investigated in two studies. One study is only reported as short abstract, thus reliability of the data cannot be assessed (Hüls 1989)). The second study (Bringmann, 1979) was performed using butyric acid under neutralized conditions (presence of butyrate salt instead of butyric acid). This result is not comparable to test results with not neutralized acid (the salt is much less toxic).

 

As valid data are not available to assess the toxicity of butyric acid to aquatic algae, data for isobutyric acid as supporting substance will be used based on following reasons.

 

Isobutyric acid is closely related to butyric acid. Both compounds are carboxylic acids with the same number of carbon atoms differing only in the position of one methyl-group (branched versus linear carbon chain). Physical properties and chemical reactivity will be quite similar. Main structural feature is the carboxylic acid group, which will have a major impact on the effects of both substances in aquatic systems. Thus, it is justified to use isobutyric as supporting substance in the evaluation of the toxicity of butyric acid to aquatic algae and cyanobacteria.

 

Supporting substance isobutyric acid

The study was performed according to a German test guideline (DIN 38412 part 9) which is comparable to OECD TG 201. Reliability is 2.

 

BASF 1990

 

In a 72-h acute toxicity study similar to OECD TG 201, cultures of Scenedesmus subspicatus (new name Desmodesmus subspicatus), strain SAG 86.81, were exposed to isobutyric acid at nominal concentrations of 0 (controls), 3.9, 7.8, 15.6, 31.25, 62.5, 125, 250, and 500 mg/L under static conditions. Cell density was recorded at 0, 24, 48, and 72 h by fluorescence measurement.

 

The 72-h EC50 was determined to be 45.1mg/L (BASF AG, 1990)

Butyric acid: toxicity to aquatic algae and cyanobacteria

As the molecular weight of both substances is the same, toxicity values have not to be converted to allow for different compounds.

The 72-h EC50 for butyric acid is assessed to be 45.1 mg/L.