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Toxicity to aquatic algae and cyanobacteria

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Description of key information

As indicated in the hydrolysis study acetone cyanhydrin hydrolyses rapidly within minutes to acetone and cyanide. The study was performed over 96 hours. Therefore the toxic principle is the cyanide ion. The hydrolysis product cyanide of acetone cyanhydrin is very toxic against the green algae Desmodesmus subspicatus.

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Additional information

As 2-hydroxy-2-methylpropanenitril (CAS-Nr. 75-86-5) hydrolyses within minutes to form acetone and HCN, the active toxic principle is the cyanide ion. Therefore it can be extrapolated that 2-hydroxy-2-methylpropanenitril (CAS-Nr. 75-86-5) would exhibit the same toxicity as potassium cyanide against green algae Desmodesmus subspicatus. The study classified as acceptable and satisfies the information needs for toxicity against green algae.The LOEC value based on cell density (turbidity of the test cultures) was 160 mg a.i./L. Potassium cyanide (CAS-No. 151-50-8) is very toxic against the green algae Desmodesmus subspicatus.

These finding are in compliance with the AEGL committee (US-NAC,
Acetone Cyanohydrin, Interim Acute Exposure Guideline Levels (AEGLs), Interim final draft, 2005) and ECETOC (JACC report No. 53, Volume I, 2007), where the toxic effects of acetone cyanhydrin are attributed to the toxic action of the free cyanide. The principle of cyanide toxicity is similar in all organisms including micro-organisms. The cyanide ion reacts with the central metal ion of metallo-enzymes to form cyanide complexes thus rendering those enzymes non-functional.

Following ECETOC JACC report No. 53, Volume I, 2007 there are only a few studies providing EC50 values for effects of cyanide on algae. None of them seems to have been conducted to current or Good Laboratory Practice (GLP) guidelines. Exposure was static in all tests. Most of the values are higher than 200
μg/l, but Krebs (1991) reported an EC50 of 45 μg/l for oxygen production of Chlorococcales (green algae) and Pablo et al (1997a) determined an EC50 of 57 μg/l for the saltwater diatom Nitzschia closterium (Both not assignable).