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Toxicity to aquatic algae and cyanobacteria

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Description of key information

Toxicity to algae: EC50 (10d) = ca. 34.5 mg/L Manganese (II) acetate for Plectomena boryanum (static, based on surviving colonies)

Key value for chemical safety assessment

EC50 for freshwater algae:
34.5 mg/L
EC10 or NOEC for freshwater algae:
5.1 mg/L

Additional information

The relevant EC50 and EC10 values were derived from the key study, which is the more relevant one. First, the test duration is prolonged which gives in general a better insight in the overall toxicity of a substance. Second, since here a "split-up" approach was used to assess the overall toxicity of Manganese (II) acetate, namely to assess both manganese cation and acetate anion separately due to the available data, the acetate anion turned out to be not harmful at all to aquatic algae (Toxicity Threshold of acetic acid = 4000 mg/L). Hence, the results for the manganese cation are the more relevant ones for further risk assessment.

Although no standard guideline was followed, two different strains of algae tested and the documentation and given results indicate that the study was well-performed and satisfies general scientific requirements. So the study is classified as reliable with restrictions and consequently, the results can considered to be reliable and used for classification. Since Plectonema boryanum is the more sensitive species, these values are used for classification: An EC50 value of approx. 0.2 mmol Mn2+, which corresponds to 34.5 mg/ Manganese (II) acetate, was determined. Taking into account that Manganese (II) acetate as an inorganis salt has no potential for bioaccumulation and no effects were observed at the hypothetic concentration of 5.1 mg/L Mn(acO)2 after 10 days in this experiment, Manganese (II) acetate does not need to be classified according to regulation 1272/2008/EC.

In general, algae serve as model organisms in order simulate actual environmental conditions and how a testing substance could affect them. Testing only on algae can only give a limited insight in real conditions, and every testing on additional species, such as aquatic plants or higher tier mesocosm studies, could enlarge the insight, how a substance would affect the environment. So every testing on other aquatic plants and organisms would decrease the possible uncertainties due to extrapolation from only one species and hence, can serve to support the results from algae toxicity testing and to lower any assessment factor.

So, three additional studies were chosen to support the results from the testing on algae and the conclusion that no classification of Manganese (II) acetate is required.

A study on Duckweed (Lemna minor) (Wang S, 1986) revealed a growth inhibition of 50% after 4 days at a concentration of 96 mg/L, corrected for Manganese (II) acetate and based on the increase of their front number. Since even the CLP regulation 1272/2008/EC mentions the possibility to base the decision, whether to classify or not, on the results derived from studies on aquatic plant, these results may perfectly serve to support the low aquatic toxicity of Manganese (II) acetate.

Two studies on aquatic protozoae, i.e. Tetrahymena pyriformis (Bogaerts P, 1998) with an IC50 (9h) of 650.6 mg/L, corrected for Manganese (II) acetate and based on growth inhibition, and Spirostomum ambiguum (Nalecz-Jawecki G, 1998) with an LC50 (48h) of 452.4 mg/L, corrected for Manganese (II) acetate and based on growth mortality, are additionally available.

Although no standard species, the test organisms Tetrahymena pyriformis and Spirostomum ambiguum broaden additionally the spectrum of aquatic organisms and, as additional models, allow a deeper insight in the expectable effects of the test substance on the aquatic environment. . Therefore, these results should be also taken into account when assessing the possible hazard of Manganese (II) acetate to the aquatic environment and consequently the need for classification.

Although the test durations and endpoints of these mentioned studies are not equal enough to allow one to plot them as a species sensitivity distribution, it can be clearly stated that the available endpoint for algae toxicity on Plectonema boryanum, i.e. an EC50 value of approx. 34.5 mg/L Manganese (II) acetate after 10 days, can considered to be the most sensitive endpoint in the most sensitive species, and should therefore be taken into account when assessing the possible hazard of Manganese (II) acetate to the aquatic environment based on the broad base of aquatic food chains.

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