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Toxicity to aquatic algae and cyanobacteria

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Description of key information

Two publications are available that are reporting 96-hr effect concentrations for the green algae Scenedesmus subspicatus Chodat:
- Geyer et al. (1985): EC10 = 0.3 - 0.6 mg/L, EC50 = 4.8 - 10 mg/L
- Friesel et al. (1984): EC10 = 0.5 - 0.7 mg/L, EC50 = 3.8 - 5.4 mg/L
Both publications report values that are within in the same range. Due to precautionary principles, the lowest reported value for the EC10 and EC50, respectively, is used as key value in the risk assessment for thiourea.

Key value for chemical safety assessment

EC50 for freshwater algae:
3.8 mg/L
EC10 or NOEC for freshwater algae:
0.3 mg/L

Additional information

Friesel at al. (1984) and Geyer et al. (1985) both report 96 -hr EC50 and EC10 values of thiourea for the green algae Scenedesmus subspicatus Chodat. In both cases results were determined in test according to the guideline "Hemmung der Zellvermehrung bei der Grünalge Scenedesmus subspicatus Chod." of the German Environmental Protection Agency (UBA; May 1982). But as the documentation of both experiments, especially with regard to the results, is insufficient for assessment, the publications are rated as Klimisch III.

According to current guidelines, the standard test duration for testing of algae toxicity is 72 hours instead of 96 hours. But as the test duration in the reported experiments is longer than in standard tests, the results are considered to rather overestimate the toxicity of thiourea to algae. Thererfore, the results are deemed sufficiently precautionary to be used in the risk assessment of the substance.

In addition, as both publications report results that are within the same range, the results are used in a weight-of-evidence approach to assess the environmental risk of thiourea:

- Geyer et al. (1985): EC10 = 0.3–0.6 mg/L, EC50 = 4.8–10 mg/L

- Friesel et al. (1984): EC10 = 0.5–0.7 mg/L, EC50 = 3.8–5.4 mg/L

Due to precautionary principles, the lowest reported value for the EC10 and EC50, respectively, is used as key value in the risk assessment for thiourea.