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Toxicity to aquatic algae and cyanobacteria

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The toxicity of CBS to aquatic algae (Selenastrum capricornutum) was tested according to OECD TG 201 "Alga, Growth Inhibition Test". After 72 hours of exposure an ECr50 of 0.15mg/L and a NOEC of 0.0084 mg/L were obtained (National Institute of Technology and Evaluation, 1996).

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Additional information

A hydrolysis half-life time of CBS of 12.5 h and a complete hydrolysis of CBS in 24.9 h were observed in deionized water buffered at pH 7.0 and at 25°C (Monsanto, 1984). National Institute of Technology and Evaluation Japan did not provide detailed information, whether the effect concentrations were nominal concentrations or from analytical monitoring. The observed effect is most probably a combined effect from both CBS and its hydrolysis products. The reported effective concentrations are lower than water solubility of CBS (i.e. 0.32 mg/l).

The absolute values of chlorophyll and/or cell numbers were not reported for control experiment in Monsanto study from 1979 and hence no exponential growth can be concluded from this study. Therefore the information provided in this study is not sufficient for assessment. Although this study was used in EU-risk assessment (2008), this study is assigned to be a reliability of 4 in the up-to-date assessment and will not be used in the risk assessment under REACH.

CBS can be rapidly degraded by hydrolysis and the main degradation products appearing during the time frame of acute tests are benzothiazole (BT) and benzothiazolone (BTon). The toxicities of BT and BTon to alga were tested according to OECD TG 201(AKZO NOBEL, 2004). The 72h-ErC50 (nominal) was 50.8 mg/l (BT) and 22.4 mg/l (BTon); and the NOEC was 19.4 mg/l (BT) and 9.9 mg/l (BTon), respectively. The nominal concentration was confirmed analytically; however the results should be used provisionally with reservations due to lack of exponential growth in the control cultures. There are some other benzothiazole derivatives as metabolites from CBS are considered as relevant for environmental exposure and risk assessment, such as MeBT, MBT, MBTS and MeSBT. The toxicities of MeBT and MeSBT to alga were tested according to OECD TG 201 and the 72h- ErC50 of 45.2 mg/l and 3.43 mg/l (nominal) were determined, respectively, which were analytically confirmed (AKZO NOBEL, 2004). Since the control cultures did not grow exponentially, in EU-risk assessment (2008), the data were re-evaluated. According to re-evaluation of the raw data and expert judgement, an ErC50 = ca. 32 mg/l is considered more defendable for MeBT. For MeSBT the re-evaluated value is ca. two times higher than the reported value, and hence the reported value was considered sufficiently protective and provisionally used with reservations. The toxicity of MBT to aquatic algae (Selenastrum capricornutum) was tested according to OECD TG 201 "Alga, Growth Inhibition Test". After 72 hours of exposure, an ECr50 of 0.5mg/L and a NOEC of 0.066 mg/L were obtained (MITI, 1999). For MBTS no acute toxic effect was observed to algae (Desmodesmus subspicatus) up to its water solubility (Bayer AG, 1992). Regarding the data summarized here, the described metabolites of CBS are less toxic to aquatic algae than CBS itself.