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Toxicity to aquatic algae and cyanobacteria

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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 (National Institute of Technology and Evaluation, 1999).
MBT can be rapidly degraded by direct photolysis in water and the main degradation products appearing during the time frame of acute tests are benzothiazole (BT) and benzothiazolone (BTon) (Brownlee, 1992). 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 MBT are considered as relevant for environmental exposure and risk assessment, such as MBTS and MeSBT. For MBTS no acute toxic effect was observed to algae (Desmodesmus subspicatus) up to its water solubility (Bayer AG, 1992). The toxicities of MeSBT to alga was tested according to OECD TG 201 and the 72h- ErC50 of 3.43 mg/l (nominal) was determined, which was analytically confirmed (AKZO NOBEL, 2004). Since the control cultures did not grow exponentially, in EU-draft risk assessment, the data were re-evaluated. 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.
Regarding the data summarized here, the described metabolites of MBT are less toxic to aquatic algae than MBT itself.

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Additional information

MBT can be rapidly degraded by direct photolysis with a half-life time of ca. 30 mins (CMA, 1989) and can be rapidly oxidized in water by atmospheric oxygen with 60% conversion in 2 hours (Hansson & Agrup, 1993). MITI online 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 MBT and its hydrolysis products like BT and its oxidation product like MBTS, however BT and MBTS have much lower toxicities to algae than MBT.