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Toxicity to aquatic algae and cyanobacteria

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toxicity to aquatic algae and cyanobacteria
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study scientifically not necessary / other information available
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An expert statement was written to support the waiving of further algal growth inhibition studies with rare earth compounds (please refer to the attached justification). In this expert statement, a thorough evaluation was performed of all available data (study reports as well as scientific literature) on the adverse effects of rare earth compounds in algae. When testing trivalent water soluble rare earth compounds, significant adverse effects on algal growth are typically observed, resulting in effect concentrations that – when expressed as initially measured concentration of dissolved rare earth compound – could trigger classification for the environment. It has to be noted that significant growth inhibition was only observed in treatments in which phosphate was depleted from the start of testing onwards and/or in treatments in which phosphate levels were significantly lower than in the control treatments at the start of testing and fully depleted shortly after start of the test (e.g. after 24 h). The strong interaction with phosphate was supported by modelling with the software Visual Minteq which confirmed that all phosphate is depleted from the test medium whenever the rare earth is in excess and vice versa. These observations provide indirect evidence that adverse effects on algal growth observed in tests with rare earth substances are due to phosphate deprivation as a result of the strong interaction of rare earths with phosphate rather than a direct toxic effect as a result of exposure to bioavailable dissolved rare earths. To date, no experiment could be identified in which a direct effect of rare earth elements on growth rate / biomass (i.e., the relevant effects to be considered in guideline studies) is convincingly demonstrated. Further, the strong interaction with phosphate in the test medium poses a technical issue hampering further research, i.e. when phosphate is in excess of the rare earth, there would be no exposure to the rare earth, and when the rare earth is in excess of the phosphate, the phosphate would be depleted from the test medium, affecting algal growth. In case phosphate deprivation is indeed responsible for the observed adverse effects on algal growth, these effects, which are observed in limited test systems, cannot be considered equally relevant at an ecosystem level. Altogether, at this point in time, it is concluded that there is no added value in performing further algal growth inhibition studies with rare earth compounds. Further, it is not considered useful to include the results of algal growth inhibition tests for classification purposes. Since either fish or daphnids have appeared to be the driver for classification for rare earth compounds, this waiving of further algal growth inhibition studies would not affect classification anyhow.

Description of key information

Based on all the available algae data on rare earth compounds it is clear that phosphate depletion and growth inhibition are concurrent and hence it is not possible to distinguish between phosphate deprivation effects in the algae and potential direct effects of the bioavailable test material on algal growth. The observed growth inhibition is therefore most likely due to phosphate deprivation of the algae.

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Additional information