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Toxicity to aquatic algae and cyanobacteria

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Coloured substances are often assumed to behave differently in the algal growth inhibition test due to shading of the algae. Coloured substances can absorb photosynthetically active light and hence limit growth of algal cultures. Absorption will be proportional to test substance concentration and as a consequence it can result in growth inhibition which is difficult to distinguish from inherent toxicity.

Several studies on algae conducted on dark dyes, including those with a modified test system for dye-stuffs, showed that the growth inhibition is mainly correlated to the reduced light absorption, caused by the stained water and not due to an effective toxicity of the dye.


A modified test design has been proposed by the cooperation of the Ecological and Toxicological Association of Dyes and Organic Pigments Manufacturers (ETAD) (Memmert et al) in order to eliminate the effect of shading. Nevertheless this method has some limitations, as it focuses on the shadow effect, giving no information on the real potential toxicity for algae of the tested substance.

Considering that the test substance is a dyestuff, which could influence the photosynthesis capacity of algae cells in the test solution, and in order to clarify the (eco)toxicological properties related to toxicity for aquatic plants, it was considered a test using Lemna minor as a test organism, according to the OECD Guideline 221, as suggested also in the Guidance on Information Requirements and Chemical Safety Assessment Chapter R.7b: Endpoint specific guidance (Version 2.0 November 2014), published by the European Chemical Agency.



Memmert U, Motschi H, Inauen J, W. Uthrich V. Inhibition of algal growth caused by coloured test substances. ETAD Project E 3023. Ecological and Toxicological Association of Dyes and Organic Pigments Manufacturers, Basel, Switzerland, 1994.