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Toxicity to aquatic algae and cyanobacteria

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Description of key information

An OECD TG 201 algal growth inhibition test (Mead, 2000) was performed with potassium zirconium carbonate. Exposure of Scenedesmus subspicatus gave an EbC50 (72h, TWA) value of 0.82 mg/L and an ErC50 (0-72h, TWA extrapolated) value of 17 mg/L. The No Observed Effect Concentration (TWA) was 0.23 mg/L. The preferred observational endpoint for risk assessment in this study is the algal growth rate inhibition of ErC50 (0-72h, TWA) value of 17 mg/L.

Key value for chemical safety assessment

EC50 for freshwater algae:
17 mg/L

Additional information

The toxicity of the test material to aquatic plants was investigated in the algae Scenedesmus subspicatus in accordance with the standardised guidelines OECD 201 and EU Method C.3.

Following a preliminary range-finding study, Scenedesmus subspicatus was exposed to an aqueous solution of the test material at concentrations of 0.50, 1.0, 2.0, 4.0 and 8.0 mg/l (three triplicates per concentration) for 72 hours, under constant illumination and shaking at a temperature of 24 ± 1ºC. Samples of the algal population were removed daily and cell concentrations determined for each control and treatment group, using a Coulter Multisizer II Particle Counter.

Exposure of Scenedesmus subspicatus to the test material gave nominal figures of EC₅₀ (72 h, biomass) value of 1.3 mg/l and an EC₅₀ (0 -72 H, growth) value of 18 mg/l. The No Observed Effect Concentration was 0.50 mg/l.

The zirconium concentration in the test preparations was determined using Inductively Coupled Plasma Atomic Emission Spectroscopy and was then converted to the concentration of test material by multiplying by the appropriate factor. Analysis of the test samples at 0 hours showed the measured test concentrations to range from 42% to 85% of nominal (based on zirconium analysis). At 72 hours the measured test concentrations ranged from 50% to 79% of nominal. The variation in measured concentrations ohof zirconium analysis were considered to be due to one or a combination of: adsorption to algal cells, sampling from a heterogeneous dispersion or an inappropriate method of analysis.

Given the variation obtained in the measured test concentrations at 0 and 72 hours, it was considered justifiable to calculate the results of the study in terms of the time-weighted mean measured test concentrations in order to give a worst case analysis of the data.

In terms of the time-weighted mean measured test concentrations, the EC₅₀ (72 h, biomass) value was determined to be 0.82 mg/l and the EC₅₀ (0 -72 h, growth) value was 17 mg/l. The No Observed Effect Concentration (NOEC) was 0.23 mg/l.

Additional information was supplied by the study sponsor indicating that zirconium salts were extremely efficient at removing phosphates from solution. Therefore it was considered possible that the reduction in algal growth observed in the definitive study was a result of phosphate starvation rather than true test material toxicity. Post study work was therefore conducted where Scenedesmus subspicatus cells were exposed to the test material at concentrations of 0.50, 2.0 and 8.0 mg/l for a period of 72 hours. After the exposure period aliquots of the test cultures were removed, used to inoculate fresh culture medium and the resulting sub-cultures incubated for a period of 96 hours. Regrowth of algal cells occurred in all test cultures after 96 hours thereby indicating that the test material was algistatic in effect. This effect was considered to be due to the test material removing phosphates from solution and hence preventing the algal cells from growing rather than true toxicity of the test material to algal cells.

The study was performed in line with GLP study and accepted standardised guidelines, with a high level of reporting. The study was assigned a reliability score of 1 according to the principles for assessing data quality set out in Klimisch (1997).