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Toxicity to aquatic algae and cyanobacteria

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Description of key information

For five category members eight reliable studies (7 times RL 1, one study RL 2) are available resulting in EC10 (72 h) values between 2.6 µg/L (C16 DMA) and 8.5 µg/L (C16-18 DMA). No obvious relationship between chain length and toxicity exists. 

Key value for chemical safety assessment

EC50 for freshwater algae:
16.5 µg/L
EC10 or NOEC for freshwater algae:
2.6 µg/L

Additional information

Dimethyl Alkyl Amines (DMA), which are cationic surfactants at pH relevant in the environment, exhibit strongsorption to test organisms and walls of test vessels due to a combination of ionic and hydrophobic interaction. The sorption coefficient was found to be concentration dependent. Due to these properties the test items are difficult to test in synthetic water and results from such tests depend on the test settings applied.In river water,which contains particulate as well as dissolved organic carbon,Dimethyl Alkyl Amines (DMA) are either dissolved in water or adsorbed to dissolved and particulate matter. This reduces the difficulties encountered in tests with synthetic water caused by the high adsorption potential (adsorption losses due to settling on surfaces). In general, the adsorbed fraction of DMA is difficult to extract from the test system, which normally leads to low analytical recoveries especially in the old media, while initially measured concentrations (fresh media) are generally within +/- 20% as recommended by the guidelines. Due to the short exposure periods applied in these tests these low recoveries cannot be explained by biodegradation. No or negligible sorption to glass ware occurs under these conditions which was confirmed by measurements. This ensures reliable as well as reproducible results and means that the test substance is present in the test system and therefore available for exposure (dissolved in water and adsorbed, also called bulk). This so called Bulk Approach is described by ECETOC (2003).Consequently, nominal concentrations were used for these tests instead of measured ones.

Therefore, reliable (without restrictions, reliability category 1) tests with river water as dilution water were newly performed (NOACK, 2012) using the green alga Desmodesmus subspicatus for four category members involving different chain length (C10 DMA, C16 DMA, C16-18 DMA, and C18 DMA). These tests (static) were performed compliant to GLP according to OECD 201 and involved analytical determination of test item adsorbed to glass walls as well as initial and final test item concentration in test water and are regarded to be of higher reliability and relevance compared to tests performed with synthetic dilution water. Natural river water from river “Innerste” (Lower Saxony) was used as dilution water in these tests. This river has been chosen due to its properties representing typical conditions of a German medium sized river. The concentration of suspended matter measured in the river water was in a range of 14.0 to 15.6 mg/L, the non-purgable organic carbon concentration was between 3.2 and 3.3 mg/L.

Sometimes mitigating effects are observed for river water tests compared to tests involving synthetic water. This was not the case for results on algal toxicity of DMA. Where reliable studies for both test types are available for comparison (C10 DMA, C16 DMA) EC10 (72 h, growth rate) values observed in the river water test were even lower than those determined using synthetic dilution water (4.31 µg/L and 12.8 µg/L, respectively for C10 DMA; 1 µg/L and 25 µg/L, respectively for C16 DMA).

In addition to the new river water tests, an older test (Noack, 2000) involving river water (Elbe and Boehme, with high DOC of 14 mg/L) as well as synthetic dilution water is available for C12-14 DMA. The test was not performed under GLP and test item concentrations were not analytically verified (RL 2). Especially due to the high DOC mitigating effects may be anticipated and the comparably high NOEC determined (72h, growth rate: 20 µg/L) points in that direction. Therefore, as key study for C12-14 the reliable study (RL 1) performed with synthetic water and low concentrations of Tween 80 (<< CMC) to reduce adsorption was chosen, resulting in an EC10 (72 h, growth rate) of 7 µg/L for C12-14 DMA.

EC10 (72 h, growth rate) values of similar magnitude were determined in river water tests (RL 1) for C16-18 DMA (8.52 µg/L) and C18 DMA (5.94 µg/L), while lower corresponding values were determined for C10 DMA (4.31 µg/L) and C16 DMA (1 µg/L).

EC50 (72 h, growth rate) values were determined in river water tests (RL 1) for C10 DMA (26.8 µg/L), C16 DMA (9.9 µg/L), C16 -18 DMA (20.1 µg/L) and C18 DMA

(14.1 µg/L).

With regard to the results of C16 DMA, these values seems to be implausibly low out of the following considerations:

Following the category approach, similar toxicity is expected for category members and no trend for algal toxicity (e.g. increasing or decreasing with chain length) is evident from the experimental results for 5 members differing in chain length as outlined above. Thus, taken for granted inherently similar algal toxicity for all category members with reliable river water data (including analytics, i.e.not including C12-14) formation of a geometric mean value over all four test results would be justified. This results in geometric mean EC10 (72 h, growth rate, n= 4) of 3.8 µg/L and in geometric mean EC50 (72h, growth rate, n=4) of 16.5 µg/L which is used as a reasonable value for classification and labelling.

C16-18 DMA (72-h ErC10: 8.52 µg/L) consists of (rounded) 30% C16 DMA and 65% C18 DMA. Even if attributing total determined toxicity to the 30% of C16 DMA, the calculated virtual worst case estimate for C16 DMA (8.52 µg/L*0.3 = 2.56 µg/L) is higher than the ErC10 (72 h) for C16 DMA experimentally determined. However, also C18 DMA is highly toxic to freshwater algae (ErC10 5.94 µg/L), and thus the calculated ErC10 for C16 DMA from the experimental result of C16-18 DMA will clearly be an overestimation of toxicity and thus a very conservative estimate, making the even lower experimentally determined value for C16 DMA improbable. Moreover, the value calculated from C16-18 DMA under worst case assumption is very close to the geometric mean EC10 (72 h, growth rate, n= 4) of 3.8 µg/L over all four reliable (RL 1) river water study results.

In conclusion, it is justified to prefer the calculated virtual worst case estimate for C16 DMA of ErC10 (72 h) = 2.56 µg/L (derived from ErC10 for C16-18 DMA, see above) over the experimentally determined value for C16 DMA. This is still the lowest 10 percent effect value determined for DMA in tests on algae. Due to algae being the most sensitive trophic level for the aquatic environment, this ErC10 of 2.56 µg/L will be the basis for derivation of PNECfreshwater.