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Toxicity to aquatic algae and cyanobacteria

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Description of key information

ErL50 (72 h) = 13 mg/L (nominal, loading rate)
NOErLR (72 h) = 4.1 mg/L (nominal, loading rate)

Key value for chemical safety assessment

Additional information

No experimental data investigating the toxicity of Glycerides, C12-18 di- and tri- (CAS No. 91744-28-4) to algae are available. Therefore, toxicity data from a structurally related category member (Glycerides, C12-18 mono- and di- (CAS No. 91052-49-2) are used as read-across according to Regulation (EC) No. 1907/2006, Annex XI, 1.5. Both substances are esters formed from the combination of C12-18 fatty acids and glycerol. Due to differences on the degree of esterification of these substances (mono- and diester for CAS No. 91052-49-2, and di- and triester for CAS No. 91744-28-4), a higher bioavailability to aquatic organisms can be expected for the source substance (CAS No. 91052-49-2). Generally, a higher degree of esterification will result in an increase of molecular size and weight of the substance. At higher molecular size and weight, the potential to cross biological membranes tends to decrease (Guidance on information requirements and chemical safety assessment, Chapter R.11 (ECHA, 2012). Considering this information, reading across from Glycerides, C12-18 mono- and di- represents a worst-case scenario for the target substance and therefore it is justified.

The toxicity of Glycerides, C12-18 mono- and di- (CAS No. 91052-49-2) to algae was evaluated in a study by Hafner (2013). This test was conducted according to OECD 201, under GLP conditions. Desmodesmus subspicatus was exposed to the test substance for 72 hours at nominal loading rates ranging from 4.1 to 99.4 mg/L (WAF). Analytical measurement of test concentrations was performed via TOC and DOC analysis at the start and at the end of the test. Measured concentrations ranged from 4.66 to 11.8 mg/L at a nominal loading rate of 99.4 mg/L and from < LOD to 1.52 mg/L for a nominal loading rate of 20.9 mg/L. Concentrations at the lowest nominal loading rate of 4.1 mg/L could not be detected (< LOD).

 

After the exposure period, significant effects in growth rate were reported in all loading rates except for the lowest, 4.1 mg/L. The resulting EL50 value (72 h) was determined to be 13 mg/L (based on growth rate, loading rate). The NOELR (72 h) was 4.1 mg/L (based on growth rate, loading rate).

 

Nevertheless, the observed effects might be caused by direct physical interference of test substance particles with algae cells, rather than intrinsic toxicity. For this test, Water Accommodated Fractions (WAFs) were prepared by adding the test material into a defined volume of test medium, stirring for a period of 48 hours, followed by a sedimentation period of 1 hour. After the sedimentation period, the WAF with the highest loading rate (99.4 mg/L) was non-homogeneous and turbid. The next two lower loading rates (45.7 and 20.9 mg/L) were turbid, and also the second lowest loading rate (9.7 mg/L) was slightly turbid. The lowest loading rate (4.1 mg/L), the only one at which no effects were observed, was reported to be clear. The WAFs were not filtered for the final test. According to the authors of the report, at the highest loading rates (45.7 mg/L and 99.4 mg/L), algae cells were smaller and had a different shape (crumpled) compared to those in the control. Furthermore, at these two loading rates, a dense emulsion of oily drops was observed. This was confirmed by microscopic observation. At the middle loading rate of 9.7 mg/L, algae cells were larger than those in the control and small drops were reported. At a loading rate of 20.9 mg/L, the number of algae cells was significantly lower and the solution was turbid. In the lowest loading rate (4.1 mg/L) no difference compared to the control was observed. Based on the above information, mechanical disturbance of cells and cell growth cannot be excluded due to emulsified test material, which is very likely to have caused the observed effects.

 

Scientific evidence showed that aquatic toxicity testing of this type of Glycerides is technically very difficult. In an article by Prajapati et al. (2012)(see IUCLID section 6.1.4), the phase behaviour of lipid/surfactant/water phases was investigated, where medium-chain (C8-10) mono-, di- and triglycerides represent the lipid. Phase boundaries between lipids (monoglycerides, diglycerides, triglycerides), surfactant (PEG-35 castor oil) and water were established by visual inspection after an equilibration period, and the results expressed in phase diagrams. Viscosity and particle size distribution were measured. The mixtures with monoglyceride displayed two predominant phases: microemulsion and emulsion phases, whereas di- and triglycerides showed additionally a gel phase. Mixtures of monoglycerides and diglycerides, and of monoglycerides and triglycerides seemed to promote an increase of the microemulsion phase (in the 4 phases equilibrium). Particle size in these mixtures was found to be much smaller than in the monoglyceride sample alone. Microemulsions are solutions with an average particle size < 0.2 µm. This particle size would not be intercepted by a standard filter used in an aquatic toxicity test (generally, pore size of 0.45 µm). Due to their small size, based on visual inspection, clear or translucent solutions might be observed even when these microemulsions are present. Glycerides, C12-18 di- and tri- contains 40-65% C12 fatty acids and formation of microemulsions in test solutions is therefore possible for this substance.

 

Based on the read-across data, the observed effects are expected to be caused by mechanical disturbance of the algae cells rather than due to intrinsic toxicity of the substance. No toxicity of Glycerides, C12-18 di- and tri- up to the highest attainable solubility (before microemulsion formation) is thus expected. Nevertheless, since the NOELR value is within the water solubility range of the substance, 4.1 mg/L (loading rate), this value is used for PNEC derivation as a worst-case approach.