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Toxicity to aquatic algae and cyanobacteria

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Reference
Endpoint:
toxicity to aquatic algae and cyanobacteria
Type of information:
(Q)SAR
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
results derived from a valid (Q)SAR model and falling into its applicability domain, with adequate and reliable documentation / justification
Justification for type of information:
The registered substance is a complex mixture whose constituents within the mixture have variable physicochemical properties. Therefore, a modeling framework is needed to account for the bioavailability of the constituents in the mixture, which was then used to predict the toxicity of the substance as a whole. A computer-based program, PETROTOX (v4.0), was used to calculate aquatic toxicity. The program is an extension of the three-phase partitioning (to calculate the partitioning of constituents in air, water, and non-aqueous phase liquid phases) and toxicity models used to predict the toxicity of complex substances, and incorporates the Target Lipid Model (TLM), which has been validated for applications to individual narcotic chemicals and mixtures. Composition information is entered as separate hydrocarbon blocks according to carbon number range and chemical classes, which comprise the substance.Representative constituents input to model located in the attachment "PETRORISK and PETROTOX block allocation" in Section 13.2 of IUCLID.

The TLM is a framework that relates toxicity to the physicochemical properties of a nonpolar organic constituent. The TLM is used to calculate the toxicity of the dissolved constituents to aquatic organisms. Toxicity of complex mixtures with common toxic modes of action is determined using toxic units (TU) that normalizes aqueous concentrations by their inherent toxicity. To compute a TU for each component, the estimated concentration, Cw, from the partitioning model is divided by the aqueous effect concentration (e.g., LC50 or NOEC). The individual toxic units are then summed to compute the toxicity of the mixture. The lethal or chronic loading is determined by iterating until a loading is determined that yields one TU.

The model contains a database of TLM parameters for more than 70 individual organisms including several fish, daphnid, and algal endpoints. The model contains a database of TLM parameters for more than 70 individual organisms including several fish, daphnid, and algal endpoints. In the recent work by McGrath et al. (2018), amongst other updates, the chronic toxicity database was revised and includes the major petroleum substance categories. The database for the ACRs was compiled from paired acute and chronic toxicity data (data generated for the same chemical and species in the same laboratory). The current database comprises 72 ACRs including paraffins, olefins, naphthenics, aliphatic, MAH and PAH classes; covers 20 different species and includes 7 taxonomic groups. The species geometric average ACR is 5.22 and was used in the PetroTox calculations. The TLM framework was validated previously for acute and chronic aquatic toxicity endpoints. As such, modeled data are considered fit for purpose for classification and labelling and risk assessment.
Principles of method if other than guideline:
The aquatic toxicity was estimated by a QSAR, the PETROTOX computer model (v 4.0). This model combines a partitioning model used to calculate the aqueous concentration of hydrocarbon components with the Target Lipid Model used to calculate acute and chronic toxicity of non-polar narcotic chemicals. PETROTOX computes toxicity based on the summation of the aqueous-phase concentrations of hydrocarbon block(s) that represent a hydrocarbon substance and membrane-water partition coefficients (Kmw) that describe the partitioning of the hydrocarbons between the water and organism. For model description and justification of QSAR prediction: see field 'Justification for type of information'.
Analytical monitoring:
not required
Test organisms (species):
Raphidocelis subcapitata (previous names: Pseudokirchneriella subcapitata, Selenastrum capricornutum)
Test type:
other: QSAR
Total exposure duration:
72 h
Key result
Duration:
72 h
Dose descriptor:
EL50
Effect conc.:
25.3 mg/L
Nominal / measured:
estimated
Conc. based on:
other: QSAR prediction
Basis for effect:
growth rate
Remarks on result:
other: QSAR predicted value
Key result
Duration:
72 h
Dose descriptor:
EL10
Effect conc.:
4.84 mg/L
Nominal / measured:
estimated
Conc. based on:
other: QSAR prediction
Basis for effect:
growth rate
Remarks on result:
other: ACR 5.22 (McGrath et al., 2018)
Remarks:
QSAR predicted value
Details on results:
Chronic endpoints were estimated using typical acute-to-chronic ratio of 5.22 (McGrath et al., 2018).
Validity criteria fulfilled:
yes
Conclusions:
The PETROTOX-estimated freshwater 72-h EL50 (Effect Loading Rate 50%) value for Pseudokirchneriella subcapitata is 25.3 mg/L based on growth rate, and the PETROTOX-estimated freshwater 72-h EL10 (Effect Loading Rate 10%) value for Pseudokirchneriella subcapitata is 4.84 mg/L based on reproduction.
Executive summary:

The aquatic toxicity was estimated using the PETROTOX computer model (v4.0), which combines a partitioning model used to calculate the aqueous concentration of hydrocarbon components as a function of substance loading with the Target Lipid Model used to calculate acute and chronic toxicity of nonpolar narcotic chemicals. PETROTOX computes toxicity based on the summation of the aqueous-phase concentrations of hydrocarbon block(s) that represent a hydrocarbon substance and membrane-water partitioning coefficients (KMW) that describe the partitioning of the hydrocarbons between the water and organism.

The PETROTOX-estimated freshwater 72-h EL50 (Effect Loading Rate 50%) value for Pseudokirchneriella subcapitata is 25.3 mg/L based on growth rate, and the PETROTOX-estimated freshwater 72-h EL10 (Effect Loading Rate 10%) value for Pseudokirchneriella subcapitata is 4.84 mg/L based on reproduction.

Description of key information

The aquatic toxicity was estimated using the PETROTOX computer model (v4.0), which combines a partitioning model used to calculate the aqueous concentration of hydrocarbon components as a function of substance loading with the Target Lipid Model used to calculate acute and chronic toxicity of nonpolar narcotic chemicals. PETROTOX computes toxicity based on the summation of the aqueous-phase concentrations of hydrocarbon block(s) that represent a hydrocarbon substance and membrane-water partitioning coefficients (KMW) that describe the partitioning of the hydrocarbons between the water and organism.

Key value for chemical safety assessment

Additional information

A supporting study has been reported for the analogue substance Pentane. This study is summarised below.

In a 72 -hour study, Pseudokirshneriella subcapitata was exposed to Pentane. the EC50 (growth)growth was reported to be 10.7 mg/l and the NOEC (growth) was reported to be 2.04 mg/l. The study follows the standard guideline with only minor deviations which are not believed to have effected the test performance.