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Bioaccumulation: aquatic / sediment

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Description of key information

Glycerides, mixed C8-10 and succinyl is expected to show low bioaccumulation potential

Key value for chemical safety assessment

Additional information

5.3.1 Aquatic bioaccumulation

No experimental data evaluating the bioaccumulation potential of Glycerides, mixed C8-10 and succinyl (CAS No. 91744-56-8) are available. However, the information gathered on environmental behaviour and metabolism in combination with the QSAR-estimated BCF values provide enough evidence (in accordance to the REACh Regulation (EC) No. 1907/2006, Annex XI General rules for adaptation of the standard testing regime set out in Annexes VII to X, 1.2, to cover the data requirements of Regulation (EC) No. 1907/2006, Annex IX) to state that this substance is likely to show low bioaccumulation potential.

 

Intrinsic properties and fate

Glycerides, mixed C8-10 and succinyl (CAS No. 91744-56-8) is readily biodegradable. According to the Guidance on information requirements and chemical safety assessment, Chapter R.7b, readily biodegradable substances can be expected to undergo rapid and ultimate degradation in most environments, including biological Sewage Treatment Plants (STPs)(ECHA, 2012). Therefore, after passing through conventional STPs, only low concentrations of this substance are likely to be (if at all) released into the environment.

The water solubility of Glycerides, mixed C8-10 and succinyl was determined to be 1-2 mg/L (at 20°C). On the other hand, the log Pow value (> 10) indicates high adsorption potential of the substance to solid particles. Therefore, besides being extensively biodegraded in STPs (due to its ready biodegradability), a significant degree of removal of this substance from the water column due to adsorption to sewage sludge can be expected (Guidance on information requirements and chemical safety assessment, Chapter R.7a (ECHA, 2012)). Discharged concentrations into the aquatic compartment are therefore likely to be low. Should the substance be released into the water phase, it will tend to bind to sediment and other particulate organic matter due to its adsorption potential, being the actual dissolved fraction available to fish via water low (Mackay and Fraser, 2000). Thus, the most relevant exposure route for aquatic organisms such as fish will be via food ingestion or contact with suspended solids.

According to the Guidance on information requirements and chemical safety assessment, Chapter R7.c (ECHA, 2012), the potential for bioaccumulation can be estimated from the log Kow value of the substance as a screening approach. Generally, at log Kow values > 6 a decrease in BCF values is observed, probably caused by the reduced uptake with the expected increasing molecular size of such substances. Even though experimental data evaluating bioaccumulation for substances with log Kow > 10 is not known, these substances are expected to have BCF values < 2000 L/kg (criterion used to consider a substance Bioaccumulative)(Guidance on information requirements and chemical safety assessment, Chapter 11 (ECHA, 2012).

Metabolism

After lipid content, the degree of biotransformation seems to be the most relevant factor regarding the bioaccumulation of organic chemicals in aquatic organisms (Katagi, 2010). Biotransformation consists in the conversion of a specific substance into another/other (metabolites) by means of enzyme-catalyzed processes (ed. van Leeuwen and Hermens, 1995). Carboxylesterases are a group of ubiquitous and low substrate specific enzymes, involved in the metabolism of ester compounds in both vertebrate and invertebrate species, including fish (Leinweber, 1987; Barron et al., 1999).

Glycerides, especially triglycerides, are the predominant lipid class in the diet of both marine and freshwater fish. Once ingested, they will be hydrolized into fatty acids and glycerol by a specific group of carboxylesterase (CaE) enzymes (lipases) as reported in different fish species (Tocher, 2003). In the case of Glycerides, mixed C8-10 and succinyl, the enzymatic hydrolysis is expected to result in C8 and C10 fatty acids, glycerol and succinic acid as transformation products. Part of the free fatty acids will be re-sterified once more with glycerol and partial acyl glycerols to form triglycerides, that will be stored as long-term energy reserves. Glycerol is naturally present in animal and vegetable fats, rarely found in free state (mostly combined with fatty acids forming triglycerides) (ed. Knothe, van Gerpen and Krahl, 2005). If freely available in aquatic organisms, it will not bioaccumulate in view of its log Kow value of -1.76 (OECD SIDS, 2002). Especially in periods in which the energy demand is high (reproduction, migration, etc.), glycerides are mobilized from the storage sites as source of fatty acids. Fatty acid catabolism is the most important energy source in many species of fish, resulting in the release of acetyl CoA and NADH (through β-oxidation) and eventually, via the tricarboxylic cycle, the production of metabolic energy in the form of ATP. This fatty acid-catabolism pathway is the predominant source of energy related to growth, reproduction and development from egg to adult fish. A similar metabolic pathway is observed in mammals (see section 7.1.1 Basic toxicokinetics).

Moreover, succinic acid is naturally present in mammals and fish. The combination of its ionized form (succinate) with coenzyme A (CoA) plays a major role as intermediate in the citric acid cycle (succinyl-CoA)(Lehninger, Nelson and Cox (1994); De Silva and Anderson, 1995). Therefore, it is expected to be extensively metabolized and not bioaccumulated in aquatic organisms.

According to the Guidance on information requirements and chemical safety assessment, Chapter R.7c (ECHA, 2008), even though ready biodegradability does not per se preclude bioaccumulation potential, generally (depending on exposure and uptake rates) ready biodegradable substances are likely to be rapidly metabolised, and therefore, concentrations stored in aquatic organisms will tend to be low.

 

Experimental data

The bioaccumulation potential of a structurally similar substance, glycerol tristearate (fatty acid component C18 triester, CAS No. 555-43-1) in fish, algae and activated sludge has been investigated in a publication by Freitag et al. (1985). Leuciscus idus melanotus, Chlorella fusca and activated sludge microorganisms were exposed for 3 days, 24 hours and 5 days respectively to the test substance, within a static water regime at a nominal concentration of 0.05 mg/L. After the exposure period, the resulting BCF values were < 10 (fish), 15840 (algae) and 3600 (activated sludge). The large differences between the results reported for fish with respect to those from algae and activated sludge suggest the role of a higher and more developed metabolic system which will allow for biotransformation as described in the previous section. In view of these results, the bioaccumulation potential of glycerol tristearate in fish species is expected to be low.

In conclusion, the experimental data available for glycerol tristearate indicate that low bioaccumulation of Glycerides, mixed C8-10 and succinyl in fish species can be expected.

QSAR data

Additional information on the bioaccumulation of Glycerides, mixed C8-10 and succinyl in fish species is available. Estimated bioconcentration (BCF) and bioaccumulation (BAF) values were calculated for all relevant components of the substance using the BCFBAF v3.01 program, assuming biotransformation (Arnot-Gobas method). Even though the components are not within the applicability domain of the model (covering only substances with log Kow values in the range 0.31-8.70), the estimated values can be considered as supporting data indicating low bioaccumulation of the substance. BCF and BAF values ranged from 0.89 to 1 L/kg.

Conclusion

Glycerides, mixed C8-10 and succinyl (CAS No. 91744-56-8) is not expected to be bioaccumulative. Due to its readily biodegradable nature, extensive degradation in conventional STPs will take place and only low concentrations are expected to be released (if at all) into the environment. If present in the aquatic compartment, Glycerides, mixed C8-10 and succinyl will be bioavailable to aquatic organisms such as fish mainly via feed and contact with suspended organic particles, due to its high adsorption potential (log Kow > 10). After uptake by fish species, extensive and fast biotransformation of the substance by carboxylesterases into fatty acids, glycerol and succinic acid is expected. On the other hand, the bioaccumulation potential of a structurally related substance, (Glycerol tristearate, CAS No. 555-43-1) was experimentally determined, resulting in a BCF < 10 in fish (Leuciscus idus melanotus). High BCF values were reported for algae (15840) and activated sludge (3600), reflecting the lack of a developed metabolic system as present in fish. These findings support the argument that in fish species, rapid metabolism takes place and therefore, low bioaccumulation is expected. The supporting BCF/BAF values estimated with the BCFBAF v3.01 program also indicate that these substances will not be bioaccumulative (0.89 to 1 L/kg).

The information above provides strong evidence supporting the statement that rapid metabolism and low bioaccumulation potential can be expected for Glycerides, mixed C8-10 and succinyl.

 

A detailed reference list is provided in the technical dossier (see IUCLID, section 13) and within the CSR.