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Toxicity to soil microorganisms

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Endpoint:
toxicity to soil microorganisms
Type of information:
experimental study
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
comparable to guideline study with acceptable restrictions
Remarks:
Study well documented, meets generally accepted specific principles, acceptable for assessment.
Principles of method if other than guideline:
This study investigated the ability of Streptomyces to utilize different chain length fatty acids as sole carbon and energy sources, and to characterize their uptake system biochemically.
GLP compliance:
not specified
Analytical monitoring:
not required
Test organisms (inoculum):
other: Streptomyces coelicolor
Remarks:
Not applicable. In-vitro assay.
Details on test conditions:
EFFECT PARAMETERS MEASURED: In-vivo fatty acid degradation
Reference substance (positive control):
no

The study indicated that S.coelicolor strain M145 can effectively utilize fatty acids of different chain length, from C4 to C18, as sole carbon energy source. The in vivo ß-oxidation studies in cells grown in the presence or absence of fatty acids (Table 1), and in vitro assay of two enzymes of the pathway, acyl-CoA synthetase and acyl-CoA dehrdrogenase (Table 2), clearly indicate that S. coelicolor constitutively expressed the enzymes of the ß-oxidation cycle, without the need for the induction by a fatty acid of any chain length. The ß-oxidation pathway in this microorganism, instead of being repressed by glucose was, at least for long-chain fatty acids, stimulated by this metabolite.

Table 1: Rate of ß-oxidation of 300 µM of labeled fatty acids by S.coelicolor M145 grown in SMM-oleate of SMM-glucose

Carbon source

Rate of ß-oxidation (nmol min-1mL-1(mg protein)-1)

 

(14C) palmitate

(14C) octanoate

Oleate

2.825

2.050

Glucose

4.500

1.950

Table 2: Acyl-CoA synthetase and acyl-CoA dehydrogenase in crude protein extracts prepared from cells of S.coelicolor M145 grown in SMM-glucose or SMM-oleate

Carbon source

Acyl-CoA synthetase (pmol min-1mL-1)

Acyl-CoA dehydrogenase (U (mg protein)-1)

Oleate

8.3 +/- 0.5

35.0 +/- 0.5

Glucose

95.0 +/- 1.0

40.0 +/- 0.5

 

Endpoint:
toxicity to soil microorganisms
Type of information:
experimental study
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
comparable to guideline study with acceptable restrictions
Remarks:
Study well documented, meets generally accepted specific principles, acceptable for assessment.
Qualifier:
no guideline followed
Principles of method if other than guideline:
The degradation of an oil additive in soil was investigated. Lysimeter was used to follow the migration and progressive biodegradation of the oils by soil microorganisms over time. Also metabolites were identified.
GLP compliance:
not specified
Analytical monitoring:
yes
Details on sampling:
- Sampling method: The lysimeters were cur at various depths into slices of 2.5, 5 or 10 cm thickness. The soil was sieved (mesh size=2 mm) and a 100 g sample was taken according to standard NF X31-100.
- Sample storage conditions before analysis: at -18 °C in a polyethylene bag
Vehicle:
yes
Details on preparation and application of test substrate:
APPLICATION OF TEST SUBSTANCE TO SOIL
- Method: Methyl oleate was applied as an emulsion in water containing 50 g/L R508 (sorbitan ester) as emulsifier.
Test organisms (inoculum):
soil
Total exposure duration:
120 d
Test temperature:
19 - 22 °C (depends on depth of soil)
Nominal and measured concentrations:
Methyl oleate was applied to the soil at its recommended rate of 2 L/ha, equivalent to 5.5 mg/10 mL of water for the surface of the lysimeter.

Degradation of methyl oleate in the soil

Totally degradation had occurred after 60 days. The half-life was determined as 7 days, during this time, it migrated by only 15 cm. The distributions of these metabolites in space and time changed in a more diffuse manner than that of the parent compound. All those found were shorter carbon-chain fatty acids (Table 1).

Table 1: Characterization of the degradation products of methyl oleate

Fatty acid

Log Kow

Maximum depth (cm)

Oleic acid

-

15

Heptadecanoic acid

-

15

Palmitic acid

7.17

25

Pentadecanoic acid

-

25

Myristic acid

6.11

30

Tridecanoic acid

-

30

Lauric acid

4.6

40

Undecanoic acid

-

50

Capic acid

4.09

50

Pelargonic acid

-

60

Caprylic acid

3.05

50

Heptylic acid

1.92

60

Caproic acid

1.87

60

Valeric acid

1.39

60

Butyric acid

0.79

60

Propionic acid

0.33

60

 

None of the metabolites were detected after 60 days, suggesting that methyl oleate was completely degraded at this point. The plant ester did not migrate very deeply in the soil because it was rapidly broken down by microorganisms in the soil and did not have time to migrate. β-oxidation and ω-oxidation led to the appearance of metabolites that migrated to depths of up to 60 cm and were completely degraded within 60 days.

Endpoint:
toxicity to soil microorganisms
Type of information:
experimental study
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Remarks:
Study well documented, meets generally accepted specific principles, acceptable for assessment.
Qualifier:
no guideline followed
Principles of method if other than guideline:
The degradation of the model molecule (pure tristearin) was investigated in three different soil types, to determine the behavior of fatty wastes.
GLP compliance:
not specified
Analytical monitoring:
yes
Vehicle:
no
Test organisms (inoculum):
soil
Total exposure duration:
8 wk
Test temperature:
20 °C
Nominal and measured concentrations:
0.2% (wt/wt)

Free lipids were extracted from representative samples and of the combined replicates of each control and supplemented soil. The concentration of free lipids extracted after 1 and 4 weeks from each series. After the first week a low diminution of total free lipids was observed (GOV: 2.7%; CHA: 2.6%; SOR: 3.5%). After 4 weeks, the amount of free lipids decreased (CHA: 11%; SOR: 8%; GOV: no variations). Fluctuations of lipid concentrations observed with time in the control were attributed to an increased activity of soil microorganisms due to the incubation. The main result was the great increase during the first week of the acid + polar fractions. This probably indicates the oxidation and hydrolysis process of the added compound. The amounts decreased when the incubation prolonged to 4 weeks. These compounds did not accumulate as they are certainly intermediate compounds in the biodegradation process. The evolution of the concentrations of monoacid and di-, keto- and hydroxy- acid fractions significantly increased during the first week. After 4 weeks a decrease of quantities was followed. The increase obtained during the first week. Monocarboxylic acids were then predominant over di-, keto- and hydroxylacids in the three soils. The results show that, due to the soil supplementation with tristearin, free fatty acids were produced. After soil microflora adaption, these compounds are utilized as they are freed by enzymatic hydrolysis. A part of the of the monocarboxyclic acids is probably oxidized to form di-, keto- and hydroxyl-acids. Contrary the acid fractions evolution, the amounts of the neutral fractions increased between 1 and 4 weeks in the supplemental soils. This is due to the increase of the quantity of alcohols and polar neutral compounds. Bio-oxidation processes seem to be more efficient after 4 weeks. After 1 week also a low decrease, compared to the controls, in the amounts of hydrocarbons consecutive to a low increase of the ester fractions.

Main result of the monoacid fractions analysis was the rapid formation of stearic acid in considerable amounts. This result showed that an intense hydrolysis reaction with specific lipase of tristearin had occurred after the soil supplementation. The investigations of ester fractions showed that new alkanoic acids (methyl stearate, ethyl stearate, and propyl stearate), not determined in the controls, were generated in the supplemented soils. Among other processes the following hypothesis to explain the formation of these compounds were proposed:

1.      Bioesterfication of a part of the free stearic acid, released by an enzymatic hydrolysis reaction

2.      Alcoholysis of the triglyceride to form esters, directly

3.      And/or direct formation of these compounds from tristearin with C-C and C-O bond cleavages

Description of key information

Taking all the available information into account effects on soil microorganisms are not expected to be of concern.

Key value for chemical safety assessment

Additional information

No experimental data investigating the effects on soil microorganisms are available for Propyleneglycol dioleate (CAS 105-62-4). Therefore, all available related data is combined in a Weight of Evidence (WoE) approach, which is in accordance to the REACh Regulation (EC) No 1907/2006, Annex XI, 1.2, to adapt the data requirements of Regulation (EC) No 1907/2006 Annex VII - X (ECHA guidance section R.7.11.5.3, page 121).

The test substance is characterized by a high log Koc indicating a potential for adsorption to the soil particles. Tests with soil-dwelling organisms that feed on soil particles are therefore most relevant for the evaluation of soil toxicity of ethylene distearate. In the absence of a clear indication of selective toxicity, an invertebrate (earthworm or collembolan) test is preferred, as outlined in ECHA guidance section R.7.11.5.3, page 122. Read-across data in accordance to Regulation (EC) No 1907/2006 Annex XI, 1.5 from the structurally related substances butylene glycol dicaprylate/dicaprate (CAS 853947-59-8) and decanoic acid, mixed diesters with octanoic acid and propylene glycol (CAS 68583-51-7) did not show any mortality to earthworms (Eisenia fetida) in acute terrestrial toxicity tests according to OECD 207 and EU Method C.8, respectively (LC50 > 1000 mg/kg soil dw). A plant study with butylene glycol dicaprylate/dicaprate according to OECD 208 with three species from different taxonomic groups resulted in an EC50 of > 100 mg/kg soil for all tested species (three plant species). Moreover, no effects were observed on aquatic microorganisms in two available read-across studies (Diefenbach, 1997 and Mead, 1997). The Guidance Document (ECHA, 2008, page 122) states that a test on soil microbial activity will only be additionally necessary for a valid PNEC derivation if inhibition of sewage sludge microbial activity has occurred and this is clearly not the case. Since the substance is readily biodegradable, it will be degraded quickly. Thus, acute tests with terrestrial organisms from different taxonomic groups in combination with chronic aquatic data and toxicity data on microorganisms indicating no effects up to the limit of water solubility are sufficient to assess that the substance have a very low toxicity to terrestrial organisms.

This is supported by further evidence from literature data. This data showed that soil microorganism communities are well capable of degrading fatty acid esters (Hita et al., 1996 and Cecutti et al., 2002) and use them as energy source (Banchio & Gramajo, 1997). Hita et al. investigated the degradation of the model molecule tristearin which is a triglyceride containing of glycerin tri-esterified with stearic acid in three different soils for 4 weeks. The amount of stearic acid increased in considerable amounts during the experiment showing the hydrolytic activity of lipases breaking the ester bonds. The investigation of ester fractions moreover showed the generation of new alkanoic acids (methyl stearate, ethyl stearate and propyl stearate) which were not determined in the controls. Nevertheless the amounts were no longer present after 4 weeks, which leads to the assumption that degradation by soil microorganisms had occurred. The same was shown by Cecutti et al. One soil sample was chosen and incubated with methyl oleate (plant oil) for 120 d. Methyl oleate and its metabolites were completely degraded after 60 d. Streptomyces coelicolor, a common gram-positive soil bacterium uses fatty acids (C4-C18) as sole carbon end energy source indicating that fatty acids are not-toxic and can be used for catabolism (Banchio and Gramajo, 1997). The available literature data shows that soil microorganisms are capable to break-up ester bonds and degrade fatty acids in significant amounts. Moreover, the data indicated the non-toxic properties of fatty acids since they can be used as energy source.

Taking all the available information into account in a Weight of Evidence approach in accordance with Annex XI, 1.2, effects on soil microorganisms are thus not expected to be of concern, and consequently, no further testing is required.