Naphthenic acids

Toxicity to microorganisms

Link to relevant study record(s)

Reference

Endpoint:

toxicity to microorganisms

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Qualifier:

no guideline followed

Principles of method if other than guideline:

Toxicity Assays.
Microtox analysis was performed on a Microtox Model 500 Analyzer using the Basic Test provided by Azur Environmental. The four monocarboxyl naphthenic acid (NA)-like surrogates were dissolved in 0.01 N NaOH, and the four dicarboxyl NA-like surrogates were dissolved in 0.1 N NaOH to produce sodium salts that were consistent with NAs in alkaline OSPW. The pH of the surrogates was adjusted to 7.5 ( 0.1 using HCl and NaOH. Following the pH titration and the 10% addition of osmotic adjusting solution, the toxicity of the NA-like surrogates was assessed by measuring bioluminescence of V. fischeri following 15 min exposure using three replicates. Nominal concentrations were serially diluted to 100%, 50%, 25%, and 12.5% of the initial test concentration for each of the NA-like surrogates. The EC50 values were calculated as the concentration of NArequired to reduce bioluminescence by 50% in relation to a 0% NA blank solution. A fourparameter logistic model was used to fit regression lines to the data.

GLP compliance:

not specified

Remarks:

Literature data

Specific details on test material used for the study:

The study of Frank et al. 2009 studied the effect of 8 different NA-like surrogates. Only the results based on Cyclohexanecarboxyclic acid (CHCA) were used as supportive results as it was deemed most coparable to the test substance, because it has more than 8 Carbon atoms, 1 ring and is a monoacid)

Test organisms (species):

Vibrio fisheri

Test type:

static

Water media type:

freshwater

Limit test:

no

Total exposure duration:

15 min

Nominal and measured concentrations:

nominal concentrations (mg/L) : 0, 100, 200, 300

Details on test conditions:

Microtox analysis was performed on a Microtox Model 500 Analyzer using the Basic Test provided by Azur Environmental (30). The four monocarboxyl NA-like surrogates were dissolved in 0.01 N NaOH, and the four dicarboxyl NA-like surrogates were dissolved in 0.1 N NaOH to produce sodium salts that were consistent with NAs in
alkaline OSPW. The pH of the surrogates was adjusted to 7.5 ( 0.1 using HCl and NaOH. Following the pH titration and the 10% addition of osmotic adjusting solution (30), the toxicity of the NA-like surrogates was assessed by measuring bioluminescence of V. fischeri following 15 min exposure using three replicates. Nominal concentrations were serially diluted to 100%, 50%, 25%, and 12.5% of the initial test concentration for each of the NA-like surrogates. The EC50 values were calculated as the concentration ofNArequired to reduce bioluminescence
by 50% in relation to a 0% NA blank solution. A fourparameter logistic model was used to fit regression lines to the data.

Key result

Duration:

15 min

Dose descriptor:

EC50

Effect conc.:

13 mg/L

Nominal / measured:

not specified

Conc. based on:

test mat.

Basis for effect:

other: reduction in bioluminescence

Remarks on result:

other: 95% CI +/-1.6 mg/L

Because of the limited availability of larger compounds, the NA-like surrogates investigated in this study had relatively low MWs (116.16-200.23 Da) in comparison to the NAs observed in OSPW (142-500 Da 13, 14). Also, the nonbranched acyclic surrogate structures were likely simpler than OSPW NAs, which are highly branched. However, the goal of this study was to determine the effect that size, structure, and carboxylic acid content had on NA toxicity; therefore, the relationships observed from the studied surrogates in this study could exist for larger compounds more commonly found in OSPW NA mixtures.

Results

NA-like surrogate	Observed EC50 for 15 -min V. fisheri assay(mM)	Observed LC50 for 48hr D. Magna acute lethality assay (mM)
Hexanoic acid (HA)	19.12 +/-2.6	10.04 +/- 1.3
Cyclohexane carboxylic acid (CHCA)	9.62 +/- 1.9	6.67 +/- 0.8
Decanoic acid (DA)	0.33 +/- 0.04	1.27 +/- 0.4
Cyclohexane pentanoic acid (CHPA)	0.07 +/- 0.01	0.59 +/- 0.2
Succinic acid (SA)	627.31 +/- 29	27.3 +/-2.2
Adipic acid (AP)	468.06 +/- 214	20.51 +/- 1.1
1,4 -Cyclohexanedicarboxylic acid (CHDCA)	465.71 +/-21	15.28 +/- 0.7
Cyclohexylsuccinic acid (CHSA)	26.09 +/- 1.9	6.71 +/- 1.2

Validity criteria fulfilled:

yes

Conclusions:

Based on the microtox assay with NA-like surrogates the EC50 found for Vibrio fisheri was 0.07mM for cyclohexane carboxylic acid, which coresponds to 13.0 +/- 1.6 mg/L.

Executive summary:

The bioassays with the NA-like surrogates indicate that toxicity is likely a function of hydrophobicity. As the MW of the surrogates increased, so too did the acute toxicity, while if an additional carboxylic group was present in the compound, the toxicity was significantly decreased. Based on the microtox assay the EC50 found for Vibrio fisheri for the NA surrogate was 0.07mM for cyclohexane carboxylic acid, which coresponds to 13.0 +/- 1.6 mg/L.

Description of key information

Data on microbial toxicity (Vibrio fisheri) are reported In literature for NA surrogates.  In the microtox assay the EC50 for cyclohexane carboxylic acid was 0.07 mM ( 13.0 +/- 1.6 mg/L). Data from biodegradation tests of commercial naphthenic acid and naphthenic acid from oil sands tailings have been used to show that microbial populations are able to survive naphthenic acid exposure. 

Key value for chemical safety assessment

EC50 for microorganisms:

13 mg/L

Additional information

A microtox bioassays on Vibrio fisheri have been performed naphthenic acid surrogate. The lowest observed EC50 is for cyclohexanepentanoic acid and is 13.0 mg/L.