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Toxicity to microorganisms

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Endpoint:
activated sludge respiration inhibition testing
Type of information:
experimental study
Adequacy of study:
weight of evidence
Study period:
1988
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Qualifier:
according to
Guideline:
OECD Guideline 209 (Activated Sludge, Respiration Inhibition Test
Deviations:
yes
Remarks:
see below
GLP compliance:
not specified
Specific details on test material used for the study:
Also other chemicals were tested in this study.
Test organisms (species):
activated sludge of a predominantly domestic sewage
Test type:
other: See below
Key result
Duration:
30 min
Dose descriptor:
EC50
Effect conc.:
520 mg/L
Nominal / measured:
nominal
Conc. based on:
test mat.
Basis for effect:
not specified
Validity criteria fulfilled:
yes
Conclusions:
EC50: 520 mg/L/30 min (activated sludges)
Endpoint:
toxicity to microorganisms, other
Type of information:
experimental study
Adequacy of study:
weight of evidence
Study period:
1991
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:
In this study, a set of chemicals (50 to 100 per species) was assayed to determine the toxicity to three groups of environmental bacteria. Testing methods for each group were designed to be as similar as possible while taking into account the differing metabolic needs of the bacteria so that the results would be com parable. The environmental bacteria tested were aerobic heterotrophs (from activated sludge wastewater treatment plant), Nitrosomonas (from the mixed liquor of an activated sludge plant treating meat-packing, rendering, and hide-curing waste water), and methanogens (anaerobic toxicity assays - ATA), key agents in the natural recycling of organic material in the environment and in wastewater treatment systems.
Moreover, Microtox® testing was performed using a Microtox® model 2055 toxicity analyzer and standard procedures recommended by the Microbics Corporation. The test is based on the bioluminescence of reconstituted freeze-dried Photobacterium phosphoreum (Microtox® bacteria) as a measure of biological activity.
Finally, acute toxicity to the fathead minnow has been tested for numerous compounds (Center for Lake Superior Environ. Studies, Volumes 1 to 5). The 96-hour flow-through LC50 toxicity data were obtained for compounds studied in the present research to compare to their toxicity to the bacteria.
GLP compliance:
not specified
Specific details on test material used for the study:
Also other chemicals were tested in this study.
Test organisms (species):
other: see summary below
Test type:
not specified
Remarks on result:
other: See table below

For 1,2-dichloropropane

LC50 fathead minnow

130 mg/L/96h

 

IC50 Microtox®

59 mg/L

 

IC50 Nitrosomonas

43 mg/L

 

IC50 Methanogens

180 mg/L

Note: the chlorinated aliphatic hydrocarbons are more toxic to the methanogens by about one order of magnitude respect to aerobic heterotrophs. See also below.

Most of the chemicals tested act by a nonreactive toxicity mechanism. Reactive toxicity is caused by specific chemical in teractions such as reaction with an enzyme or interference with a metabolic pathway. In contrast, nonreactive toxicity (also called nonspecific toxicity or narcosis, a sort of baseline toxicity) is a function of the quantity of toxicant that partitions into the biophase.

Of the toxicants we considered, those that have been previously identified as known or suspected reactive toxicants are the al dehydes (Kamlet, 1987), acrylates (Russom et al, 1988), acry lonitrile (Kamlet, 1987), and compounds with low pKa values (Scherrer and Howard, 1979, and Schultz, 1987).

We found that chemicals with nitro functional groups and chlorinated aliphatic hydrocarbons and chlorinated alcohols showed enhanced toxicity to the methanogens only. Chemicals from these classes were not included in comparisons and correlations.

Results of t-tests conducted to compare the sensitivities of each pair of bacteria and fish indicate whether the log IC50 values are greater than, less than, or not significantly different from each other at the 95% confidence level and the best estimate of the mean difference between the log IC50 values for the pair of organisms and the 95% confidence interval for that estimate.

Nitrosomonas, Microtox®, and the fathead minnow all had equal sensitivities and were about one order of magnitude more sensitive than the equally sensitive aerobic heterotrophs and methanogens.

Correlations made between toxicity data for each pair of bacteria and fish were involved in successful correlations.

The aerobic heterotrophs and methanogen toxicity values, with the exception of the chlorinated aliphatic hydrocarbons, are of the same magnitude as each other and well correlated. Excluded compounds are more toxic to the methanogens by about one order of magnitude. The Microtox® bacteria, Nitrosomonas, and the fathead minnow are more sensitive than the other two bacteria by about one order of magnitude and all have similar sensitivities to one another.

There is no significant difference between the sensitivities of the aerobic heterotrophs and the methanogens for most classes of chemicals tested. Anaerobic processes have a poor reputation for being more susceptible to process upsets. Our research in dicates that the cause of these upsets may not be toxicity to methanogens. Solids retention time and other operating con ditions may be primary determinants of process stability. The important caveat to this conclusion is that for chlorinated ali phatic hydrocarbons and alcohols, the methanogens are more sensitive. When these compounds are present, aerobic processes will be more resistant to toxic upsets. The best estimate of the mean difference in log IC50 values between aerobic heterotrophs and methanogens for the chlorinated aliphatic hydrocarbons and alcohols is 1.22 +/0.43.

The finding that aerobic heterotrophs are significantly less sensitive (approximately one order of magnitude) than Nitrosomonas supports the common observation that in aerobic processes combining carbon oxidation and ammonia conversion, the nitrification process is more susceptible to upsets than carbon oxidation. This phenomenon is caused or exacerbated by the lower growth rate of the nitrifiers. Our research indicates that greater susceptibility to toxicity may also be important in nitrification upsets.

The greater sensitivity of Microtox® bacteria compared to aerobic heterotrophs also confirms previous observations (Dutka et al, 1983; King, 1984; and Reteuna et al, 1986). Because Microtox® is more sensitive, it can be used as a screening measurement of toxicity to the aerobic heterotrophs. A toxicant concentration that does not inhibit Microtox® will generally not inhibit the less sensitive aerobic heterotrophs.

In conclusion, aerobic heterotrophs and methanogens showed the same sensitivity to nonreactive toxicants, with the exception of the enhanced toxicity of the chlorinated aliphatic hydrocarbons and chlorinated alcohols to the methanogens. Nitrosomonas, Microtox®, and the fathead minnow showed the same sensitivity to toxicants, which was significantly greater than the sensitivity of the aerobic heterotrophs and methanogens.

Validity criteria fulfilled:
not applicable
Conclusions:
In conclusion, aerobic heterotrophs and methanogens showed the same sensitivity to nonreactive toxicants, with the exception of the enhanced toxicity of the chlorinated aliphatic hydrocarbons and chlorinated alcohols to the methanogens. Nitrosomonas, Microtox®, and the fathead minnow showed the same sensitivity to toxicants, which was significantly greater than the sensitivity of the aerobic heterotrophs and methanogens.

For 1,2-dichloropropane:
LC50 fathead minnow: 130 mg/L/96h
IC50 Microtox®: 59 mg/L
IC50 Nitrosomonas: 43 mg/L
IC50 Methanogens: 180 mg/L

Description of key information

First study:

EC50 activated sludge: 520 mg/L/30 min (OECD 209)

Second study:

LC50 fathead minnow: 130 mg/L/96h

IC50 Microtox®: 59 mg/L

IC50 Nitrosomonas: 43 mg/L

IC50 Methanogens: 180 mg/L

Microtox® bacteria and fathead minnow compared to aerobic heterotrophs are more sensitive. Moreover, the chlorinated aliphatic hydrocarbons are more toxic to the methanogens by about one order of magnitude respect to aerobic heterotrophs. Although a IC50 value of 1,2-dichloropropane for aerobic heterotrophs is not available, it is expected that it would be higher than the value for methanogens.

 

The IC50 Microtox® of 59 mg/L can be used as a worst case toxicity value.

Key value for chemical safety assessment

EC50 for microorganisms:
59 mg/L

Additional information