Mecoprop

Additional information on environmental fate and behaviour

Administrative data

Endpoint:

additional information on environmental fate and behaviour

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

Data source

Reference

Reference Type:

publication

Title:

Enantioselective degradation of the herbicide mecoprop [2-(2-methyl-4-chlorophenoxy) propionic acid] by mixed and pure bacterial cultures.

Author:

Tett VA, Willetts AJ & Lappin-Scott HM

Year:

1994

Bibliographic source:

FEMS Microbiology Ecology (14): 191-200

Materials and methods

Test guideline

Qualifier:

no guideline followed

Principles of method if other than guideline:

A consortium of three bacteria was isolated from top soil through their capacity to utilise the chlorinated, aromatic herbicide test material as a single growth substrate.
The paper describes a novel laboratory study of test material degradation by both pure and mixed microbial cultures. This is an indicator of the potential of microorganisms to degrade the test material in situ. This work also includes an investigation of the ability of a microbial community to distinguish between the (±) stereoisomers of the test material. Finally, the ability of the consortium to degrade structurally related compounds that are often introduced into the environment as mixed formulations with the test material, to maximise weed control, was reported on.

GLP compliance:

not specified

Type of study / information:

Study of test material degradation by both pure and mixed microbial cultures

Test material

Test material information

Constituent 1

Reference substance name:

Mecoprop

EC Number:

230-386-8

EC Name:

Mecoprop

Cas Number:

7085-19-0

Molecular formula:

C10H11ClO3

IUPAC Name:

2-(4-chloro-2-methylphenoxy)propanoic acid

Test material form:

not specified

Results and discussion

Any other information on results incl. tables

An inoculum taken from garden soil was incubated in batch culture using test material as the sole source of carbon and energy. Progressive enrichment resulted in the isolation of a consortium of three bacteria capable of growth on racemic test material. The consortium comprised Alcaligenes denitrificans, Pseudomonas glycinea and Pseudomonas marginalis. The organisms formed a tight association and because of their similar nutritional characteristics were difficult to separate. Growth of the consortium was monitored on the racemic herbicide and both the (R)-(+)- and (S)-(-)-enantiomers in independent experiments using an equivalent amount of total carbon for comparative purposes. During growth on the racemate, onset of chloride ion release occurred after a lag phase of 8 h. Degradation of the aryl moiety as monitored by the stoichiometric release of chloride ions took 37 h to complete.

The final accumulated chloride ion concentration equalled half the maximum calculated value for the complete degradation of the supplied racemic test material. Growth of the consortium supported by the (R)-(+)-enantiomer followed a similar time course, although in this case the final chloride ion concentration equalled the theoretical maximum present from the added herbicide. When growth on the (R)-(+)-enantiomer and racemate of test material had ceased, the maximum culture absorbance was 0.95 and 0.62 respectively, which corresponded to 0.63 and 0.37 g/L wet weight biomass. In both types of media, there was no significant difference between the maximum specific growth rate and release of chloride ions (P = 0.167) and the maximum population phase was reached at the time chloride ion release was completed. There was no growth of the consortium on the (S)-(-)-enantiomer and no subsequent release of chloride ions. Autoclaved, uninoculated growth media containing test material was used as a chloride ion release control to demonstrate there was no abiotic transformation of the herbicide.

Spectrophotometric scans of culture supernatants taken from both types of media were determined with time over the range 200 - 500 nm. During growth and chloride ion release, there was a concomitant decrease in the size of the 279 nm peak, probably demonstrating the disruption of the conjugated aryl moiety in the primary test material structure. During growth on the (R)-(+)-enantiomer, the decrease in A279 corresponded to a reduction in concentration from 1 to 0.03 g/L. This represents a 97 % reduction in the initial herbicide concentration as compared with only a 55 % decrease in test material concentration during growth on the racemate. The absorption maximum of media containing the racemate and the single isomer both remained at 279 nm until after 14 h of exponential growth when it shifted to 286 and 291 nm, respectively. After 35 - 40 h into the growth cycle, there was a noticeable decrease in UV absorption of the culture supernatant formulated with the (R)-(+)-enantiomer. The racemic carbon source showed no equivalent change and the residual test material in this culture remained stable on extended incubation. The decrease in the size of the 279 nm peak during incubation of the consortium on the (S)-(-)-enantiomer was negligible confirming chloride ion and culture absorbance analyses that it could not support growth.

The three-member community was also able to grow on the other herbicides as sole carbon and energy sources.

In closed culture, the proportion of the community formed by each member varied with the age of the culture. Typically, a culture had the following composition after the completion of test material degradation: A. denitrificans (2.2 x 10^9 cfu/mL), P. marginalis (2.4 x 10^7 cfu/mL) and P. glycinea (2.6 x 10^5 cfu/mL). The consortium remained able to degrade test material after twenty passages through nutrient broth. After this period of non-selective culture, growth on test material required 72 h for completion and there was a slight increase in the lag phase. Samples were periodically removed to test for pure culture degradation of test material. Initially, none of the isolates was able to degrade test material in pure culture or in combination with any other single member of the consortium. However, after five months of sub-culturing and after approximately 80 days of chemostat growth, A. denitrificans acquired the complete capacity to degrade the test material as a single substrate. The isolate from the batch culture was chosen for further study. It formed recognisable colonies on test material selective agar. Growth, expressed as an increase in culture absorbance, occurred after a lag phase of 5 h and was associated with a drop in pH from 7.1 to 6.4. Release of chloride ions was initiated 12 h into the growth cycle. The doubling time of the organism during the exponential growth phase in batch culture was 6.5 h. There was no significant difference between growth rates of the consortium and pure culture (P = 0.184).

In chemostat culture with (R)-(+)-test material at a growth limiting concentration of 0.56 g of carbon/L and a dilution rate of 0.024/h, the three-member community was maintained as a stable association for over two months of continuous growth. After an initial fall in cfu, possibly attributed to wash-out of the inoculum, all three isolates stabilised with A. denitrificans remaining the dominant organism in the culture. The absorbance of the culture was constant at 0. 74 ± 0.04, a value that corresponded to approximately 90 % breakdown of the herbicide enantiomer in batch culture. After this period of growth in the chemostat, A. denitrificans was able to grow independently on racemic and (R)-(+)-test material as the sole carbon and energy source. The other two bacteria were not washed out of the chemostat but remained as a tight consortium.

Applicant's summary and conclusion

Conclusions:

The culture exclusively degraded the (R)-(+)-isomer of the test material while the (S)-(-)-enantiomer remained unaffected. Initially, no single member of the bacterial consortium was able to degrade the test material as a pure culture but after prolonged incubation, A. denitrificans was able to grow on the herbicide as the sole source of carbon and energy.

Executive summary:

A consortium of three bacteria was isolated from top soil through their capacity to utilise the chlorinated, aromatic herbicide test material as a single growth substrate.

This paper describes a novel laboratory study of test material degradation by both pure and mixed microbial cultures. This is an indicator of the potential of microorganisms to degrade the test material in situ. This work also includes an investigation of the ability of a microbial community to distinguish between the (±) stereoisomers of the test material.

The culture exclusively degraded the (R)-(+)-isomer of the test material while the (S)-(-)-enantiomer remained unaffected. Initially, no single member of the bacterial consortium was able to degrade the test material as a pure culture but after prolonged incubation, A. denitrificans was able to grow on the herbicide as the sole source of carbon and energy.