Degradation of Phenoxyalkanoic acid Herbicides by Sphingomonas herbicidovorans MH in soil
Application of herbicides on agricultural areas, golf courses, lawns, ornamental and sports turf, in low or high doses can lead to a contamination of the soil or even of thegroundwater, where the substances can delude the quality needed for water treatment.
To avoid undesired contamination of herbicides in the underground this research was performed mainly with the idea to increase the possiblities of bacterial degradation of the herbicides. Herbicides are known to be biodegradable in soils but usually not with sufficiently high degradation rates. Specific bacterial strains have been isolated, which have the ability for fast degradation of herbicides. Sphingomonas herbicidovorans strain MH was isolated in the beginning of the nineties is among those herbicidedegrading microorganisms. S. herbicidovorans MH is able to degrade a variety of phenoxy acetic acids, among which the chiral (RS)-2-(4-chloro-2-methylphenoxy) propionic acid (mecoprop). Several studies on the degradation of mecoprop and on the ability of strain MH suggested that strain MH might be a good candidate for targeted or enhanced mecoprop bioremediation (or bioaugmentation).
In the present study, the ability of S. herbicidovorans MH to degrade mecoprop in soil cosms was investigated. Mecoprop was extracted from soil with water and measured by high-pressure liquid chromatography (HPLC). (2,4-dichlorophenoxy) acetic acid) (2,4-D) was taken as an internal standard. Strain MH was detected in soil with a DNA-isolating procedure and with PCR amplification of the chlorocatecholdioxygenase gene (clcA1), which is responsible for the degradation of mecoprop
First liquid cultivation was used to find an optimal culture condition which would lead to an active mecoprop degradation capacity of the cells and sufficient cell biomass. Such culture conditions were found in a combination of pyruvate and - mecoprop mineral medium. Afterwards, laboratory soil microcosms were carried out, which were either inoculated or not, to observe the degradation of mecoprop by strain MH. This replicated experiment was very successful and indictaed that strain MH can result in five times faster degradation rates for mecoprop in nonsterile soil microcosms. Finally, a mesocosm experiment was carried out in garden beds close to the EAWAG. Different treatments of strain MH and mecoprop were applied to confirm first the degradation of mecoprop under those conditions and secondly to select a best application method. Unfortunately, this experiment did not lead to any positive results. The mecoprop was degraded in the soil, but there was no influence of addition of strain MH on the observed degradation rates. It was assumed that the inoculated cells were not active enough, but this could not be repeated or tested.
At the end, ideas for further experiments in the examination of S. herbicidovorans MH as a mecoprop degrader in soils are proposed and shortly discussed.