Microbial degradation of the herbicide mecoprop by a rhizosphere community

Rhizosphere microorganisms were grown in batch culture with mecoprop, a selective herbicide, as the sole carbon and energy source. Growth was detected only if the mecoprop concentration was 2.08 mM or less and lag phases of up to 37 days were required before growth commenced. After consecutive subculturing the length of the lag phase decreased to less than 24 hours. Degradation of the herbicide was followed by monitoring inorganic chloride ions released from the mecoprop structure, culture absorbance increases and the disappearance of the maximum ultra-violet absorption peak of mecoprop. The culture enriched to degrade 2.08 mM was able to utilize mecoprop concentrations up to 66.7 mM, but prolonged lag phases were recorded. Analysis of the mecoprop-degrading culture revealed that it contained two Pseudomonads, an Alcaligenes species, Acinetobacter calcoaceticus and a Flavobacterium species. None of the pure cultures was able to utilize mecoprop but some combinations of two or more organisms degraded the herbicide stressing the importance of interacting communities for the degradation of xenobiotic compounds. The two Pseudomonads made up 83.5% of the community, the remaining organisms were minor components. A specific relationship existed between the Pseudomonads based on the provision of a growth factor. The community was a stable association as subculturing for over 300 days failed to reduce the complexity. Growth studies using mixed carbon sources demonstrated that the two Pseudomonads and Alcaligenes species were able to utilize mecoprop when succinate was present as a co-substrate. Pronounced two-phase growth curves were produced with mecoprop degradation occurring after initial growth on succinate. A biodegradation pathway for mecoprop was proposed based upon degradation studies of the structurally related herbicides 2,4-D, MCPA and dichlorprop by the community and the oxidation of these herbicides by mecoprop-grown cells