ENHANCEMENT OF 2,4-DICHLOROPHENOXYACETIC ACID (2,4-D) DEGRADATION IN SOIL BY DISSEMINATION OF CATABOLIC PLASMIDS

Few studies have been done to evaluate the transfer of catabolic plasm ids from an introduced donor strain to indigenous microbial population s as a means to remediate contaminated soils. In this work we determin ed the effect of the conjugative transfer of two 2,4-D degradative pla smids to indigenous soil bacterial populations on the rate of 2,4-D de gradation in soil. We also assessed the influence of the presence of 2 ,4-D on the number of transconjugants formed. The two plasmids used, p EMT1k and pEMT3k, encode 2,4-D degradative genes (tfd) that differ in DNA sequence as well as gene organisation, and confer different growth rates to Ralstonia eutropha JMP228 when grown with 2,4-D as a sole ca rbon source. In an agricultural soil (Ardoyen) treated with 2,4-D (100 ppm) there were ca. 10(7) CFU of transconjugants per gram bearing pEM T1k as well as a high number of pEMT3k bearing transconjugants (ca. 10 (6) CFU/g). In this soil the formation of a high number of 2,4-D degra ding transconjugants resulted in faster degradation of 2,4-D as compar ed to the uninoculated control soil. In contrast, only transconjugants with pEMT1k were detected (at a level of ca. 10(3) CFU/g soil) in the untreated Ardoyen soil. High numbers of transconjugants that carried pEMT1k were also found in a second experiment done using forest soil ( Lembeke) treated with 100 ppm 2,4-D. However, unlike in the Ardoyen so il, no transconjugants with pEMT3k were detected and the transfer of p lasmid pEMT1k to indigenous bacteria did not result in a higher rate o f decrease of 2,4-D. This may be because 2,4-D was readily metabolised by indigenous bacteria in this soil. The results indicate that bioaug mentation with catabolic plasmids may be a viable means to enhance the bioremediation of soils which lack an adequate intrinsic ability to d egrade a given xenobiotic.