Methane oxidation as a method to evaluate the removal of 2,4-dichlorophenoxyactic acid (2,4-D) from soil by plasmid-mediated bioaugmentation

Abstract

The herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) is known to inhibit methanotrophic bacteria. Methane oxidation was therefore used as a parameter to evaluate the residual 2,4-D after bioaugmentation of an agricultural soil. Several strains harbouring catabolic plasmids which code for the degradation of this pesticide, were compared for their potential to alleviate the negative impact of 2,4-D on methane oxidation by soil microorganisms. Three indigenous soil bacteria which contain the 2,4-D degradative plasmid pEMT1k, obtained from a donor by in situ plasmid transfer in previous experiments, were compared with Ralstonia eutropha JMP134, which harbours the well studied 2,4-D degradative plasmid pJP4. In addition a Pseudomonas putida UWC3(pEMT1k), which does not degrade 2,4-D, was used as donor to investigate the potential bioaugmentation through in situ transfer of the catabolic genes towards the indigenous soil bacteria. Both the strains that can degrade 2,4-D as well as the P. putida donor strain could enhance the recovery of methane oxidation by increasing the rate of degradation of 2,4-D and thus removing its toxic effect on the methane oxidising microbial populations. In all cases the time needed to oxidise methane was consistently shorter (4–10 days) in a 2,4-D treated soil inoculated with the strains, than in the non-inoculated 2,4-D treated soil, but still longer (5–10 days) than in the soil without 2,4-D. These data indicate that pesticide residues as well as their toxic effect on important soil microbial processes could be successfully removed from the soil by addition of well adapted specialised strains with the genetic information required to degrade the applied pesticides.