A simple solid phase assay for the detection of 2,4-D in soil

Contaminated soils are usually characterized using chemical analyses. However, these do not assess the bioavailability of pollutants, a factor which may be important in estimating the risks associated with contamination. Thus there is a need to support chemical analyses with information on biological effects to determine the potential risks a pollutant may pose in the soil. Although bacterial bioreporters have been used to detect the presence of contaminants in soils, in general these studies have been carried out in slurries or soil extracts rather than soil itself. The following study presents the development of a simple solid-phase bioassay for the direct detection of the herbicide 2,4-dichlorophenoxy acetic acid (2,4-D) in soil using Ralstonia eutropha JMP 134-32, a luxCDABE-based 2,4-D whole cell bioreporter. The bioreporter was spotted onto glass microfibre filter discs that allowed its retrieval and analysis after exposure to 2,4-D amended soils. These disc-fixed cells responded in a concentration dependent manner to 2,4-D in solution (0-25 mg/L) and in spiked soil (0-50 mg/kg). The influence of environmental factors on bioavailability was demonstrated in soil with a low moisture content which prevented 2,4-D-induced bioluminescence but which did not affect bioluminescence from already induced cells. This rapid and low cost bioassay provides a proof of concept demonstrating that retrievable disk-fixed cells can be induced in soil, thus providing a measure of solid-phase bioavailability. This method overcomes some of the limitations associated with the inoculation and monitoring of bioreporters directly in soil. Additionally, this simple system should be amenable to use with other bioreporters.