Evaluation of the potential for rhizoremediation in the breakdown of pesticides
An evaluation of the potential breakdown of xenobiotics compounds such as atrazine, naphthalene and pentachlorophenol (PCP) using bioaugmentation was carried out. Results showed that Pseudomona putida F1 Tn5 luxCDABE a poor degrader of both atrazine and naphthalene, the first two target compounds. Pentachlorophenol (PCP) was the third compound used and the organism selected was Sphingomonas chlorophenolica ATCC 39723. Results showed that the presence of an additional carbon source (glutamate) and rhizosphere exudates significantly stimulated the degradation capability of S. chlorophenolica. The specific degradation rates decreased as the PCP concentration increased. At lower concentrations, PCP degradation was more effective when in the presence of a supplementary source of carbon, while at higher PCP concentrations the degradation was more effective in their absence. Hydroponic studies were carried out to assess the sensitivity of a range of plants such as maize, winter wheat, peas and rice to PCP (20 and 30 mg 1-1). Rice and peas were shown to be more tolerant to PCP while maize and winter wheat were more sensitive as measured by the toxicity response. The presence of the inoculum in this situation did not increase the tolerance of the organism. Degradation was better performed at a lower concentration (20 mg 1-1) in the presence of inoculum and plant (winter wheat and peas) when compared to the plant alone. A stimulation of bacterial populations of S. chlorophenolica to plant exudation in hydroponic cultures was also observed. In order to evaluate the enhancement of degradation of PCP in the rhizosphere and to assess the performance of S. chlorophenolica ATCC 39723 as a bioaugmentation agent, soil experiments were carried out in presence and absence of plants. Measurements of soil dehydrogenase activity and PCP concentrations were carried out. Enumeration of soil organisms and S. chlorophenolica was monitored. An acute toxicity assay of PCP in the soil was performed using Escherichia coli HB101 pUCD607. In Boyndie soil (loamy sandy soil), degradation of PCP occurred completely within 6 days when the plant and degrader were present, although PCP had no significant effect on dehydrogenase activity and biomass C content. In addition to enhancing the degradation rate, the presence of the inoculum S. chlorophenolica protected the plant against the toxicity of PCP.