The role of Sphingobium chlorophenolicum in pentachlorophenol degradation in different systems: batch culture, soil and hydroponic systems

Sphingobium chlorophenolicum is well known as pentachlorophenol (PCP) degrader. The objective of this study was to evaluate the PCP degradation in different systems: batch culture, soil and hydroponic systems using phytoremediation and bioaugmentation. In batch culture, a basal minimal medium with or without rhizosphere exudates extracted from winter wheat was used. For the degradation experiments loamy and sandy soils were selected. The plants used were winter wheat, maize, rice and peas. Measurements of PCP concentrations were carried out using high performance liquid chromatography analyses (HPLC). The toxic effect of PCP on plants was studied through the monitoring of weight plant. The biodegradation of PCP by S. chlorophenolicum was assessed with a bioluminescence assay of Escherichia coli HB101 pUCD607. Bacterial analyses were carried out by plating on Mineral Salt Medium (MSM) for S. chlorophenolicum, MSM for PCP-degrading/tolerant organisms and Trypticase Soy Broth Agar (TSBA) for heterotrophic organisms. In batch culture, rhizosphere exudates stimulated the growth of the cells of S. chlorophenolicum at concentrations of 50 and 80mg kg dry wt soil ^-1 as well as stimulating the ability of S. chlorophenolicum to degrade PCP at a concentration of 80mg Kg dry wt soil ^-1. In addition, pentachlorophenol had an adverse effect on the growth of S. chlorophenolicum. Under hydroponic conditions, S. chlorophenolicum was able to degrade 16 mg L^-1 PCP. 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 this organism. A stimulation of bacterial population of S. chlorophenolicum to plant exudation in hydroponic cultures was also observed. In soil systems, the introduction of S. chlorophenolicum into a loamy soil with plants showed a faster degradation when compared to the non inoculated soil. Plants acted as a vector to the target compound, PCP. The monitoring of the plant growth showed a protective role of S. chlorophenolicum against the toxicity of PCP. In the sandy soil, there was no significant PCP degradation since an estimated 75 % of PCP added was sorbed to the soil and unavailable for degradation. PCP had an accentuated effect on plant growth of maize than winter wheat as shown by plant growth measurements. In both soils tested, there was a significant increase of organisms tested in the roots in comparison to those in the soil. The bioassay confirmed that initial toxicity was lowered while degradation progressed. This study showed that the presence of S. chlorophenolicum enhanced the PCP degradation in a loamy soil with a protective role to prevent phytotoxic effects of PCP on plant growth. The combined use of bioaugmentation and plants suggests that the rhizosphere of certain plant species may be important for facilitating microbial degradation of pesticides in soil with important implications for using vegetation to stabilize and remediate surface soils.