Trichloroethylene cometabolic degradation by Rrhodococcus sp. L4 induced with plant essential oils and their components

Abstract

Contamination of soil and groundwater with trichloroethylene (TCE) has become an important problem because of its toxicity and persistence. Cometabolic degradation of TCE by toluene-degrading bacteria has the potential for being a cost-effective bioremediation technology. However, the application of toluene may pose environmental problems. In this study, several plant essential oils and their components were examined as alternative inducer for TCE cometabolic degradation in resting cells assay of a toluene-degrading bacterium, Rhodococcus sp. L4. Using the initial aqueous TCE concentration of 14 µM, lemon and lemongrass oil-grown cells were capable of 20% and 27 % TCE degradation, which were lower than that of toluene-grown cells (57 %). The ability of TCE degradation increased to 36% when the bacterium was induced with cumin oil. The induction of TCE-degrading enzymes was suggested to be due to the presence of citral, cumin aldehyde, cumene, and limonene in these essential oils. In particular, the efficiency of cumin aldehyde and cumene as inducers for TCE cometabolic degradation was similar to toluene. TCE transformation capacities (Tc) for these induced cells were between 9.4-15.1 µg of TCE mg cells-1. However, these induced cells were able to effectively stimulate TCE degradation only for a short period. Immobilization of Rhodococcus sp. L4 on plant materials rich in essential oils i.e. cumin seeds, orange peels and lemon grass leaves was believed to help producing and maintaining TCE-degrading enzyme. Of all materials, cumin seeds- immobilized cells were more effective to sustain TCE degradation as well as to protect the bacteria from high TCE concentrations. A maximum Tc of 60 µg of TCE mg cells-1 was found from the cumin seeds-immobilized cells. Moreover, the immobilized cells could be reused for TCE biodegradation after a reactivation in mineral salts medium for 12 hrs. Enzymatic study found that the activity of toluene dioxygenase (TDO) enzyme from Rhodococcus sp. L4 was decreased in the presence of TCE. However, TDO activity could be recovered after removing TCE from the system. The addition of cumene to enzymatic reactions could protect TDO enzyme inactivation. These findings supported the whole-cells and immobilization study, in which the repeated addition of essential oil component i.e. cumene, cumin aldehyde, limonene and citral could maintain TCE-degrading activity of Rhodococcus sp. L4.