Substrate interactions in BTEX and MTBE mixtures by an MTBE-degrading isolate

Groundwater contaminant plumes from recent accidental gasoline releases oft en contain the fuel oxygenate MTBE (methyl tert-butyl ether) together with BTEX (benzene, toluene, ethylbenzene, o-xylene, m-xylene and p-xylene) comp ounds. This study evaluates substrate interactions during the aerobic biotr ansformation of MTBE and BTEX mixtures by a pure culture, PM1, capable of u tilizing MTBE for growth. PM1 was unable to degrade ethylbenzene and two of the xylene isomers at concentrations of 20 mg/L following culture growth o n MTBE. In addition, the presence of 20 mg/L of ethylbenzene or the xylenes in mixtures with MTBE completely inhibited MTBE degradation. When MTBE-gro wn cells of PM1 were exposed to MTBE/ benzene and MTBE/toluene mixtures, MT BE degradation proceeded, while the degradation of benzene and toluene was delayed for several hours. Following this initial lag, benzene and toluene were degraded rapidly, while the rate of MTBE degradation slowed significan tly. MTBE degradation did not increase to previous rates until benzene and toluene were almost entirely degraded. The lag in benzene and toluene degra dation was presumably due to the induction of the enzymes necessary for BTE X degradation. Once these enzymes were induced, sequential additions of ben zene or toluene were degraded rapidly, and growth on benzene and toluene wa s observed. The results of this study suggest that BTEX and MTBE degradatio n occurs primarily via two independent and inducible pathways. If subsurfac e microbial communities behave similarly to the culture used in this study, the observed severe inhibition of MTBE degradation by ethylbenzene and the xylenes and the partial inhibition by benzene and toluene suggest that the biodegradation of MTBE in subsurface environments would most likely be del ayed until MTBE has migrated beyond the BTEX plume.