The primary objective of this study was to evaluate the impact of substrate interactions on the biotransformation rates and mineralization potentials of gasoline monoaromatics and methyl tert-butyl ether (MTBE), compounds that commonly co-exist in groundwater contaminant plumes. A mixed culture was derived from gasoline-contaminated aquifer material using toluene as the enrichment substrate. Two pure cultures, Rhodococcus sp. RR1 and RR2, were isolated from the mixed culture. The three

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... rown cultures were shown to biotransform all of the six BTEX compounds (benzene, toluene, ethylbenzene, o-xylene, m-xylene, and p-xylene), both individually and in mixtures, over a broad range of concentrations. The mixed culture was shown to degrade all of the BTEX compounds to 14 CO 2, while the two isolates mineralized BTE(m-/p-)X, but biotransformed o-xylene without production of CARBON DIOXIDE. Studies to evaluate substrate interactions caused by the concurrent presence of multiple BTEX compounds during their biodegradation revealed a number of patterns ,including competitive inhibition and cometabolism. Ethylbenzene was shown to significantly inhibit BTX degradation in mixtures. MTBE was not biodegraded by any of the three toluene-grown cultures over a range of MTBE concentrations. Furthermore, the presence of MTBE at concentrations of 2 to 100 mg/L had no effect on BTEX biotransformation rates.