CENSUS – Gasoline – MTBE and TBA Table
|Detect and quantify bacteria responsible for biodegradation of Gasoline – MTBE and TBA|
In the early 1990s, methyl tert-butyl ether (MTBE) was added to gasoline at concentrations from 11% to 15% (v/v). With increased use, MTBE has become one of the most commonly detected groundwater contaminants at sites impacted by petroleum products. The compound tert-butyl alcohol (TBA) was present as an impurity in the commercial grade MTBE used in reformulated gasoline. As described below however, TBA is also an intermediate in the degradation of MTBE and is frequently detected in impacted groundwater at concentrations far exceeding the contributions of the original gasoline. This net accumulation of TBA can indicate slow or incomplete biodegradation of MTBE under existing site conditions.
Biodegradation of MTBE and TBA
Aerobic biodegradation of MTBE and the intermediate TBA has been extensively studied in the laboratory and documented in the field. However, Methylibium petroleiphilum PM1 remains one of the few bacterial cultures isolated to date that utilizes MTBE and TBA as growth supporting substrates. The key steps in aerobic MTBE/TBA biodegradation by strain PM1 is the initial oxidation mediated by a monooxygenase and the continued biodegradation of TBA catalyzed by TBA hydroxylase. The net result is complete oxidation to CO2 without accumulation of TBA. Anaerobic biodegradation of MTBE has been observed but as yet is poorly understood. Furthermore, while complete mineralization of MTBE has been noted, significant accumulation of TBA particularly under methanogenic conditions is also frequently reported. Taken as a whole, available field evidence suggests that complete biodegradation of MTBE and TBA under anaerobic conditions is likely to be quite site specific.
Biodegradation of ETBE and other Fuel Oxygenates
While MTBE may be the most common, additional fuel oxygenates including ethyl tert-butyl ether (ETBE) and tert-amyl methyl ether (TAME) were also employed to increase combustion efficiency and decrease exhaust emissions particularly outside the US. Bacteria, primarily of the genera Rhodococcus and Gordonia, capable of utilizing ETBE as a growth supporting substrate have been isolated. Aerobic ETBE biodegradation is initiated by a cytochrome P450 monooxygenase (ethB). As with many of the monooxygenases, ethB exhibits relaxed specificity and will co-oxidize MTBE and TAME to TBA and tert-amyl alcohol (TAA), respectively. Most ETBE utilizing isolates grow on the ethoxy group releasing TBA which is not further degraded. The notated exception is Aquincola teriaricarbonis L108 which constitutively expresses ethB and possesses pathways for complete mineralization of ETBE, MTBE, and TAME.
CENSUS Targets for Aerobic Gasoline – MTBE and TBA
The following table describes the individual CENSUS targets, their imporatance in evaluating aerobic MTBE and TBA biodegradation, and provides guidelines for integrating CENSUS results into routine groundwater monitoring for common corrective actions.