Microbial degradation and fate in the environment of methyl tert-butyl ether and related fuel oxygenates

Oxygenates, mainly methyl tert-butyl ether (MTBE), are commonly added to ga soline to enhance octane index and improve combustion efficiency. Other oxy genates used as gasoline additives are ethers such as ethyl tert-butyl ethe r (ETBE), tert-amyl methyl ether (TAME), and alcohols such as tert-butyl al cohol (TBA). As a result of its wide use, MTBE has been detected, mainly in the USA, in groundwater and surface waters, and is a cause of concern beca use of its possible health effects and other undesirable consequences. MTBE is a water-soluble and mobile compound that generates long pollution plume s in aquifers impacted by gasoline releases from leaking tanks. Field obser vations concur in estimating that, because of recalcitrance to biodegradati on, natural attenuation is slow (half-life of at least 2 years). However, q uite significant advances have been made in recent years concerning the mic robiology of the degradation of MTBE and other oxygenated gasoline additive s. The recalcitrance of these compounds results from the presence in their structure of an ether bond and of a tertiary carbon structure. For the most part, only aerobic microbial degradation systems have been reported so far . Consortia capable of mineralizing MTBE have been selected. Multiple insta nces of the cometabolism of MTBE with pure strains or with microflorae, gro wing on n-alkanes, isoalkanes, cyclohexane or ethers (diethyl ether, ETBE), have been described. MTBE was converted into TBA in all cases and was some times further degraded, but it was not used as a carbon source by the pure strains. However, mineralization of MTBE and TBA by several pure bacterial strains using these compounds as sole carbon and energy source has recently been reported. The pathways of metabolism of MTBE involve the initial atta ck by a monooxygenase. In several cases, the enzyme was characterized as a cytochrome P-450. After oxygenation, the release of a Cl-unit as formaldehy de or formate leads to the production of TBA, which can be converted to 2-h ydroxyisobutyric acid and further metabolized. Developments in microbiology make biological treatment of water contaminated with MTBE and other oxygen ates an attractive possibility. Work concerning ex situ treatment in biofil ters by consortia and by pure strains, and involving or not cometabolism, i s under way. Furthermore, the development of in situ treatment processes is a promising goal.