Biodegradation 11: 171–186, 2000. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. 171 Aerobic MTBE biodegradation: an examination of past studies, current challenges and future research directions Rula A. Deeb 1,∗ , Kate M. Scow 2 & Lisa Alvarez-Cohen 1 1 Department of Civil and Environmental Engineering, 631 Davis Hall, MC 1710, University of California, Berke- ley, CA 94720-1710, U.S.A.; 2 Department of Land, Air and Water Resources, One Shields Avenue, University of California, Davis, CA 95616-8627, USA ( ∗ author for correspondence: current address: Malcolm Pirnie, Inc., 180 Grand Ave., Ste. 1000, Oakland, CA 94612, USA, e-mail: rdeeb@pirnie.com) Accepted 19 September 2000 Key words: aerobic, bioremediation, biodegradation, gasoline, MTBE, TBA Abstract With the current practice of amending gasoline with up to 15% by volume MTBE, the contamination of ground- water by MTBE has become widespread. As a result, the bioremediation of MTBE-impacted aquifers has become an active area of research. A review of the current literature on the aerobic biodegradation of MTBE reveals that a number of cultures from diverse environments can either partially degrade or completely mineralize MTBE. MTBE is either utilized as a sole carbon and energy source or is degraded cometabolically by cultures grown on alkanes. Reported degradation rates range from 0.3 to 50 mg MTBE/g cells/h while growth rates (0.01–0.05 g MTBE/g cells/d) and cellular yields (0.1–0.2 g cells/g MTBE) are generally low. Studies on the mechanisms of MTBE degradation indicate that a monooxygenase enzyme cleaves the ether bond yielding tert-butyl alcohol (TBA) and formaldehyde as the dominant detectable intermediates. TBA is further degraded to 2-methyl-2-hydroxy-1- propanol, 2-hydroxyisobutyric acid, 2-propanol, acetone, hydroxyacteone and eventually, carbon dioxide. The majority of these intermediates are also common to mammalian MTBE metabolism. Laboratory studies on the degradation of MTBE in the presence of gasoline aromatics reveal that while degradation rates of other gasoline components are generally not inhibited by MTBE, MTBE degradation could be inhibited in the presence of more easily biodegradable compounds. Controlled field studies are clearly needed to elucidate MTBE degradation poten- tial in co-contaminant plumes. Based on the reviewed studies, it is likely that a bioremediation strategy involving direct metabolism, cometabolism, bioaugmentation, or some combination thereof, could be applied as a feasible and cost-effective treatment method for MTBE contamination. Abbreviations: BTEX compounds (benzene, toluene, ethylbenzene, o-xylene, m-xylene and p-xylene); DIPE (diisopropyl ether); ETBE (ethyl tert-butyl ether); HIBA (2-hydroxyisobutyric acid); MHP (2-methyl-2-hydroxy- 1-propanol; also known as 2-methyl-1,2-propanediol); MTBE (methyl tert-butyl ether); ORC (oxygen release compounds); PHS (peat humic substances); TAME (tert-amyl methyl ether); TBA (tert-butyl alcohol). Introduction History of problem Fuel oxygenates, including methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), tert-amyl methyl ether (TAME), diisopropyl ether (DIPE), tert- butyl alcohol (TBA), methanol and ethanol (Zogorski et al. 1997), are added to gasoline to increase com- bustion efficiency and to reduce air pollution. These compounds are used worldwide in quantities that vary by season and country. Among fuel oxygenates, MTBE is most commonly used because of its high octane level, low production cost, ease of blending with gasoline, and ease of transfer and distribution (Nakamura 1994; Piel & Thomas 1990). MTBE was