Degradation of fuel oxygenates and their main intermediates by Aquincola tertiaricarbonis L108

Abstract

Growth of Aquincola tertiaricarbonis L108 on the fuel oxygenates methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE) and tert-amyl methyl ether (TAME), as well as on their main metabolites tert-butyl alcohol (TBA), tert-amyl alcohol (TAA) and 2-hydroxyisobutyrate (2-HIBA) was systematically investigated to characterize the range and rates of oxygenate degradation by this strain. The effective maximum growth rates for MTBE, ETBE and TAME at pH 7 and 30 degrees C were 0.045 h(-1), 0.06 h(-1) and 0.055 h(-1), respectively, whereas TAA, TBA and 2-HIBA permitted growth at rates up to 0.08 h(-1), 0.1 h(-1) and 0.17 h(-1), respectively. The experimental growth yields with all these substrates were high. Yields of 0.55 g dry mass (dm) (g MTBE)(-1), 0.53 g dm (g ETBE)(-1), 0.81 g dm (g TAME)(-1), 0.48 g dm (g TBA)(-1), 0.76 g dm (g TAA)(-1) and 0.54 g dm (g 2-HIBA)(-1) were obtained. Maximum specific degradation rates were 0.92 mmol MTBE h(-1) (g dm)(-1), 1.11 mmol ETBE h(-1) g(-1), 0.66 mmol TAME h(-1) g(-1), 1.19 mmol TAA h(-1) g(-1), 2.82 mmol TBA h(-1) g(-1), and 3.27 mmol 2-HIBA h(-1) g(-1). The relatively high rates with TBA, TAA and 2-HIBA indicate that the transformations of these metabolites did not limit the metabolism of MTBE and the related ether compounds. Despite the fact that these metabolites still carry a tertiary carbon atom that is commonly suspected to confer recalcitrance to the ether oxygenates, the transformation rates were in the same range as those with succinate and fructose. With MTBE, strain L108 grew at pHs between 5.5 and 8.0 at near-maximal rate, whereas no growth was found below pH 5.0 and above pH 9.0. The optimum growth temperature was 30 degrees C, but at 5 degrees C still about 15 % of the maximum rate remained, whereas no growth occurred at 42 degrees C. This indicates that MTBE metabolites are valuable substrates and that A. tertiaricarbonis L108 is a good candidate for bioremediation purposes. The possible origin of its exceptional metabolic capability is discussed in terms of the evolution of enzymic activities involved in the conversion of compounds carrying tertiary butyl groups.