# Enhanced Methyl _Tert_-Butyl Ether Removal by Mixed Consortium: Performance and Adaptability
## 1 Introduction  
Methyl _tert_-butyl ether (MTBE) has been commonly used as an oxygenated additive to increase the octane number of gasoline [1]; the global MTBE market is expected to reach $22.9 billion by 2026 [2]. It is the second most frequently detected volatile organic compound in shallow groundwater due to the accidental leakage of gasoline [3]. Moreover, it has a high solubility in water and is hardly adsorbed by soil particles [4; 5]. The persistent contamination caused by MTBE is responsible for the irritation to skin and eyes. Further, it has been reported to be genotoxic, and has the possibility to depress the nervous system [3; 6]. As a result, the remediation of MTBE contamination is required. Various techniques including adsorption, air stripping, advanced oxidation, and biological processes have been applied for MTBE treatment. However, there are some drawbacks in chemical and physical methods for MTBE removal, such as spent adsorbent disposal [3]. The low Henry's law constant of MTBE limits the efficiency and increases the cost of air stripping [7]. Moreover, the formation of by-products during MTBE decomposition is a major disadvantage in oxidation process since more toxic compounds are formed [3]. As a result of these challenges, the biological process is considered as a promising technology due to its low cost, environmental friendliness, no secondary pollution, and ease of operation [8]. MTBE can be broken down into carbon dioxide and water as well as harmless metabolites through aerobic biodegradation [8; 9]. There are some pure cultures that can degrade MTBE through direct metabolism, such as _Methylbium petroleiphulum_ PM1 [10], _Aquincola tertiaricarbonis_ L108 [11], _Hydrogenophaga fava_ ENV735 [12], _Mycobacterium autrofrakenum_ IFP2012 [13], _Rhodococcus_ sp. EH831 [14], _Enterobacter_ sp. NKNU02 [15], and_Achromobacter xylos oxidans_ MCM1/1 [16]. Among them, NKNU02 isolated from gasoline-contaminated water could grow with MTBE as the sole carbon and energy source, reaching a removal efficiency of 60% at 100 mg/L [15]. At the same concentration, higher removal was obtained by MCM1/1, which biodegraded up to 78% MTBE in a mineral medium in 5 days [16]. Although pure cultures have been shown to degrade MTBE, the degradation rate and concentration range are limited. At present, the application of the mixed consortium for pollutant degradation has attracted increasing attention because of its advantages over pure cultures, such as metabolic diversity, stronger adaptability, and higher decomposition efficiency [17; 18]. The removal of pollutants is enhanced by the synergism between different strains in the mixture [19]. Most studies have focused on the screening and domestication of natural mixed bacteria existing in the environment, but selective mixing of different pure cultures has rarely been studied. On the basis of determining the characteristics of several bacteria, the composition of the mixture can be designed, and the proportion of strains can be controlled. That is, the simultaneous cultivation of different strains facilitates the investigation of biodegradation mechanism. [PERSON] et al. [17] screened four strains from an aged oil-contaminated area and found that a mixture of four bacteria raised the removal efficiency of crude oil by about 11% compared to the best pure isolates. Two strains of _Acinetobacter baumanmii_ and _Talaromyces_ sp. were isolated from oil-contaminated soil and sludge and combined in a ratio of 1:1, and the obtained co-cultures displayed a higher degrading ability to n-alkanes and excellent alkali resistance [20]. In this way, a new idea for MTBE bioremediation was developed. Furthermore, studies on the impact of co-contaminants on MTBE biodegradation are limited. Considering the composition of gasoline, it is speculated that alkanes, BTEX, and heavy metals may co-exist in MTBE-contaminated soil or water. [PERSON] et al.