# Enhanced Methyl _Tert_-Butyl Ether Removal by Mixed Consortium: Performance and Adaptability
## 3 Result and Discussion
### The Mixture of Three Pure Cultures Effectively Removed MTBE in the Presence of Heavy Metals
[41] selected several metal-resistant bacteria from naturally polluted environments and determined the minimum inhibitory concentrations of Cu\({}^{2+}\), Cd\({}^{2+}\), and Zn\({}^{2+}\) for _Pseudomonas aeruginosa_ (strain S7) were 1.6, 1.5 and 1.5 mg/L, respectively. Further, _Pseudomonas aeruginosa_ (strain S6) isolated from raw wastewater in Tunis city exhibited a relatively high resistance to various metals. Bioassays were performed to assess the responses of _Microococcus_ and _Pseudomonas_ to the metals added singly or in combination to a crude oil/mineral salts medium. It was found that Zn\({}^{2+}\) and Pb\({}^{2+}\) induced significant decreases in the density of the two bacteria, while Cu\({}^{2+}\) and Mn\({}^{2+}\) did not [42]. [PERSON] et al. [40] illustrated that the addition of 1 mg/L Mn\({}^{2+}\) offered a slight stimulating effect on MTBE degradation, while Zn\({}^{2+}\) reduced MTBE removal at the same concentration. The degradation rate of MTBE decreased at Zn\({}^{2+}\) concentrations of 10 mg/L and 50 mg/L, and the inhibition was stronger at high concentration (50 mg/L). Unlike organics, metals are not biodegradable, so they inhibit microbial growth when accumulated to toxic levels. While some metal ions are necessary for bacteria metabolism as they provide essential elements for synthesis and action of enzymes and proteins, they may be replaced by metal ions existing in the environment, resulting in the loss of functionality [43]. The promotion of MTBE biodegradation is mainly due to the better cell growth and enzyme synthesis [44]. However, the toxicity of metal ions dominants at high concentrations and their binding to groups on enzyme molecules associated to MTBE biodegradation or cellular metabolism leads to the inactivation and denaturation of enzymes [44]. Moreover, the sequestration of metal ions prevents cell metabolism, such as abiotic oxidation followed by precipitation. The reduction in MTBE removal is the result of adsorption of metal ions into microbial cells and precipitation of metal ions on the cell surfaces [44]. More seriously, some metal ions change the transcription mechanism in cells, thus fundamentally suppressing the cell division [45]. Therefore, the original bacteria are partially inactivated in the presence of metals. In addition, the effect of metals on MTBE biodegradation was similar in terms of either the three pure strains or the mixed consortium. The degrading ability of bacteria was unaffected until the threshold concentration of metals was reached. The threshold required for inhibition depends on the concentration and type of metals as well as the bacteria. Within the appropriate concentration, the enzymatic system in propagated cells is not suppressed, and the metabolic reaction proceed normally. Above the threshold concentration, the activity of cells is restrained and no new cells can be synthesized, resulting in MTBE being degraded only after additional bacteria reproduction [40]. What is more, compared with pure strains, the degradation efficiency of MTBE by mixed consortium was higher with the addition of 20 mg/L Mn\({}^{2+}\), Pb\({}^{2+}\) and Cu\({}^{2+}\). That is, mixed consortium had stronger resistance to metal ions, especially for the high concentration (20 mg/L) of Mn\({}^{2+}\), Pb\({}^{2+}\), and Cu\({}^{2+}\). The results revealed that mixing three pure cultures effectively removed MTBE in the presence of heavy metals, and attention should be paid to MTBE bioremediation in gasoline-contaminated environment with the presence of various organics and heavy metals.