## 4 Discussion
### The Influencing Factors on Microbial Degradation of UDMH  
The two strains in solution and soil environment displayed different degradation ability. P4 could not biodegrade UDMH as a sole carbon source in aqueous MSM, and M12 could not biodegrade UDMH in soil. Three important factors--initial UDMH concentration, oxygen content,  
Figure 6: The influence of extra carbon and organic matter on the biodegradation of UDMH. and co-metabolism--were chosen to be discussed, which may help with finding the biodegradation pathway of UDMH. The results are shown in Figures 7 and 8. According to the results of the influences of different initial UDMH concentrations given in Figures 7a and 8a, UDMH was proven to inhibit metabolism of the strains, as had been found early  
Figure 8: The influencing factors on UDMH degradation for P4: (**a**) the concentration of UDMH in soil; (**b**) the extra carbon source in MSM. Figure 7: The influencing factors on UDMH degradation for M12 in liquid MSM: (**a**) the concentration of UDMH; (**b**) the shaking speed. in 1979 [32; 43]. It could carry the double mutation of \(recA^{-}\) and \(\
u rA^{-}\) by interacting with different compounds of the cell, including lipids, proteins, and nucleic acids [52]. The inhibition influence was more severe in aqueous MSM than in soil. This was because the biodegradation of UDMH in the soil was complex, which was affected by organic matter, autochthonous microorganisms, and metal ions. P4 had high tolerance for UDMH in soil, and could degrade 100-600 mg\(\cdot\)kg\({}^{-1}\) UDMH, reflecting a good soil degradation capability. Although M12 could biodegrade UDMH as a sole carbon source, the oxygen concentration in solution might influence this strain to biodegrade UDMH effectively. The content of oxygen in solution was mainly controlled by the shaking speed. To prove that oxygen was an important influencing factor for M12, the effect of the shaking speed is discussed in this study. The degrading percentages of UDMH in aqueous MSM with different shaking speeds are listed in Figure 7b. The faster the shaking speed, the higher the UDMH degrading percentage was. When the shaking speed was 80 r\(\cdot\) min\({}^{-1}\), the degrading percentage of UDMH was close to that of the control sample. When the shaking speed was 160 r\(\cdot\) min\({}^{-1}\), the degradation of UDMH was greatly improved in the initial 20 h. Then the degradation of UDMH trended to be stable. The great improvement of the degrading capacity was probably due to the air oxidation of UDMH [53]. Oxygen could help M12 resist the toxicity of UDMH by reducing the concentration of UDMH, and it was also essential for the growth and metabolism of this strain. After UDMH was air oxidized, small molecules of carbonaceous and nitrogenous compounds could provide the carbon and nitrogen needed for the growth of the bacterium (Figure 9). Based on the above, UDMH in soil could not be degraded by the M12 strain, most likely due to low oxygen levels in the soil, which caused M12 to reduce the capacity of biodegradation for UDMH. Co-metabolism with a substrate might cause the P4 strain to biodegrade UDMH more effectively. To resist the toxic inhibition of UDMH, co-metabolism with a substrate was adopted to improve the biodegradation rate of UDMH. Figure 8b shows the co-metabolism effect of two kinds of medium in aqueous solution. It can be seen that the co-metabolism medium is very useful for degrading UDMH for P4, but a broth medium is a more effective choice. The key enzyme should be the basis of UDMH biodegradation for P4 (Figure 10). When UDMH was the sole carbon source, the toxicity effect of UDMH was embodied in the cells of P4. Fortunately,  
Figure 9: The influence of air oxidation on UDMH degradation for M12 strain in solution. the microorganism had a self-detoxification mechanism and a self-recovery function [54]. As soon as the microorganism was supplied with enough energy, the self-recovery would be carried out after a lag period.