## 3 Results and Discussion
### Bacterial Strain Tolerance to Hg
All of the experiments were performed four times and the average values were used. The results showed that the growth curves of BL21-7 cultured in LB containing 0 \(\upmu\)mol/L, 5 \(\upmu\)mol/L and 10 \(\upmu\)mol/L HgCl\({}_{2}\) were very similar (Figure 5a). As shown in Figure 5, they could not be distinguished. The growth curves of BL21-7 cultured in LB containing 0 \(\upmu\)mol/L, 5 \(\upmu\)mol/L and 10 \(\upmu\)mol/L MMC were also very similar (Figure 5b). For BL21-7, the MICs of Hg\({}^{2+}\), MMC and PMC were 100 \(\upmu\)mol/L, 60 \(\upmu\)mol/L and 80 \(\upmu\)mol/L, respectively (Figure 5a-c). BL21-7 could remove approximately 80% of the Hg\({}^{2+}\) in the medium when the Hg\({}^{2+}\) concentration was 80 \(\upmu\)mol/L (Supplementary Materials Figure S3a). BL21-7 could effectively accumulate MMC and PMC when the concentrations of MMC and PMC were approximately 40 \(\upmu\)mol/L and 60 \(\upmu\)mol/L, respectively (Supplementary Materials Figure S3b,c). For BL21-7 on LB plates, the MICs of HgCl\({}_{2}\), MMC and PMC were 100 \(\upmu\)mol/L, 60 \(\upmu\)mol/L and 80 \(\upmu\)mol/L, respectively. These MICs were consistent with the measured MIC of the Hg remaining in the supernatant. To characterize the MICs of BL21-7 when the nutrition in the medium was poor, this strain was cultured on PB plates containing different Hg derivatives. The results showed that for BL21-7 on PB plates, the MICs of HgCl\({}_{2}\), MMC and PMC were 30 \(\upmu\)mol/L, 20 \(\upmu\)mol/L and 30 \(\upmu\)mol/L, respectively. Figure 4: _E. coli_ strains’ tolerance to organic Hg. The OD\({}_{600}\) values were measured after the strains were cultured in LB liquid medium containing MMC or PMC for 120 h.  
An engineered bacterial strain, MSR33 has been reported, with MICs of HgCl\({}_{2}\) and MMC of 120 umol/L and 80 umol/L, respectively [8]. CM111 and HG3 also had high HgCl\({}_{2}\) MICs [10]. Mt-1 could effectively accumulate Hg from medium containing 120 umol/L HgCl\({}_{2}\)[6]. These bacteria all had higher MICs than that of BL21-7. However, the mechanism of MSR33 for removing Hg involves volatilization, which releases Hg into the air and pollutes the environment. The tolerance of CM111 and HG3 to HgCl\({}_{2}\) was thought to not be due to Hg(0) volatilization [10]. However, the detailed mechanisms by which these two strains tolerated HgCl\({}_{2}\) remained unclear. Although mt-1 had a high tolerance to HgCl\({}_{2}\), it remains unknown whether it can accumulate organic Hg. Comparatively, BL21-7 not only exhibited high MICs of HgCl\({}_{2}\), MMC and PMC (Figure 5) but could also store Hg  
Figure 5: Growth curves of BL21-7 cultured in LB liquid medium containing mercurial derivatives. **(a)** shows BL21-7 cultured in LB liquid medium containing HgCl\({}_{2}\). **(b)** represents BL21-7 cultured in the presence of MMC. **(c)** shows BL21-7 cultured in the presence of PMC. in the cell (Supplementary Materials S3). BL21-7 not only accumulated Hg\({}^{2+}\) but also effectively accumulated organic Hg (MMC and PMC) (Supplementary Materials S3). [PERSON] et al. constructed an engineered bacterial strain harboring the pMKB18 plasmid, which contained _merT-merP-merB1-ppk_[9]. After the engineered bacteria were cultured in LB medium containing 80 \(\upmu\)mol/L HgCl\({}_{2}\) for 5 h, the Hg remaining in the medium was nearly 40 \(\upmu\)mol/L [9]. However, after BL21-7 was cultured in LB medium containing 80 \(\upmu\)mol/L HgCl\({}_{2}\) for 5 h, the Hg remaining in the medium was only approximately \(25\pm 4\)\(\upmu\)mol/L (Supplementary Materials Figure S3a). BL21-7 could also remove more phenylmercury from the medium than _E. coli_ harboring pMKB18 (Supplementary Materials Figure S3b) [9]. These findings demonstrated that BL21-7 had greater Hg-accumulation capacity than _E. coli_ harboring pMKB18.