Uptake and efflux of mercury sulfide nanoparticles in Escherichia coli

Mercury sulfide nanoparticles (HgS NPs) are ubiquitous in nature and are methylated to neurotoxic methylmercury (MeHg) by microorganisms. However, there is a paucity of knowledge regarding the biokinetics of HgS NPs in microorganisms. Here, the biokinetic uptake and efflux of HgS NPs in the model bacterium (Escherichia coli) were quantified and compared with those of Hg-dissolved organic matter complexes (Hg-DOM). The results demonstrate that the uptake of HgS NPs was time- and concentration-dependent. The uptake rate constant of HgS NPs was 0.031 ± 0.006 L g^-1 h^-1, which was 2.4-fold higher than that of Hg-DOM. Meanwhile, the internalized Hg in E. coli was depurated, as evidenced by the presence of Hg-containing NPs in the culture media. The efflux rate constant of HgS NPs and Hg-DOM were comparable (0.028 ± 0.007 h^-1 vs. 0.031 ± 0.009 h^-1). Consequently, HgS NPs exhibited a significantly higher bioconcentration factor (BCF; 1107 ± 63 L kg⁻¹) than Hg-DOM (419 ± 57 L kg⁻¹), suggesting their enhanced bioavailability and therefore greater methylation potential in Hg-methylating bacteria. These results demonstrate the unique characteristics of HgS NP uptake kinetics, which are essential for capturing their bioaccumulation mechanisms and providing more reliable and accurate risk assessments.