“Trojan Horse” Type Internalization Increases the Bioavailability of Mercury Sulfide Nanoparticles and Methylation after Intracellular Dissolution

Mercury sulfide nanoparticles (HgS_NP), as natural metal-containing nanoparticles, are the dominant Hg species in anoxic zones. Although the microbial Hg methylation of HgS_NP has been previously reported, the importance of this process in Hg methylation has yet to be clarified due to the lack of knowledge on the internalization and transformation of HgS_NP. Here, we investigated the internalization and transformation of HgS_NP in microbial methylator Geobacter sulfurreducens PCA through total Hg analysis and different Hg species quantification in medium and cytoplasm. We found that the microbial uptake of HgS_NP, via a passive diffusion pathway, was significantly higher than that of the Hg²⁺-dissolved organic matter (Hg²⁺-DOM) complex. Internalized HgS_NP were dissolved to Hg²⁺ in cytoplasm with a maximal dissolution of 41%, suggesting a “Trojan horse” mechanism. The intracellular Hg²⁺ from HgS_NP exposure at the initial stage (8 h) was higher than that in Hg²⁺-DOM group, which led to higher methylation of HgS_NP. Furthermore, no differences in methylmercury (MeHg) production from HgS_NP were observed between the hgcAB gene knockout (ΔhgcAB) and wild-type strains, suggesting that HgS_NP methylation may occur through HgcAB-independent pathways. Considering the possibility of a broad range of hgcAB-lacking microbes serving as methylators for HgS_NP and the ubiquity of HgS_NP in anoxic environments, this study highlights the importance of HgS_NP internalization and methylation in MeHg production and demonstrates the necessity of understanding the assimilation and transformation of nutrient and toxic metal nanoparticles in general.