Some anaerobic bacteria can take up dissolved elemental mercury, Hg(0), oxidize it to Hg(II), and methylate the product into methylmercury (MeHg), a bioaccumulating neurotoxin. Work published in 2013 established this trait in laboratory cultures and raised a bundled mechanistic and ecological question that remains only partly answered: by what molecular route neutral Hg(0) enters anaerobic cells and is oxidized, which cellular components catalyze that oxidation, how the Hg(0)-derived Hg(II) reaches the HgcAB methylation machinery, and how much this pathway contributes to MeHg production in nature. Later studies resolved several parts: Hg(0) oxidation is widespread among anaerobes including non-methylators; in Desulfovibrio desulfuricans ND132 it is intracellular and thiol-mediated; Hg(II) uptake in that strain appears passive rather than active, although transporter-mediated uptake has been neither identified nor excluded even there and remains contested across organisms; and the HgcAB complex, with S-adenosyl-L-methionine as the methyl donor, is now biochemically characterized in vitro. Still unresolved are the identity of the Hg(0)-oxidizing thiols and the electron acceptors of that oxidation, the uptake route of Hg(0) itself and any transporter contribution to it, the delivery of Hg(0)-derived Hg(II) to HgcAB in vivo, and the prevalence and environmental significance of Hg(0)-derived methylation. The problem asks for the complete activation pathway and its environmental significance.

Background

Methylmercury (MeHg) is a potent neurotoxin that bioaccumulates and biomagnifies in aquatic food webs, making its environmental production a global health concern. The main source of MeHg in nature is microbial methylation of inorganic mercury by anaerobic microorganisms living in sediments, wetlands, and oxygen-depleted waters, chiefly sulfate-reducing and iron-reducing bacteria and some methanogens. Mercury occurs in two interconvertible inorganic forms: divalent mercuric mercury, Hg(II), a positively charged ion that strongly binds sulfide and thiol (sulfhydryl, R–SH) groups; and elemental mercury, Hg(0), a neutral, volatile species that is sparingly soluble in water. Hg(0) dominates at contaminated sites (liquid mercury waste, artisanal gold mining) and in the atmosphere, so whether microbes can convert it directly into MeHg matters for risk assessment. The genetic basis of methylation was identified in 2013: the gene pair hgcAB, present in all known methylators, encodes HgcA, a membrane-associated corrinoid (vitamin-B12-cofactor) methyltransferase, and HgcB, a ferredoxin-like protein with two [4Fe–4S] iron–sulfur clusters that reactivates the cofactor. In 2024 the purified HgcAB complex was shown to transfer a methyl group from S-adenosyl-L-methionine (SAM, the universal biological methyl donor) to Hg(II) through a methylcobalamin intermediate — resolving the chemistry of the methylation step itself for Hg(II) substrates. Against this backdrop, Hu and colleagues (Nature Geoscience 6:751–754, 2013) showed that Desulfovibrio desulfuricans ND132 can oxidize dissolved Hg(0) and methylate it (at about one-third the rate of Hg(II) methylation), that Desulfovibrio alaskensis G20 oxidizes Hg(0) but cannot methylate it, and that Geobacter sulfurreducens PCA oxidizes and methylates Hg(0) only when the thiol amino acid cysteine is present. They proposed that methylating and non-methylating bacteria may jointly enhance MeHg formation, the latter oxidizing Hg(0) into a bioavailable Hg(II) pool. Separately, thiols were shown to abiotically oxidize dissolved Hg(0) under anoxic (oxygen-free) conditions, implying a plausible chemical route for 'thiol-induced oxidative complexation' on cell surfaces and in natural exudates. How Hg species enter cells was contested: one line of evidence (competitive inhibition by zinc) suggested active, transporter-mediated uptake of Hg(II), while other experiments pointed to passive adsorption and ligand exchange. The paper's discussion therefore posed the questions that define this problem: the molecular and biochemical pathway of Hg(0) uptake (and whether oxidation occurs intracellularly), how uptake handles Hg(0) versus Hg(II), which gene products or transport systems enable Hg(0)-derived methylation, the chemical steps and stoichiometries of thiol-driven oxidation on real microbial surfaces, how widespread and environmentally significant the trait is across communities and geochemical regimes, and how the laboratory rates could be translated into predictive models of MeHg production.

Problem Statement

Determine the complete molecular pathway by which dissolved elemental mercury is activated for microbial methylation under anoxic conditions, and the environmental significance of that pathway for methylmercury production. Concretely: (1) identify the route and mechanism of Hg(0) entry into anaerobic methylating cells — passive membrane diffusion versus any transporter-mediated process — and how this uptake differs from, or does not differ from, that of ionic Hg(II); (2) identify the cellular thiol-bearing components responsible for oxidizing Hg(0) to Hg(II), together with the chemical steps, stoichiometries, and electron acceptors of that oxidation, and establish the compartment in which it occurs (intracellular versus cell surface/exudate), including whether the intracellular localization reported for one model methylator holds generally across anaerobic taxa rather than being a single-strain observation; (3) establish how the Hg(0)-derived Hg(II) pool is transferred in vivo to the HgcAB methylation complex — including which gene products or transport systems enable Hg(0)-derived methylation — and how the efficiency with which the internally generated Hg(0)-derived Hg(II) pool is methylated compares with that of ambient Hg(II), including the mechanistic basis of any difference and whether the comparison reported for a single model strain extends more broadly; and (4) determine the extent to which Hg(0) oxidation is coupled to Hg(0)-derived methylation across anaerobic bacterial strains and species, and assess the significance of the Hg(0)-derived pathway for methylmercury formation in anaerobic environments, so that it can be represented in predictive models of methylmercury production in sediments and waters. The mechanistic parts (1)–(3) are not definitively resolved in the literature: the evidence permits a best-supported, evidence-graded pathway rather than definitive identification, with the oxidizing thiols, the oxidation stoichiometry, the electron acceptor and the transporter remaining open wherever the evidence does not pin them down. Parts (1)–(3) are mechanistic and together resolve the activation pathway: the Hg(0) uptake route, the oxidizing thiol components with their electron acceptor and the compartment of oxidation, and the in vivo delivery of Hg(0)-derived Hg(II) to HgcAB. Part (4) is a bounded assessment — how far Hg(0)-derived methylation extends across the strains and species for which data exist, and how large the pathway's contribution to methylmercury production is relative to other mercury sources — and does not require a complete predictive model of methylmercury production. The in vitro methylation chemistry of Hg(II) by the purified HgcAB complex with S-adenosyl-L-methionine as methyl donor is already characterized (Zheng et al. 2024) and is not the target; the target is the activation and delivery pathway upstream of that chemistry and the pathway's environmental significance.

References

  1. Hu, H., Lin, H., Zheng, W., Tomanicek, S. J., Johs, A., Feng, X., Elias, D. A., Liang, L., Gu, B. (2013). Oxidation and methylation of dissolved elemental mercury by anaerobic bacteria. Nature Geoscience 6, 751–754. DOI: 10.1038/ngeo1894. https://doi.org/10.1038/ngeo1894
  2. Colombo, M. J., Ha, J., Reinfelder, J. R., Barkay, T., Yee, N. (2013). Anaerobic oxidation of Hg(0) and methylmercury formation by Desulfovibrio desulfuricans ND132. Geochimica et Cosmochimica Acta 112, 166–177. DOI: 10.1016/j.gca.2013.03.001. https://doi.org/10.1016/j.gca.2013.03.001
  3. Zheng, W., Lin, H., Mann, B. F., Liang, L., Gu, B. (2013). Oxidation of dissolved elemental mercury by thiol compounds under anoxic conditions. Environmental Science & Technology 47, 12827–12834. DOI: 10.1021/es402697u. https://doi.org/10.1021/es402697u
  4. Colombo, M. J., Ha, J., Reinfelder, J. R., Barkay, T., Yee, N. (2014). Oxidation of Hg(0) to Hg(II) by diverse anaerobic bacteria. Chemical Geology 363, 334–340. DOI: 10.1016/j.chemgeo.2013.11.020. https://doi.org/10.1016/j.chemgeo.2013.11.020
  5. Parks, J. M., Johs, A., Podar, M., Bridou, R., Hurt, R. A., Smith, S. D., Tomanicek, S. J., Qian, Y., Brown, S. D., Brandt, C. C., Palumbo, A. V., Smith, J. C., Wall, J. D., Elias, D. A., Liang, L. (2013). The genetic basis for bacterial mercury methylation. Science 339, 1332–1335. DOI: 10.1126/science.1230667. https://doi.org/10.1126/science.1230667
  6. Lin, H., Lu, X., Liang, L., Gu, B. (2015). Thiol-facilitated cell export and desorption of methylmercury by anaerobic bacteria. Environmental Science & Technology Letters 2, 292–296. DOI: 10.1021/acs.estlett.5b00209. https://doi.org/10.1021/acs.estlett.5b00209
  7. Wang, Y., Schaefer, J. K., Mishra, B., Yee, N. (2016). Intracellular Hg(0) oxidation in Desulfovibrio desulfuricans ND132. Environmental Science & Technology 50, 11049–11056. DOI: 10.1021/acs.est.6b03299. https://doi.org/10.1021/acs.est.6b03299
  8. Liu, Y.-R., Lu, X., Zhao, L., An, J., He, J.-Z., Pierce, E. M., Johs, A., Gu, B. (2016). Effects of cellular sorption on mercury bioavailability and methylmercury production by Desulfovibrio desulfuricans ND132. Environmental Science & Technology 50, 13335–13341. DOI: 10.1021/acs.est.6b04041. https://doi.org/10.1021/acs.est.6b04041
  9. An, J., Zhang, L., Lu, X., Pelletier, D. A., Pierce, E. M., Johs, A., Parks, J. M., Gu, B. (2019). Mercury uptake by Desulfovibrio desulfuricans ND132: passive or active? Environmental Science & Technology 53, 6264–6272. DOI: 10.1021/acs.est.9b00047. https://doi.org/10.1021/acs.est.9b00047
  10. Poulin, B. A., Ryan, J. N., Tate, M. T., Krabbenhoft, D. P., Hines, M. E., Barkay, T., Schaefer, J. K., Aiken, G. R. (2019). Geochemical factors controlling dissolved elemental mercury and methylmercury formation in Alaskan wetlands of varying trophic status. Environmental Science & Technology 53, 6203–6213. DOI: 10.1021/acs.est.8b06041. https://doi.org/10.1021/acs.est.8b06041
  11. Regnell, O., Watras, C. J. (2019). Microbial mercury methylation in aquatic environments: a critical review of published field and laboratory studies. Environmental Science & Technology 53, 4–19. DOI: 10.1021/acs.est.8b02709. https://doi.org/10.1021/acs.est.8b02709
  12. Yin, X., Wang, L., Zhang, L., Chen, H., Liang, X., Lu, X., DiSpirito, A. A., Semrau, J. D., Gu, B. (2020). Synergistic effects of a chalkophore, methanobactin, on microbial methylation of mercury. Applied and Environmental Microbiology 86, e00122-20. DOI: 10.1128/AEM.00122-20. https://doi.org/10.1128/AEM.00122-20
  13. Zheng, K., Rush, K. W., Date, S. S., Johs, A., Parks, J. M., Fleischhacker, A. S., Abernathy, M. H., Sarangi, R., Ragsdale, S. W. (2024). S-adenosyl-L-methionine is the unexpected methyl donor for the methylation of mercury by the membrane-associated HgcAB complex. Proceedings of the National Academy of Sciences 121, e2408086121. DOI: 10.1073/pnas.2408086121. https://doi.org/10.1073/pnas.2408086121
  14. Zhang, Z., Zhang, Z., Zhang, C., Chang, Q., Fang, Q., Liao, C., Chen, J., Alvarez, P. J. J., Chen, W., Zhang, T. (2024). Simultaneous reduction and methylation of nanoparticulate mercury: the critical role of extracellular electron transfer. Environmental Science & Technology 58, 18368–18378. DOI: 10.1021/acs.est.4c07573. https://doi.org/10.1021/acs.est.4c07573
  15. Wang, Y. L., Ikuma, K., Brooks, S. C., Varonka, M. S., Deonarine, A. (2024). Non-mercury methylating microbial taxa are integral to understanding links between mercury methylation and elemental cycles in marine and freshwater sediments. Environmental Pollution 346, 123573. DOI: 10.1016/j.envpol.2024.123573. https://doi.org/10.1016/j.envpol.2024.123573
  16. Lim, S. H., Kim, Y., Motta, L. C., Yang, E. J., Rhee, T. S., Hong, J. K., Han, S., Kwon, S. Y. (2024). Near surface oxidation of elemental mercury leads to mercury exposure in the Arctic Ocean biota. Nature Communications 15, 7598. DOI: 10.1038/s41467-024-51852-2. https://doi.org/10.1038/s41467-024-51852-2
  17. Santos, J. P., Tessier, E., Le Faucheur, S., Amouroux, D., Slaveykova, V. I. (2025). Comparative analysis of species-specific dissolved gaseous mercury oxidation in phytoplankton cultures. Environmental Research 279, 121764. DOI: 10.1016/j.envres.2025.121764. https://doi.org/10.1016/j.envres.2025.121764
  18. Lavoie, N. C., Poulain, A. J. (2025). Anaerobic HgII reduction is driven by cellular HgII-thiol interactions. Access Microbiology 7, 000932.v3. DOI: 10.1099/acmi.0.000932.v3. https://doi.org/10.1099/acmi.0.000932.v3
  19. Meng, F., Skyllberg, U., Li, Y., Hayama, S., Björn, E., Song, Y. (2026). Subcellular thiol functional group distribution in Geobacter sulfurreducens determined by Hg LIII-edge EXAFS. Frontiers in Microbiology 16, 1728775. DOI: 10.3389/fmicb.2025.1728775. https://doi.org/10.3389/fmicb.2025.1728775
  20. Fan, W., Kou, M., Wang, Y., Liu, Y., Zhang, Q., Guo, Y., Yin, Y., Cai, Y., Jiang, G. (2026). Microbial methylation of particulate-bound mercury in water column and its underlying mechanisms: a critical review. Water Research 289, 124871. DOI: 10.1016/j.watres.2025.124871. https://doi.org/10.1016/j.watres.2025.124871