Mercury methylation is the process of forming methylmercury (MeHg). The methylation of mercury can occur abiotically or biotically. Biotically, the primary methylators of mercury are sulfate-reducing and iron-reducing bacteria.[1] Three mechanisms have been proposed for the biotic methylation of mercury by sulfate-reducing bacteria.[2] Mercury methylation can be problematic as methylmercury is toxic and can be bio-magnified through the food web.[2]

Chemistry

Chemical elements on Earth cycle through atmospheric, terrestrial, and aquatic environments in a process called biogeochemical cycling.[3] Mercury goes through its own version of biogeochemical cycling named the mercury cycle where it circulates through the environment and changes between oxidation states: Hg(0), Hg(I), Hg(II).[3][4] When mercury is present in the environment microbial organisms can uptake the elemental form of mercury.[2] This signals the transcription of the genes hgcA and hgcB are transcribed to synthesize the HgcA and HgcB proteins.[4] These proteins can then start the methylation reaction to form methylmercury.[4] 

Biochemical

Microbial

Species from all three domains of life have been found to play a role in the methylation of mercury. More species have been discovered that genetically are capable of mercury methylation due to the discovery of the hgcAB genes.[5] It is not known if the HgcA and HgcB proteins create a multienzyme complex or work sequentially. It has also been shown that deletion of either gene results in the complete loss of the ability to methylate mercury.[6]

Bacterial species currently known to methylate mercury include the major of Desulfovibrio spp. (i.e. Desulfovibrio desulfuricans).[5][7] and Geobacter spp. (i.e. Geobacter sulfurreducens)[5][7] Other species with the hgcAB genes that suspected to produce MeHg include Bacteroidota, Chloroflexota, and Nitrospirota.[7]

Archaeal species known to methylate mercury include the majority of species of methanogen class Methanomicrobia, however, class Thermoplasmata has been found to carry the hgcAB genes. No other species of methanogens have been found with the ability of mercury methylation.[7]

Reactions

pH influences on mercury methylation can be variable depending on the species that are undergoing the reactions. Some findings demonstrate that an increase in the hydrogen ion concentration resulted in large increases of the Hg(II) uptake, leading to potential impacts on the actual methylation of mercury.[8] Another finding demonstrated that the decrease in pH leads to a shift in the production of methyl mercury species. Specifically, the production of dimethylmercury decreases and the production of monomethylmercury increases, but total remains essentially constant.[2]

Enough adequate studies on the temperature influences on the methylation of mercury have not been published. Mercury methylation reaches maximum activity in the summer[2] but this enhanced methylation may be due to other factors unrelated to temperature. However, it is evident that temperature affects microbial activity which will correspond to an impact on the subsequent biochemical reactions that lead to methylation of mercury.

Similar to the pH effects, different concentrations of available mercury ion lead to different products and complexes of mercury being produced.[9] In addition, the enzymes HgcA and HgcB have a very low Km and will therefore readily bind to the available mercury even at very low concentrations.[9]

Transport into cell

Before mercury can be methylated, it must be transported into the cell through the lipid membrane. Mercury ions are bound by a mercury scavenger protein, MerP. MerP transfers the mercury ion to a cytoplasmic membrane transporter, MerT, then to the active site of mercuric reductase or mercury(II) reductase in the cytoplasm.[2]

Normally mercury would be toxic to the cell, but some microorganisms are resistant to mercury ion due to an inducible mer operon. Translation of the operon results in the synthesis of mercuric reductase. Mercuric reductase will reduce the mercury ion into elemental mercury, which is volatilized from the cell.[2] If mercuric reductase is not employed, methylation of mercury can occur via three identified pathways.[2]

Biochemical pathway

Cultures of sulfate reducing bacteria grown without the presence of sulfate will not methylate mercury. It is a possibility that the respiration of these cells is coupled to mercury methylation.[2]

The Acetyl-CoA pathway for mercury methylation is done by sulfate reducing bacteria and is catalyzed by a corrinoid dependent protein. Through this pathway, the methyl group is proposed to originate from C-3 serine. A transfer of the methyl group from CH3-Tetrahydrofolate to the corrinoid protein requires the genes hgcA and hgcB .[4] The methyl group now on the corrinoid protein will then be transferred to mercury ion.[2] This activity was shown to decrease in aerobic environments, suggesting that the methylation occurs anaerobically.[2]

Acetate Metabolic pathway (methyl-transferase enzymes) is very similar to the acetyl CoA pathway, where methyltransferase enzymes involving tetrahydrofolate intermediates are utilized.[2][10] It was shown that methylation of mercury was greater by three orders of magnitude in cells that were capable of utilizing acetate.[2]

Methylation of mercury can also occur using a cobalamin dependent methionine synthase. The cobalamin dependent process requires the use of the substrate S-adenosylmethionine, a biological methylating agent.[2] As methionine synthase was used, it is possible that the enzyme that methylates mercury is also able to transfer methyl groups from CH3-Tetrahydrofolate to thiols.[2]

Environmental impact

Animal health

Methyl mercury is a toxic substance to living organisms. The toxicity of methylmercury in humans is due to methyl mercury crossing the blood-brain barrier and causing cell lysis in the central nervous system. The cell damage is irreversible. The half-life of methylmercury in human tissue is 70 days, which allows it ample time to accumulate to toxic levels. Humans are exposed to methyl mercury from the consumption of aquatic species. As mercury bioaccumulates through the food chain, the amount of methyl mercury increases to these toxic levels.[10][11][9]

See also

References

  1. ↑ Fleming EJ, Mack EE, Green PG, Nelson DC (January 2006). "Mercury methylation from unexpected sources: molybdate-inhibited freshwater sediments and an iron-reducing bacterium". Applied and Environmental Microbiology. 72 (1): 457–64. doi:10.1128/AEM.72.1.457-464.2006. PMC 1352261. PMID 16391078.
  2. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Bystrom E. Assessment of mercury methylation and demethylation with focus on chemical speciation and biological processes (PDF) (Ph.D. thesis). Georgia Institute of Technology.
  3. 1 2 Selin, Noelle E. (2009-10-15). "Global Biogeochemical Cycling of Mercury: A Review". Annual Review of Environment and Resources. 34 (1): 43–63. doi:10.1146/annurev.environ.051308.084314. ISSN 1543-5938.
  4. 1 2 3 4 Poulain AJ, Barkay T (March 2013). "Environmental science. Cracking the mercury methylation code". Science. 339 (6125): 1280–1. Bibcode:2013Sci...339.1280P. doi:10.1126/science.1235591. PMID 23493700. S2CID 206547954.
  5. 1 2 3 Ghimire PS, Tripathee L, Zhang Q, Guo J, Ram K, Huang J, Sharma CM, Kang S (2019-12-20). "Microbial mercury methylation in the cryosphere: Progress and prospects". Science of the Total Environment. 697: 134150. Bibcode:2019ScTEn.697m4150S. doi:10.1016/j.scitotenv.2019.134150. ISSN 0048-9697. PMID 32380618. S2CID 202870490.
  6. ↑ Date SS, Parks JM, Rush KW, Wall JD, Ragsdale SW, Johs A (2019-01-04). "Kinetics of enzymatic mercury methylation at nanomolar concentrations catalyzed by HgcAB: Supplementary information". bioRxiv: 510180. doi:10.1101/510180.
  7. 1 2 3 4 Gilmour CC, Bullock AL, McBurney A, Podar M, Elias DA (April 2018). Lovley DR (ed.). "Robust Mercury Methylation across Diverse Methanogenic Archaea". mBio. 9 (2): e02403–17, /mbio/9/2/mBio.02403–17.atom. doi:10.1128/mBio.02403-17. PMC 5893877. PMID 29636434.
  8. ↑ Kelly CA, Rudd JW, Holoka MH (July 2003). "Effect of pH on mercury uptake by an aquatic bacterium: implications for Hg cycling". Environmental Science & Technology. 37 (13): 2941–6. Bibcode:2003EnST...37.2941K. doi:10.1021/es026366o. PMID 12875398.
  9. 1 2 3 Date SS, Parks JM, Rush KW, Wall JD, Ragsdale SW, Johs A (July 2019). Kivisaar M (ed.). "Kinetics of Enzymatic Mercury Methylation at Nanomolar Concentrations Catalyzed by HgcAB". Applied and Environmental Microbiology. 85 (13): e00438–19, /aem/85/13/AEM.00438–19.atom. doi:10.1128/AEM.00438-19. PMC 6581168. PMID 31028026.
  10. 1 2 An J, Zhang L, Lu X, Pelletier DA, Pierce EM, Johs A, et al. (June 2019). "Mercury Uptake by Desulfovibrio desulfuricans ND132: Passive or Active?". Environmental Science & Technology. 53 (11): 6264–6272. Bibcode:2019EnST...53.6264A. doi:10.1021/acs.est.9b00047. OSTI 1530103. PMID 31075193. S2CID 149454648.
  11. ↑ Zhang L, Wu S, Zhao L, Lu X, Pierce EM, Gu B (March 2019). "Mercury Sorption and Desorption on Organo-Mineral Particulates as a Source for Microbial Methylation". Environmental Science & Technology. 53 (5): 2426–2433. Bibcode:2019EnST...53.2426Z. doi:10.1021/acs.est.8b06020. OSTI 1509536. PMID 30702880. S2CID 73449064.
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