Overview of Methylation and Demethylation Mechanisms and Influencing Factors of Mercury in Water
Abstract
:1. Introduction
2. Methylation of Mercury in the Aquatic Environment
2.1. Microbial Pathway Methylation
2.2. Abiotic Pathway Methylation
3. Demethylation of Mercury in Aquatic Environments
4. Factors Influencing Mercury Methylation and Demethylation
4.1. Redox Conditions
4.2. Organic Substances
4.3. Sulphide
4.4. Temperature
4.5. pH
4.6. Iron and Manganese Oxides
5. Conclusions and Outlook
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Type | Average Methylmercury Content | Reference |
---|---|---|
Freshwater Crab | 0.028 mg/kg | [32] |
Freshwater Fish | 0.034 mg/kg | [32] |
Marine Fish | 0.031 mg/kg | [32] |
Rice | 0.00137 ± 0.00118 mg/kg | [33] |
Beryx splendens | 0.78 ± 0.56 mg/kg | [34] |
Atlantic Thunnus thynnus | 0.42 ± 0.06 mg/kg | [34] |
Thunnus obesus | 0.98 ± 0.34 mg/kg | [34] |
Tetraptrus audax | 0.51 ± 0.08 mg/kg | [34] |
Hypophthalmichthys moritrix | 0.18 ± 0.09 mg/kg | [35] |
Sediment | 0.06 mg/kg to 1.38 mg/kg | [35] |
Australian Reed | 0.618 mg/kg | [36] |
Carps | 0.019 mg/kg to 0.063 mg/kg | [37] |
Long-tailed Tuna | 0.180 mg/kg to 1.460 mg/kg | [38] |
Sri Lankan Rice | 0.0051 ± 0.37 mg/kg | [39] |
Type of Demethylation | Mechanism of Action | Characteristics | References |
---|---|---|---|
Mer operon demethylation | Genes encode lytic and reductive enzymes that lyse methylmercury to methyl and mercury(II) and reduce mercury(II) to mercury(0) | Widespread in mercury-resistant bacteria | [61,62] |
Demethylation of methane nutrients | Bacteria produce methanobactin molecules to promote methane oxidation and methylmercury degradation | Uses the methyl group in methylmercury as an auxiliary C1 carbon source for microorganisms | [65] |
Phytoplankton demethylation | Phytoplankton utilise endogenous reactive oxygen species as the main driver of demethylation | Can degrade methylmercury by dark reaction | [66,67] |
Selenium amino acid demethylation | Methylmercury and selenoamino acids through the formation of bis(methylmercury) selenide and dimethylmercury as intermediates, with HgSe(s) as the final degradation product | Laboratory stage, not proven in natural environment | [70,71] |
Photodemethylation pathway 1 | Methylmercury causes C-Hg bond breaking by direct absorption of light energy | Most important demethylation pathway in the aquatic environment, about 56–80% of methylmercury is photodegradable (pathway1, 2, 3) | [85] |
Photodemethylation pathway 2 | Reactive oxygen species (ROS) and other photoactive substances, produced by organic molecules, ions, suspended solids, etc., attack the C-Hg bond and degrade MeHg when exposed to sunlight | [86,87] | |
Photodemethylation pathway 3 | When MeHg is complexed with photochemically active dissolved organic matter (DOM) and exposed to light, the excited DOM-MeHg complex may undergo intramolecular electron transfer, leading to C-Hg bond breaking | [88] |
Influencing Factors | Effects on Methylation |
---|---|
Redox conditions | The anoxic environment is the primary environment for methylation, but methylation under anoxic and oxidative conditions both may occur. |
Organic substances | Several recent studies have demonstrated a clear correlation between certain organic substances and methylmercury concentrations. Organic substances act in a number of ways, by stimulating microbial activity and by providing methyl groups. |
Sulphide | Increased sulphate may stimulate methylation. |
Temperature | The rate of methylmercury production is generally positively correlated with temperature. |
PH | May be negatively correlated, with higher levels of methylmercury prevalent at low pH. |
Iron and manganese oxides | Methylation levels are highest in areas rich in dissolved iron and organic matter, but it is still not possible to determine their exact effect or impact. |
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Zhao, W.; Gan, R.; Xian, B.; Wu, T.; Wu, G.; Huang, S.; Wang, R.; Liu, Z.; Zhang, Q.; Bai, S.; et al. Overview of Methylation and Demethylation Mechanisms and Influencing Factors of Mercury in Water. Toxics 2024, 12, 715. https://doi.org/10.3390/toxics12100715
Zhao W, Gan R, Xian B, Wu T, Wu G, Huang S, Wang R, Liu Z, Zhang Q, Bai S, et al. Overview of Methylation and Demethylation Mechanisms and Influencing Factors of Mercury in Water. Toxics. 2024; 12(10):715. https://doi.org/10.3390/toxics12100715
Chicago/Turabian StyleZhao, Wenyu, Runjie Gan, Bensen Xian, Tong Wu, Guoping Wu, Shixin Huang, Ronghua Wang, Zixuan Liu, Qin Zhang, Shaoyuan Bai, and et al. 2024. "Overview of Methylation and Demethylation Mechanisms and Influencing Factors of Mercury in Water" Toxics 12, no. 10: 715. https://doi.org/10.3390/toxics12100715