Mercury methylation is the process of creating methylmercury (MeHg). This process can happen without living organisms (abiotically) or with the help of living organisms (biotically). When living organisms are involved, the main types of bacteria that add methyl groups to mercury are sulfate-reducing bacteria and iron-reducing bacteria. Scientists have identified three ways that sulfate-reducing bacteria may methylate mercury. In environments without oxygen, such as water or soil, sulfate-reducing bacteria, iron-reducing bacteria, and methanogens are often found to create methylmercury. These bacteria are frequently the main producers of methylmercury in areas without oxygen. However, recent research has shown that methylmercury can also form in oxygen-rich water through the same process involving the hgcAB gene pair.
Methylmercury is harmful because it can build up in the food chain and accumulate in the bodies of living organisms.
Chemistry
Chemical elements on Earth move through the air, land, and water in a process called biogeochemical cycling. Mercury follows its own version of this process, called the mercury cycle, where it moves through the environment and changes between different forms: Hg(0), Hg(I), and Hg(II). When mercury is in the environment, certain microorganisms can take in the elemental form of mercury. This process activates genes called hgcA and hgcB, which create proteins named HgcA and HgcB. These proteins begin a chemical reaction that forms methylmercury.
Organisms from all three major groups of life (bacteria, archaea, and eukaryotes) are involved in mercury methylation. Scientists have found more species that can methylate mercury because they carry the hgcAB genes. It is not yet known whether the HgcA and HgcB proteins work together as a group or one after the other. Studies show that if either gene is missing, the organism cannot methylate mercury.
Bacteria known to methylate mercury include species like Desulfovibrio desulfuricans and Geobacter sulfurreducens. Other bacteria with the hgcAB genes, such as those in the groups Bacteroidota, Chloroflexota, and Nitrospirota, are suspected to produce methylmercury.
Archaeal species that methylate mercury include most species in the Methanomicrobia class. The Thermoplasmata class also carries the hgcAB genes, but no other methanogen species have been found to methylate mercury.
In bacteria, most HgcA proteins are selenoproteins, which have a special part at the beginning that includes a CU (cysteine-selenocysteine) pair. A smaller number of HgcB proteins are also selenoproteins.
The effects of pH on mercury methylation vary depending on the species involved. Some studies show that higher hydrogen ion levels increase the uptake of Hg(II), which may affect how much mercury is methylated. Other studies found that lower pH levels change the types of methylmercury produced, with less dimethylmercury and more monomethylmercury, though the total amount remains similar.
Few studies have been published about how temperature affects mercury methylation. Mercury methylation is most active in summer, but this may be due to other factors, not just temperature. Temperature does affect microbial activity, which can influence the chemical reactions that lead to mercury methylation.
Similar to pH effects, the amount of available mercury ions affects the types of mercury products formed. The enzymes HgcA and HgcB have a very low Km, meaning they can bind to mercury even at very low concentrations.
Recent studies show that increased levels of terrestrial dissolved organic matter (tDOM), caused by more runoff from climate change in the Northern Hemisphere, lead to more bacterial activity and increased mercury methylation. Studies also show that higher methylmercury levels are linked to organic matter in ocean waters, where microorganisms break down particles in low-oxygen environments, allowing mercury-methylating bacteria to thrive.
Before mercury can be methylated, it must enter the cell through the cell membrane. Mercury ions are captured by a protein called MerP, which passes them to another protein, MerT, and then to an enzyme called mercuric reductase inside the cell.
Some microorganisms resist mercury toxicity through a system called the mer operon. This system produces mercuric reductase, which changes mercury ions into elemental mercury, which is then released from the cell. If mercuric reductase is not used, mercury can be methylated through three known pathways.
Sulfate-reducing bacteria grown without sulfate do not methylate mercury. It is possible that their respiration process is connected to mercury methylation.
The Acetyl-CoA pathway for mercury methylation is used by sulfate-reducing bacteria and involves a corrinoid-dependent protein. In this pathway, the methyl group comes from C-3 serine. Genes hgcA and hgcB are needed to transfer the methyl group from CH3-Tetrahydrofolate to the corrinoid protein, which then gives it to mercury. This process happens in low-oxygen environments.
The Acetate Metabolic pathway is similar to the Acetyl-CoA pathway, using methyltransferase enzymes with tetrahydrofolate intermediates. Studies show that mercury methylation is up to 1,000 times higher in cells that can use acetate.
Mercury can also be methylated using a cobalamin-dependent methionine synthase. This process uses S-adenosylmethionine, a natural methylating agent. Since methionine synthase is involved, the enzyme that methylates mercury might also transfer methyl groups from CH3-Tetrahydrofolate to thiols.
Methylmercury is harmful to living organisms. In humans, it crosses the blood-brain barrier, causing cell damage in the central nervous system. This damage is permanent. Methylmercury stays in human tissue for about 70 days, allowing it to build up to dangerous levels. People are exposed to methylmercury by eating fish and other aquatic animals. Mercury accumulates through the food chain, increasing to toxic levels in larger predators.