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How does it make it from its source?
How does it make it from its source?
Rigorously speaking, anoxic zones in the sediments (or even in the same water column) are the source of methylmercury, where Hg-methylating microorganisms thrive and are capable of dealing with the mercury entering their metabolism. They release CH3Hg+ as a by-product, which diffuses to the oxygen-rich water phase.
At this point, methylmercury can basically take one of three paths:
Become dimethylmercury, not bioaccumulate, volatilize and leave the aquatic system.
Oxidize back to inorganic Hg (aka demethylation, a process that can also be microbially mediated).
Lose its charge by joining chloride or hydroxyl anions and make its way diffusing and permeating the cellular membranes of plankton communities.
Mercury cycle in an aquatic environment: Hg(II) becomes MeHg due the action of microbial activity. it is then transported towards the water column where it enters the food web, increasing its concentration as the trophic level gets higher.
Source: Global Biogeochemical Cycling of Mercury: A Review, Noelle E. Selin, Annual Review of Environment and Resources 2009 34:1, 43-63
Source: Global Biogeochemical Cycling of Mercury: A Review, Noelle E. Selin, Annual Review of Environment and Resources 2009 34:1, 43-63
Following path No. 3... a low pH favors the complexation of methylmercury with chloride. This new compound -CH3HgCl- has a lipid solubility (i.e., how likely it is to partition in a cell's cytosol) and permeability through the cell membrane that are similar to that of HgCl2 [3]. This is measured via the octanol-water partition coefficient which in this case is around Kow = 1.7, meaning that CH3HgCl can easily sneak into the bodies of aquatic cells (like plankton).
... but notice that following this logic, inorganic mercury in the form of HgCl2 would bioaccumulate as well...
Global mercury cycle:
Source: Morel, F. M. M., Kraepiel, A. M. L., & Amyot, M. (1998). The Chemical cycle and bioaccumulation of mercury. Annual Review of Ecology and Systematics, 29(1), 543–566. DOI: 10.1146/annurev.ecolsys.29.1.543
Source: Morel, F. M. M., Kraepiel, A. M. L., & Amyot, M. (1998). The Chemical cycle and bioaccumulation of mercury. Annual Review of Ecology and Systematics, 29(1), 543–566. DOI: 10.1146/annurev.ecolsys.29.1.543
Big scale
(Not in our scope but worth to mention?)Big scale
In the big scale picture, we can consider to main routes for inorganic mercury reaching the zones where methylation occurs in aquatic systems:
One is the atmospheric mercury that is emitted naturally from volcanic activity and from anthropogenic sources like energy production from coal burning . These emissions are transported by wind movement to eventually deposit in soils and water bodies. Another concern in the global mercury cycle is the stored mercury in permafrost whose thawing would represent a massive addition of mercury into the cycle [8].
The other route is from point sources where the intensive use of mercury ends up directly in near-by streams, transported and deposited downstream afterwards. This is the case of industrial processes releasing the mercury to the environment, for instance, the Minamata disaster or -currently- artisanal gold mining involving mercury amalgamation.
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