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What are the processes governing its journey?
What are the processes governing its journey?
At this point, the processes involving methylmercury moving in an aquatic system have been:
Inorganic mercury deposits in sediments until reaching anoxic regions.
Hg(II) enters microorganisms capable of its methylation and MeHg is released.
It leaves the sediment beds via diffusing towards the water column or aided by some perturbation of the sediments (like bioturbation).
MeHg complexates with (mainly) chloride anions.
This form of MeHg can permeate and partition in cell bodies.
After this, we were kept wondering... if inorganic mercury also permeates and partitions into cells, why is MeHg worst?
Turns out it all depends where in the cell each of the mercury species is fixed. Hg(II) binds better to particulate material (i.e., membranes) whereas MeHg associates with the soluble fraction of the cell. When a higher organism consumes a cell with both Hg(II) and methylmercury, only MeHg will be absorbed because the particulate cellular material is excreted [3]. Thanks (ad)sorption.
MeHg and Hg(II) fates after a trophic level jump: At the right, MeHg associates with the soluble part of a diatom while Hg(II) does it with its particulate portion. At the left, a copepod eats the diatom, absorbing its soluble part but excreting the particulate portion.
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
Overall mercury speciation in different trophic levels: Whilemethylmercury in the water column contitues around 10% of the total mercury in an aqueous system, MeHg accounts for more of 90% of the total mercury found in fish muscles and lipids.
Source: Built upon data from · 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 and · Hong, Y. S., Kim, Y. M., & Lee, K. E. (2012). Methylmercury exposure and health effects. Journal of preventive medicine and public health = Yebang Uihakhoe chi, 45(6), 353–363. DOI: 10.3961/jpmph.2012.45.6.353
Source: Built upon data from · 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 and · Hong, Y. S., Kim, Y. M., & Lee, K. E. (2012). Methylmercury exposure and health effects. Journal of preventive medicine and public health = Yebang Uihakhoe chi, 45(6), 353–363. DOI: 10.3961/jpmph.2012.45.6.353
This phenomenon is referred to as methylmercury having a greater efficiency of transfer. CH3HgCl continues its journey up the food web thanks to its lipid solubility: it binds to the lipids in fish and other higher animals where it accumulates over time [3]. It is important to mention that most methylmercury in higher animals comes from ingesting methylmercury rich food, rather than the MeHg directly going from the aqueous solution into the animals body.
But! When we reach the fish level in a food web, the amount of methylmercury in muscle tissue is greater than in lipids, meaning that another mechanism -besides MeHg lipid solubility- is involved in the mercury bioaccumulation: affinity of fish intestines to absorb MeHg.
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