Methylmercury trophic transfer

Figure 11. Uptake rates of inorganic Hg (a) and of methylmercury (b) by a marine alga as a function of the octanol-water distribution ratio of the Hg-species under various conditions of pH and chloride concentrations. The neutral species HgCl and CH5HgClH diffuse through the membranes. Reprinted with permission from [79] Mason, R. P. et al. (1996). Uptake, toxicity, and trophic transfer in a coastal diatom , Environ. Sci Technol., 30, 1835-1845 copyright (1996) American Chemical Society...

O2 concentrations, such as found in marine wetlands. High biomethylation rates have also been observed in coastal sediments. Because methylmercury is transferred up the food chain, the marine fish that occupy high trophic levels have very high mercury concentrations. In some cases, such as for tuna and swordfish, concentrations are high enough to pose human health risks. 

Most of the discrimination between inorganic and methylmercury thus occurs during trophic transfer, while the major enrichment factor is between water and the phytoplankton. This also has been reported for the diatom Thalassiosura weissflogii in a marine food chain (Mason et al. 1996). Methylmercury was accumulated in the cell cytoplasm, and its assimilation by copepods was 4 times more efficient than the assimilation of inorganic mercury. Bioaccumulation has been demonstrated for predator fish in both freshwater and marine systems and in marine mammals (see Section 5.4.4). Bioaccumulation of methylmercury in aquatic food chains is of interest, because it is generally the most important source of nonoccupational human exposure to this compound (EPA 1984b WHO 1990, 1991). 

Methylmercury is produced by methylation of inorganic mercury present in both freshwater and saltwater sediments, and accumulates in aquatic food chains in which the top-level predators usually contain the highest concentrations. The percent of total mercury accounted for by methylmercury generally increases with higher trophic levels, confirming that methylmercury is more efficiently transferred to higher trophic levels than inorganic mercury compounds. 

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