Mercury, methylation

The next eight chapters will be devoted to the ecotoxicology of groups of compounds that have caused concern on account of their real or perceived environmental effects and have been studied both in the laboratory and in the field. These are predominantly compounds produced by humans. However, a few of them, for example, methyl mercury, methyl arsenic, and polycyclic aromatic hydrocarbons (PAHs), are also naturally occurring. In this latter case, there can be difficulty in distinguishing between human and natural sources of harmful chemicals. 

Choi S-C, TT Chase, R Bartha (1994) Metabolic pathways leading to mercury methylation in Desulfovibrio desulfuricans LS. Appl Environ Microbiol 60 4072-4077. 

Ekstrom EB, FM Morel, JM Benoit (2003) Mercury methylation independent of the acetyl-coenzyme A pathway in sulfate-reducing bacteria. Appl Environ Microbiol 69 5414-5422. 

Fleming EJ, EE Mack, PG Green, DC Nelson (2006) Mercury methylation from unexpected sources molybdate-inhibited freshwater sediments and an iron-reducing bacterium. Appl Environ Microbiol 72 457-464. 

Pak K-R, R Bartha (1998) Mercury methylation and demethylation in anoxic lake sediments by strictly anaerobic bacteria. Appl Environ Microbiol 64 1013-1017. 

Furutani A, Rudd JWM. 1980. Measurement of mercury methylation in lakewater and sediment samples. Appl Environ Microbiol 40 770-776. 

Krabbeiihoft DP, Orem W, Aiken G, Gilmour CG. 2004. Unraveling the complexities of mercury methylation in the Everglades the use of mesocosms to test the effects of new mercury, sulfate, and organic carbon. Proc 7th Int Conf Mercury Pollut, RMZ-MG, 51(l-3) June2004. 

St. Louis VL, Rudd JWM, Kelly CA, Bodaly RA, Paterson MJ, Beaty KG, Hesslein RH, Heyes A, Majewski AR. 2004. The rise and fall of mercury methylation in an experimental reservoir. Environ Sci Technol 38 1348-1358. 

Cleckner LB, Gihnom CC, Krabbenhoft DP, Hmley JP. 1999. Mercury methylation in periphyton of the Florida Everglades. Limnol Oceanogr 44 1815-1825. 

Miskimmin BM, Rudd JWM, Kelly CA. 1992. Influence of dissolved organic carbon, pH, and microbial respiration rates on mercury methylation and demethylation in lake water. Can J Fish Aquat Sci 49 17-22. 

Akagi, H.. Mortimer, D.C., and Miller. D.R. Mercury methylation and partition in aquatic systems. Bull Environ. Contam. Toxicol., 23(2) 372-376, 1979. 

Microbial methylation is a reaction that affects mainly properties of toxic, inorganic hace elements, which involves the addition of a methyl group to the contaminant molecule. It occurs under aerobic or anaerobic conditions. Mercury methylation, for example, occurs under both conditions and leads to the release of mercury into the atmosphere. 

There are several different types of organic mercury, but by far the most important in terms of health effects is methyl mercury. When atmospheric mercury is deposited on the ground or in the water, it is converted to methyl mercury by bacteria. Mercury compounds are very toxic and this is the bacteria s way to detoxify mercury. Small animals then consume the bacteria, along with the methyl mercury and bigger animals in turn consume the smaller animals, thus increasing the concentrations of methyl mercury. Methyl mercury accumulates in the larger carnivorous animals, most important of which are fish such as tuna, pike, and shark. 

Mercury accumulates in the muscle of the fish, which makes it all but impossible to avoid consumption of the methyl mercury. Methyl mercury is readily absorbed from the intestine and crosses the blood-brain barrier and the placenta. 

The above reaction proceeds through the formation of mercury methyl chloride as an intermediate (not shown above). 

Schroeder HA, Mitchener M. 1975. Life-term effects of mercury, methyl mercury and nine other trace metals on mice. J Nutr 105 452-458. 

Natural waters Mercury Methyl, ethyl and inorganic Helium plasma TM10i cavity 0.05 ng T1 methyl and ethyl mercury Emteborg et al. (1993)... 

Methyl mercury is of much greater concern when health effects are considered, as it is much more toxic than ionic mercury or free mercury. Methyl mercury is also much more likely to be bioaccumulated, leading to serious contaminations, especially of fish. The speciation for mercury can be accomplished by derivatizing the methyl mercury and Hg2+ with sodium tetraethylborate, NaBEt4. The volatile MeHgEt, from methyl mercury, and HgEt2, from Hg2+, species formed are purged from the sample solution and separated in a GC column. An atomic emission spectrometer is used as a detector. 

Gilmour, C.C., Henry, E.A., and Mitchell, R. (1992) Sulfate stimulation of mercury methylation in freshwater sediments. Environ. Sci. Technol. 26, 2281-2288. 

Minamata is an industrial city on the Yatsushiro coast of Japan on the southernmost island (K5rushu). In the city there was a factory that manufactured the chemicals vinyl chloride (used to make the plastic PVC see pp. 168-71) and acetaldehyde for many years. The processes used inorganic mercury (mercuric oxide) as a catalyst. The effluent from the factory contained inorganic mercury and perhaps also some organic mercury (methyl mercury), produced as a by-product in the chemical reaction in the plant. This effluent was discharged into the waters of Minamata Bay. 

Environmental Mercury Methylation 9.04.5.LI Near-shore regions... 

Figure 8 Sulfate/Sulfide controls on mercury methylation in aquatic environments —the Gilmour curve. At relatively low sulfate concentrations (most freshwaters), methylation of mercury is limited hy the rate of sulfate reduction. At higher sulfate concentrations (saltwaters), sulfide buildup from relatively high rates of sulfate reduction results in decreased bioavailahility of mercury (figure from Danger et al. (2001) after Gilmour and Henry (1991).

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