Methyl bioaccumulation

Demethylation in the water column and sediments is receiving increasing attention. Both abiotic (e.g.. Sellers et al., 1996, 2001) and biotic (e.g., Pak and Bartha, 1998 Marvin-Dipasquale and Oremland, 1998 Marvin-Dipasquale et al., 2000 Hintehnann et al., 2000) processes are imphcated. The result is that MMHg accumulation in aquatic systems represents a balance between methylation, bioaccumulation, and the demethylation processes. In sediments, MMHg decomposition is particularly important, and it is possible that some sediments represent net sinks, rather than net sources, for MMHg in the water column. 

Food Chain Bioaccumulation. There are a few studies to determine residues of methyl parathion in organisms in the environment. These have consistently shown low methyl parathion residues, indicating that methyl parathion does not bioconcentrate to a significant extent in aquatic organisms, plants, or animals (Crossland and Bennett 1984 Sabharwal and Belsare 1986). The methyl parathion that does get into organisms is rapidly degraded (Sabharwal and Belsare 1986). Some recent analyses of fish in a... 

In a report from the U.S. EPA (1980), fish contained between 10,000 and 100,000 times the concentration of methyl mercury present in ambient water. In a study of methyl mercury in fish from different oceans, higher levels were reported in predators than in nonpredators (see Table 8.2). Taken overall, these data suggest that predators have some four- to eightfold higher levels of methyl mercury than do nonpredators, and it appears that there is marked bioaccumulation with transfer from prey to predator. 

In a laboratory study (Borg et al. 1970), bioaccumulation of methyl mercury was studied in the goshawk (Accipiter gentilis). The details are shown in Table 8.3 below. 

Thus, chickens bioaccumulated methyl mercury to about twice the level in their food, whereas goshawks bioaccumulated methyl mercury to about four times the level present in the chicken upon which they were fed. The period of exposure was similar... 

Mercury, tin, lead, arsenic, and antimony form toxic lipophilic organometallic compounds, which have a potential for bioaccumulation/bioconcentration in food chains. Apart from anthropogenic organometallic compounds, methyl derivatives of mercury and arsenic are biosynthesized from inorganic precursors in the natural environment. 

Harvey J, JJ Dulka, JJ Anderson (1985) Properties of sulfometuroin methyl affecting its environmental fate aqueous hydrolysis and photolysis, mobility and adsorption on soils, and bioaccumulation potential. J Agric Food Chem 33 590-596. 

Letcher RJ, RJ Norstrom, DCG Muir (1998) Biotransformation versus bioaccumulation sources of methyl sulfone PCB and 4,4 -DDE metabolites in polar bear food chain. Environ Sci Technol 32 1656-1661. 

In the 1960s, organic residues (e.g. DDT, PCBs, methyl mercury) began to be detected in several species of shellfish, fish and fish-eating birds [1,2]. Since then, assessment of the bioaccumulation of chemicals has been considered decisive for determining the potential hazard and environmental risk it is regulated by various official organisations such as the OECD [3], EPA [4,5] and ASTM [6],... 

General facts bacteria convert inorganic mercury to methyl mercury, then it enters the food chain (bioaccumulation)... 

The primary human exposure to methyl mercury is from consumption of contaminated fish. The most sensitive population is the developing fetus or infant due to the effects of methyl mercury on the nervous system (neurotoxic) and developmental effects. Exposure limits and fish consumption advisories are directed at pregnant women, women of childbearing age, and children. All agencies also recognize that fish consumption has many nutritional benefits and is an important part of many people s diet. Nevertheless, the widespread distribution of mercury and subsequent bioaccumulation of methyl mercury requires that many agencies have developed recommendation for levels of mercury in fish. Below is a list of some of these recommendations, but it is very important to consult the local fish consumption advisories. 

It turns out that most of these compounds have similar characteristics that contribute to their toxicity to both humans and other species of plants and animals. First, the compounds are environmentally persistent. Many of the early pesticides, and certainly the metals, do not break down in the environment or do so only very slowly. If persistent chemicals are released continually to the environment, the levels tend to rise ever higher. This means they are available to cause harm to other organisms, often not even the target of the pesticide. Second, the early pesticides were broad acting and toxic to many species, not just the target species. These poisons often killed beneficial insects or plants. Third, many of these compounds would bioaccumulate or concentrate in species as they moved up the food chain. The chlorinated pesticides accumulate in the fat of animals. Animals that consumed other animals accumulated more and more of these pesticides. Most species could not metabolize or break down the compounds. Lead accumulates in bone and methyl mercury in muscle. And finally, because of their persistence in the environment and accumulation in various species, the persistent toxicants spread around the world even to places that never used them. Animals at the top of the food chain, such as polar bears and beluga whales, routinely have fat PCB levels greater that 6 ppm. 

Bioaccumulate The ability of some organisms to accumulate specific compounds Fish accumulate methyl mercury DDT or PCBs accumulate in fat... 

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