Selenium compounds, biological methylation

It is now well established that organometallic compounds are formed in the environment from mercury, arsenic, selenium, tellurium and tin and hence were also deduced on the basis of analytical evidence for lead, germanium, antimony and thallium. Biological methylation of tin has been demonstrated by the use of experimental organisms. Methylgermanium and methyllead were widely found in the environment but it is debatable whether germanium and lead are directly methylated by biological activity in natural environment. 

Phytovolatilization involves the use of plants and plant-associated soil microbes to take up contaminants from the soil, transform them into volatile forms, and release them into the atmosphere (Lin, 2008). Phytovolatilization occurs as growing trees and other plants take up water and the organic and inorganic contaminants. Metalloids, such as selenium. As, and tin, can be methylated to volatile compounds or mercury that can be biologically transformed to elemental Hg. Phytovolatilization has been primarily used for the removal of mercury and selenium. 

Selenium is present in biological materials in a number of compounds that correspond in their structure to sulfur compounds. The amino acid L-selenocysteine (see Section 2.2.1.1.1) bound in proteins arises from selane and O-acetyl-L-serine, methyla-tion yields Se-methyl-L-selenocysteine, reaction with O-succinyl-L-homoserine leads to L,L-selenocystathionine, hydrolysis of the last compound provides i-selenohomocysteine and methylation... 

Studies of biological selenium conversions show that microorganisms can metabolize selenium. Chau et al. [12] found that several types of aquatic bacteria converted seleno-compounds to volatile methylated forms. In nature, all valence forms of selenium exist. These forms depend on the solubility and oxidation reduction reactions possible in the environment. Figure 1 illustrates the pH and pE relation-... 

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