X

a With permission from Klein and Thayer (1990).

a With permission from Klein and Thayer (1990).

Alcaligenes, and Acinetobacter might also be included based on Hg resistance, which correlates with Hg2+-reducing ability.

Biomethylation Transformations

Microorganisms have been shown to be intimately involved in the interconversions of metals between inorganic and organic compounds. Many metals and metalloids with toxic potential, such as Hg, As, Cd, Pb, Se, Sn, and Te, are subject to alkylation. Binding a C atom to the metal changes an element's volatility, solubility, and toxicity. All biotic alkylations are accomplished by one of three coen-zymes: N-methyltetrahydrofolate, S-adenosylmethionine, or methylcobalamine. Methylation is accomplished by a wide array of fungi and bacteria (Table 15.13), and a given species of microorganism may methylate several different metals. Methylation affects elements differently. Cadmium alkyls are water insoluble, while the methylated forms of As are soluble and highly toxic. Similarly to Hg, methylated Se undergoes atmospheric transformations that foster detoxification, deposition, and bioaccumulation.

The methylation of Hg can occur under aerobic or anaerobic conditions, but the process was found to occur more rapidly under the latter conditions (Robinson and Tuovinen, 1984). Aerobic organisms such as Pseudomonas spp., Bacillus mega-terium, E. coli, En. aerogenes, and even fungi such as A. niger and Scopulariopsis brevicaulis have been found capable of Hg methylation. However, sulfate reducers such as Desulfovibrio desulfuricans appear to be the principal methylators of Hg. The reaction occurs through the transfer of methyl groups from methylcobal-amine (cobalt prophyrin), or from methylated vitamin B12, to Hg2+. The normal role of methylcobalamine in D. desulfuricans is to provide the methyl group in acetate synthesis from CO2 using the acetyl-SCoA pathway. The Hg2+ acts as a competing methyl acceptor during acetate formation (Choi et al., 1994). The micro-bially mediated reaction is a protection mechanism for the microorganism as it increases the solubility and volatility of the Hg. However, methylmercury is lipophilic and can be accumulated in higher organisms.

environmental significance of p, s, and metal biogeochemistry

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