Selenium and Arsenic

Selenium is, in contrast to mercury, a metalloid that is essential for the proper functioning of living organisms. It is involved in enzymatic and other biochemical reactions. Rotruck et al. (1973) showed that selenium has a protective function against oxidative stress. In proteins, selenium is a component of the twenty-first amino acid (selenocysteine), where it functions as a redox-sensitive centre. Another difference from mercury is that the methylation of selenium does not lead to a more toxic product; instead, this is considered to be a detoxification reaction.

Rosenheim (1902) performed biological experiments with the fungus Scopulariopsis brevicaulis. He applied different arsenic species and observed a garlic smell. This smell was not released when he used pure arsenic. Challenger and North (1934) identified dimethylselenide as a reaction product. Challenger (1945) proposed a reaction mechanism for the methylation of selenium that is now known as the Challenger mechanism. It is a combination of reduction and methylation reactions that is mainly observed when S-adenosylmethionine is available as a donor for the methyl group. Another possible pathway is comparable to the methylation of mercury, where methylcobalamin can transfer the methyl group to selenium and produce organic selenium species (McBride and Wolfe 1971; Thompson-Eagle et al. 1989).

Thompson-Eagle et al. (1989) investigated the volatilisation of selenium by the fungus Alternaria alternata. For this study, water samples from different sites were collected containing between 0.005 and 5 mg Se per litre were collected. The authors were able to isolate A. alternata from the samples, and they measured the production of the volatile dimethylselenide under different conditions (pH values, temperatures, Se substrates and methyl donors). The optimal conditions for methy-lation by A. alternata were 30°C and a pH value of 6.5 using Se(VI) as the source of selenium. For the methyl donors, only a small difference between L-methionine and methyl cobalamin was measured. Another work with A. alternata that investigated the bioavailability of inorganic selenium adsorbed to different kinds of soil samples was performed by Peitzsch (2008).

As in the case of mercury, a lot of studies have been performed on the methylation and demethylation of arsenic in ecosystems such as freshwaters (Bohari et al. 2001) and wastewaters (Segura et al. 2002). Data on the methylation of arsenic species in soil habitats are scarce. Just as it is with selenium, the methylation of arsenic is considered a detoxification reaction. The proposed pathway to the methylation of arsenic is (just like selenium) the Challenger mechanism. In the work of Pinel-Raffaitin et al. (2007), the release of inorganic and organic arsenic from landfill leachates and biogases was measured. Hirner et al. (2000) investigated the amounts of organometallic species and metalloids like arsenic in various contaminated soil samples and directly in shredder (fresh car metal and electronic waste). All methylated forms of arsenic were found in the contaminated soil, in contrast to the shredder. The importance of microorganisms in the biomethylation of arsenic was demonstrated by these authors. Duester et al. (2005) measured the concentrations of organic arsenic species in urban soils near German cities. The highest concentrations were observed in agricultural and garden soils. Soils from abandoned industrial sites showed lower concentrations. This can be explained by the increased biological activity in agricultural and garden soil. Because of their higher levels of contamination, their artificial substrates and also the destruction of the natural soil structure, the activities in the industrial soils were significant lower.

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