Introduction

Microorganisms play an essential role in the recycling of various elements. The carbon, nitrogen and sulfur cycles are well known, but biochemical cycles of heavy metals also occur in the aquatic and terrestrial environment. In the case of the microbial methylation of metals, a lot of studies have been performed, but these have largely focussed on aquatic systems. A well-studied example is the methylation of mercury. The accident in the 1950 in Minamata, Japan, prompted intense research into the influence of organic mercury in humans (Ekino et al. 2007). On the other hand, biomethylation pathways also generated special interest after the accident. In the following years, many studies provided information on alkylation mechanisms (Mason et al. 1995a-c) and the mercury cycle. These investigations led to detailed knowledge of bioaccumulation in aquatic invertebrates and in fish (Westoo 1966; Mason et al. 2000; Hightower and Moore 2003). As well as mercury, other metals such as bismuth and metalloids like arsenic or selenium have also become targets for biomethylation research. There is special interest in selenium because it is essential (in small amounts) to proper functioning in living organisms (Rotruck et al. 1973). In contrast to the aquatic pathways of alkylation, data on alkylation pathways in terrestrial ecosystems and their organisms are scarce. Just like in aquatic systems, microorganisms like bacteria and fungi are mainly responsible for the alkylation of metals and also their biochemical pathways in terrestrial habitats. Cobalamin is involved in the transfer of methyl groups to inorganic mercury (Bertilsson and Neujahr 1971; Choi and Bartha 1993). In the case of arsenic and selenium, the

Institute of Microbiology and Wine Research, Johannes Gutenberg-University,

Becherweg 15, 55099, Mainz, Germany e-mail: [email protected]; hkö[email protected]

This article is dedicated to Prof. Dr. Ajit Varma on the occasion of his 70th birthday.

I. Sherameti and A. Varma (eds.), Soil Heavy Metals, Soil Biology, Vol 19, DOI 10.1007/978-3-642-02436-8_14, © Springer-Verlag Berlin Heidelberg 2010

methylation is achieved by S-adenosylmethionine (Challenger 1945). Another possible biochemical pathway for methyl group transfer was postulated by Choi et al. 1994. In this work, it was demonstrated that the methyl group is transferred to mercury in Desulfovibrio desulfuricans strain LS by methyltetrahydrofolate via methylcobalamin.

Besides methylating metals, a few microorganisms are also able to reduce them (Lloyd et al. 2003). In the case of mercury, for example, some species of bacteria can reduce Hg2+ to nontoxic, volatile, elemental mercury. A gene called the mer gene is linked to this reaction. This ability exhibited by a few species of mercury-resistant bacteria is used for the bioremediation of mercury-contaminated soil, and also as biosensors that are used to monitor heavy metals (Bontidean et al. 2002; Lloyd and Lovley 2001).

Recently, the biomethylation of terrestrial microbial flora has been investigated in more depth. For example, Hirner et al. (2000) and Meyer et al. (2007) studied soil samples with different origins and degrees of contamination for their metal alkylation potentials. Recently, Raposo et al. (2008) demonstrated a relationship between the alkylation of mercury and microbiological activity in soil samples. However, there is still a lot of work to be done before we can fully comprehend the terrestrial alkylation pathways of metals.

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