Net production of the toxic and bioaccumulating methyl mercury (MeHg) molecule in wetlands is a major environmental health issue. In this paper factors and processes in control of MeHg net production in wetlands are covered. Of major importance are methylation, demethylation and photo-induced redox processes. All these reactions are linked to the biogeochemistry of C, S and Fe via an intricate interplay with iron(III) and sulphate reducing bacteria. A special focus is put on the link between the chemical speciation of Hg(II) and uptake of bioavailable forms of Hg(JJ) by methylating bacteria. A key role is played by organic substances. They control the transport of Hg(II) and MeHg, as well as the bioavailability of Hg(II) and the energy source for methylating bacteria. Research needs are identified. In particular, limited fundamental knowledge about demethylation processes may restrict society to make correct decisions regarding e.g. restoration efforts of wetlands.
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