Methylmercury formation in boreal wetlands in relation to chemical speciation of mercury(II) and concentration of low molecular mass thiols.

Methylmercury (MeHg) is a neurotoxin formed from inorganic divalent mercury (Hg) via microbial methylation, and boreal wetlands have been identified as major sources of MeHg. There is however a lack of studies investigating the relationship between the chemical speciation of Hg and MeHg formation in such environments, in particular regarding to role of thiol compounds. We determined Hg methylation potentials, k, in boreal wetland soils using two Hg isotope tracers: Hg(OH)(aq) and Hg bonded to thiol groups in natural organic matter, Hg-NOM(ads), representing Hg sources with high and low availability for methylation. The Hg(OH)(aq) tracer was consistently methylated to a 5-fold higher extent than Hg-NOM(ads), independent of environmental conditions. This suggests that the concentration of Hg in porewater was a decisive factor for Hg methylation. A comprehensive thermodynamic speciation model (including Hg complexes with inorganic sulfide (HS), polysulfides (HS), thiols associated with natural organic matter (NOM-RSH) and specific low molecular mass thiols (LMM-RSH) provided new insights on the speciation of Hg in boreal wetland porewaters, but did not demonstrate any clear relationship between k and the calculated chemical speciation. In contrast, significant positive relationships were observed between k and the sum of LMM thiol compounds of biological origin. We suggest two possible mechanisms underlying these correlations: 1) LMM thiols kinetically control the size and composition of the Hg pool available for microbial uptake, and/or 2) LMM thiols are produced by microbes such that the correlation reflects a relation between microbial activity and MeHg formation.