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

Van Liem-Nguyen; Ulf Skyllberg; Erik Björn

doi:10.1016/j.scitotenv.2020.142666

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

Sci Total Environ. 2021 Feb 10;755(Pt 2):142666. doi: 10.1016/j.scitotenv.2020.142666. Epub 2020 Oct 3.

Authors

Van Liem-Nguyen¹, Ulf Skyllberg², Erik Björn³

Affiliations

¹ Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden; Laboratory of Advanced Materials Chemistry, Advanced Institute of Materials Science, Ton Duc Thang University, Ho Chi Minh City, Viet Nam.
² Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, SE-901 83 Umeå, Sweden.
³ Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden. Electronic address: erik.bjorn@umu.se.

PMID: 33059147
DOI: 10.1016/j.scitotenv.2020.142666

Abstract

Methylmercury (MeHg) is a neurotoxin formed from inorganic divalent mercury (Hg^II) 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^II and MeHg formation in such environments, in particular regarding to role of thiol compounds. We determined Hg^II methylation potentials, k_meth, in boreal wetland soils using two Hg^II isotope tracers: ¹⁹⁸Hg(OH)₂(aq) and Hg^II bonded to thiol groups in natural organic matter, ²⁰⁰Hg^II-NOM(ads), representing Hg^II sources with high and low availability for methylation. The ¹⁹⁸Hg(OH)₂(aq) tracer was consistently methylated to a 5-fold higher extent than ²⁰⁰Hg^II-NOM(ads), independent of environmental conditions. This suggests that the concentration of Hg^II in porewater was a decisive factor for Hg^II methylation. A comprehensive thermodynamic speciation model (including Hg^II complexes with inorganic sulfide (H₂S), polysulfides (H₂S_n), thiols associated with natural organic matter (NOM-RSH) and specific low molecular mass thiols (LMM-RSH) provided new insights on the speciation of Hg^II in boreal wetland porewaters, but did not demonstrate any clear relationship between k_meth and the calculated chemical speciation. In contrast, significant positive relationships were observed between k_meth 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^II 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.

Keywords: Bioavailability; Kinetics; Mercury methylation; Soil; Thermodynamics.