Methanogens: Principal Methylators of Mercury in Lake Periphyton

Abstract

Mercury methylation and demethylation rates were measured in periphyton biofilms growing on submerged plants from a shallow fluvial lake located along the St. Lawrence River (Quebec, Canada). Incubations were performed in situ within macrophytes beds using low-level spikes of (199)HgO and Me(200)Hg stable isotopes as tracers. To determine which microbial guilds are playing a role in these processes, methylation/demethylation experiments were performed in the absence and presence of different metabolic inhibitors: chloramphenicol (general bacteriostatic inhibitor), molybdate (sodium molybdate, a sulfate reduction inhibitor), BESA (2-bromoethane sulfonic acid, a methanogenesis inhibitor), and DCMU (3-(3,4-dichlorophenyl)-1,1 dimethyl urea, a photosynthesis inhibitor). Active microbes of the periphytic consortium were also characterized using 16S rRNA gene sequencing. Methylation rates in the absence of inhibitors varied from 0.0015 to 0.0180 d(-1) while demethylation rates ranged from 0.083 to 0.217 d(-1), which corresponds to a net methylmercury balance of -0.51 to 1.28 ng gDW periphyton(-1) d(-1). Methylation rates were significantly decreased by half by DCMU and chloramphenicol, totally inhibited by BESA, and were highly stimulated by molybdate. This suggests that methanogens rather than sulfate reducing bacteria were likely the primary methylators in the periphyton of a temperate fluvial lake, a conclusion supported by the detection of 16S rRNA gene sequences that were closely related to those of methanogens. This first clear demonstration of methanogens' role in mercury methylation in environmental periphyton samples expands the known diversity of microbial guilds that contribute to the formation of the neurotoxic substance methylmercury.

The fate of mercury in Arctic terrestrial and aquatic ecosystems, a review

“…There are several possible environmental sources of MeHg, but few studies have measured production rates from these various sources in Arctic environments, especially in marine settings. In temperate marine and fresh water environments, wetlands and benthic sediments are major MeHg sources and this is thought to be due to the activity of sulfate-and iron-reducing bacteria [140][141][142] and methanogens [143] in these anoxic environments. MeHg can also be produced during detrital remineralisation in oxic marine waters, associated with mid-depth nutrient maxima and oxygen utilisation.…”

Persistent Hg contamination and occurrence of Hg-methylating transcript (hgcA) downstream of a chlor-alkali plant in the Olt River (Romania)

“…The highest MeHg production occurs mostly in sediments at the oxic/anoxic transition zones during the anaerobic degradation of natural organic matter (Compeau and Bartha 1985;Gray et al 2004;Bravo et al 2014). Several guilds of bacteria are known to methylate IHg, notably iron-reducing bacteria (FeRB), sulfate-reducing bacteria (SRB), and methanogens (Fleming et al 2006;Kerin et al 2006;Gilmour et al 2011Gilmour et al , 2013Hamelin et al 2011;Yu et al 2013). The specific mechanisms and metabolic pathways involved in Hg methylation remains undefined, but two genes in particular (cluster hgcA and hgcB) have been shown to be required for Hg methylation (Gilmour et al 2013;Parks et al 2013).…”

Acerca de la biotecnología ambiental

“…La correlación positiva entre la producción de metilmercurio y la actividad de bacterias reductoras de SO 4 2-y Fe 3+ tiene una base molecular consistente en la presencia de los genes hgcA, que codifica una proteína con un grupo prostético corrinoide, y hgcB, que codifica una proteína con 2 centros 4Fe4S, necesarias para la metilación del mercurio en Desulfovibrio desulfuricans ND 132 y Geobacter sulfurreducens PCA (Parks et al, 2013). La metanogénesis también está relacionada con la producción de metilmercurio, ampliando la interconexión entre ciclos biogeoquímicos y los problemas de contaminación ambiental (Hamelin et al, 2011).…”