Mercury Methylation by the Methanogen Methanospirillum hungatei

Abstract

c Methylmercury (MeHg), a neurotoxic substance that accumulates in aquatic food chains and poses a risk to human health, is synthesized by anaerobic microorganisms in the environment. To date, mercury (Hg) methylation has been attributed to sulfateand iron-reducing bacteria (SRB and IRB, respectively). Here we report that a methanogen, Methanospirillum hungatei JF-1, methylated Hg in a sulfide-free medium at comparable rates, but with higher yields, than those observed for some SRB and IRB. Phylogenetic analyses showed that the concatenated orthologs of the Hg methylation proteins HgcA and HgcB from M. hungatei are closely related to those from known SRB and IRB methylators and that they cluster together with proteins from eight other methanogens, suggesting that these methanogens may also methylate Hg. Because all nine methanogens with HgcA and HgcB orthologs belong to the class Methanomicrobia, constituting the late-evolving methanogenic lineage, methanogenic Hg methylation could not be considered an ancient metabolic trait. Our results identify methanogens as a new guild of Hg-methylating microbes with a potentially important role in mineral-poor (sulfate-and iron-limited) anoxic freshwater environments. M ercury (Hg) is a global environmental contaminant whose concentration in the biosphere is increasing as a result of industrial activity. Mercury enters the biosphere mostly in its inorganic form, Hg(II), but public health concerns are focused primarily on the neurotoxic substance monomethylmercury (MeHg). Since MeHg is environmentally persistent and is biomagnified in aquatic food webs (1), in situ methylation reactions critically affect the ecosystem and human health consequences of Hg contamination.Methylation of Hg takes place in anoxic environments and is attributed largely to anaerobic microbes (2). To date, bacteria affiliated with the Deltaproteobacteria, including sulfate-and ironreducing bacteria (SRB and IRB, respectively) have been identified as Hg methylators in environmental incubations and in pure cultures (3-5). Indeed, coexisting SRB and IRB may simultaneously contribute to MeHg production in river sediments (6). Evidence that other anaerobic microorganisms are capable of methylating Hg has remained elusive for decades. Methanogens were initially proposed to be Hg methylators as a conclusion of experiments showing MeHg synthesis in cell extracts of Methanobacterium bryantii (7-9). This idea was rejected after later studies identified SRB (3) and IRB (4-6) as the principal Hg methylators in salt marsh and freshwater sediments and failed to show methylation by methanogens in pure cultures (10). However, Hamelin et al. (11) recently showed that Hg methylation in periphyton collected from a freshwater lake could be attributed to methanogens, since methylation was abolished by the addition of 2-bromoethane sulfonic acid, a specific inhibitor of methanogenesis, and was stimulated 45-fold over methylation in unamended controls by the addition of molybdate. A corresponding change in the active-commu...

Patterns of Bacterial Diversity Along a Long-Term Mercury-Contaminated Gradient in the Paddy Soils

“…The resistance of bacteria to inorganic Hg 2+ is associated with the presence of the Hg resistance gene (merA) [16,17], which has been considered widely distributed in the phyla of Firmicutes, Actinobacteria and Proteobacteria [16]. Meanwhile, ionic Hg in anaerobic habitats may also be potentially methylated to highly toxic methylmercury (MeHg) by Hg-methylating microbes [18,19], such as sulphate-reducing bacteria (SRB) [20,21], iron-reducing bacteria (FeRB) [22,23] and methanogens [24,25]. However, little is known regarding the responses of these Hg-methylating bacteria to long-term soil Hg contamination.…”

Site-Directed Mutagenesis of HgcA and HgcB Reveals Amino Acid Residues Important for Mercury Methylation

“…Anaerobic Deltaproteobacteria are thought to be the major contributors of MeHg found in the environment (7,8), although potentially significant contributions by methanogens and fermenting bacteria have recently been discovered (9)(10)(11). In 2013, we showed that the proteins encoded by two genes in two bacteria of the Deltaproteobacteria phylum, Desulfovibrio desulfuricans ND132 and Geobacter sulfurreducens PCA, are essential for the conversion of Hg(II) to MeHg (12).…”

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).…”