Pelobacteraceae and Geobacteraceae are important members of mercury-methylating microbial communities of ferruginous sediments

Microbial mercury (Hg) methylation in sediments can result in bioaccumulation of the neurotoxin methylmercury in aquatic food webs. Recently, discovery of the gene hgcA, required for Hg methylation, revealed that the diversity of Hg-methylators is much broader than previously thought. However, little is known about the identity of Hg-methylating microbial organisms and the environmental factors controlling their activity and distribution in lakes. Here, we combined high-throughput sequencing of 16S rRNA and hgcA genes with chemical characterization of sediments impacted by a wastewater treatment plant that releases significant amounts of organic matter and iron, resulting in ferruginous conditions ideal for Hg-methylation. The microbial community was composed of microorganisms important in Fe and S cycling, syntrophs and methanogens. Deltaproteobacteria, notably the fermenting Pelobacteracea and the iron-reducing Geobacteraceae, dominated the richness and diversity of hgcA carrying communities, while sulfate reducers constituted only a minor component, despite being considered the dominant Hg-methylators in many anoxic aquatic environments. Because Fe is widely applied in wastewater treatment, the importance of Pelobacteracea/Geobacteraceae for Hg methylation and the complexity of Hg-methylating communities reported here are likely to occur worldwide in sediments impacted by wastewater treatment plant discharges and Fe-rich sediments in general.