Methanogens and iron-reducing bacteria are major contributors to mercury methylation in boreal lake sediments

Methylmercury is a potent human neurotoxin easily biomagnified in aquatic food webs. Anaerobic microorganisms containing the hgcA gene potentially mediate the formation of methylmercury in natural environments, but the diversity of microbial mercury methylating communities in the environment remains largely unexplored. The diversity of mercury methylating microbial communities in boreal lake sediments, encompassing a wide range of mercury methylation rates, was characterized using barcoded amplification and high throughput sequencing of 16S rRNA and hgcA genes. Previously, most studies in aquatic ecosystems have implicated sulphate-reducing bacteria as the main mercury methylators. Here we demonstrate that Methanomicrobiales contribute significantly to the hgcA gene pool in boreal lake sediments. Experimental mercury isotope tracer incubations confirmed that, while sulphate-reducers contribute to mercury methylation, methanogens and iron reducers are also quantitatively important for the process. Our results change the way we perceive the local and global diversity of mercury methylating microbial communities in boreal lake sediments and call for further studies on the environmental factors regulating mercury methylation.