Investigating the Microbial Mechanisms of Mercury Methylation: Community Composition and Biogeochemical Interactions

Mercury (Hg) methylation is a complex biogeochemical process influenced by microbial interactions, environmental conditions, and genetic mechanisms. This dissertation investigates the microbial ecology of Hg methylation and demethylation across diverse ecosystems, exploring the roles of both Hg-methylating and non-methylating taxa, the genetic basis of Hg transformations, and the environmental drivers of these processes. Statistical and network analyses of microbial communities in marine and freshwater sediments revealed that non-Hg methylating taxa play a supporting role in Hg cycling. These taxa were found to be closely associated with Hg methylators, suggesting syntrophic interactions and functional contributions to sulfur, carbon, and nitrogen cycling, which could indirectly influence Hg methylation potential. A major conclusion from this study was the differences between marine and freshwater environments, with salinity being an important factor. Expanding on this, a large-scale metagenomic analysis of 281 global freshwater and marine sediment samples identified novel Hg methylators and widespread horizontal gene transfer (hgcA/B), indicating that Hg methylation potential is shaped by both environmental selection (such as marine and freshwater) and microbial genetic exchange. The discovery of new hgcA-carrying taxa, including anammox bacteria, further suggests diverse biochemical pathways supporting Hg methylation. To further expand the hgcA and merB carriers in other habits, the role of wastewater treatment plants (WWTPs) in Hg cycling was investigated through metagenomic analysis of microbial communities in 61 WWTPs worldwide. Results demonstrated that WWTPs harbor both Hg-methylating and demethylating microbes, including novel merB carriers, and serve as hotspots for horizontal gene transfer. This highlights WWTPs as potential sources of Hg-cycling microbes to downstream ecosystems, influencing Hg transformation processes in receiving waters and sediments. Finally, controlled flooding experiments with lake sediments demonstrated that hydrological fluctuations significantly impact Hg methylation and demethylation rates. Flooding-induced redox shifts created anaerobic conditions that promoted Hg methylation while suppressing demethylation, potentially lead to increased MeHg accumulation. These interconnected results provide new insights into microbial and environmental controls on Hg methylation, revealing the widespread influence of microbial interactions and genetic exchange in shaping Hg cycling. By linking microbial ecology, genetic adaptation, and environmental perturbations, this research enhances our ability to predict Hg dynamics under changing climate conditions and informs mitigation strategies for Hg-contaminated environments.