Microbial methylation of particulate-bound mercury in water column and its underlying mechanisms: A critical review

Due to the bioaccumulation and biomagnification of methylmercury (MeHg) within aquatic food web, it poses a significant threat to ecology and human health. Consequently, the methylation process of inorganic mercury (Hg) in aquatic systems has long been a focus of research. Although early research suggested that bottom sediments were the principal hotspot for microbial MeHg production, recent evidence highlighted that Hg methylation in water column constituted a non-negligible source, especially in oceans and deep lakes, where suspended particulate matter plays a crucial role in this process. This review summarizes current fundamental understanding of the methylation of particulate-bound mercury (Hg_P), particularly in marine and deep lakes. Some studies reported both significantly higher MeHg concentrations and methylation rates/ratios in suspended particulate matter compared to underlying sediments. Methylation on suspended particulate matter in the ocean appeared primarily mediated by specific Nitrospina, rather than the previously emphasized Deltaproteobacteria, Firmicutes, and Chloroflexi. Furthermore, this review details the possible mechanisms facilitating Hg_P methylation. We propose that suspended particulate matter may enhance Hg methylation through microbe-Hg interaction by several pathways: (1) Providing anaerobic microenvironments and essential nutrients to support the growth of methylators, (2) Facilitating ligand exchange between Hg_P and methylators via intimate contact, (3) Enabling direct uptake of small-sized particulate matter, and (4) Reducing Hg aging on bacterial surfaces. Elucidating these mechanisms in further study would advance our understanding of methylation of Hg_P in the water column.