Organomercurials. Their formation and pathways in the environment

Area of Science:

• Environmental Chemistry
• Microbiology
• Toxicology

Background:

• Monomethylmercury (MMHg) is the most significant mercury species in the environment.
• MMHg is organically formed from mercuric mercury (Hg²⁺) via microbial methylation, primarily in anoxic aquatic settings.
• Terrestrial systems contribute minimally to MMHg production.

Purpose of the Study:

• To detail the formation pathways and environmental significance of monomethylmercury (MMHg).
• To explain the biomagnification and human health risks associated with MMHg.
• To describe the natural demethylation processes that counteract MMHg accumulation.

Main Methods:

• Review of microbial methylation processes in various aquatic environments (sediments, wetlands, lakes).
• Analysis of MMHg biomagnification across aquatic food webs.
• Examination of bacterial demethylation mechanisms and regulation.

Main Results:

• Anoxic aquatic environments, particularly sediments and wetlands, are hotspots for MMHg formation by sulfate-reducing and iron-reducing bacteria.
• MMHg biomagnifies significantly (over seven orders of magnitude) up the food chain, reaching high concentrations in fish.
• Bacterial demethylation, regulated by the mer operon, acts as a detoxification pathway, reducing MMHg levels.

Conclusions:

• MMHg is a potent neurotoxin with substantial biomagnification potential, necessitating human health advisories regarding fish consumption.
• Understanding methylation and demethylation processes is crucial for managing mercury contamination in aquatic ecosystems.
• Dimethylmercury (DMHg) may serve as a precursor to MMHg in marine and polar regions.