Methylmercury Lecture Notes

Overview

• Methylmercury (CH3Hg+): An organometallic cation; simplest organomercury compound.
• Toxicity: Extremely toxic, bioaccumulative environmental toxicant with a 50-day half-life.
• Source of Organic Mercury: Major source for humans, linked to Minamata disease.
• Formation: Bioaccumulation and bi-magnification in aquatic food webs.

Chemical Structure and Properties

• Formula: CH3Hg+
• Charge: +1 with Hg in the +2 oxidation state.
• Complexes: Exists as [MeHgL]+ (L = Lewis base) and MeHgX (X = anion).
• Affinities: Particularly binds with sulfur-containing anions, notably thiols.

Environmental Sources

• Microbial Action: Converted from inorganic mercury by microbes in aquatic systems (lakes, rivers, etc.).
  • Bacteria Involved:
    • Sulfate-reducing bacteria (SRB)
    • Iron-reducing bacteria (FeRB)
    • Methanogens
• Anthropogenic Sources: Includes coal burning and waste burning, releasing elemental mercury into the atmosphere.
• Natural Sources: Includes volcanic activity, forest fires, ocean volatilization.
• Mercury Methylation: Occurs after permafrost thaw and reservoir creation.

Dietary Sources

• Food Chain Accumulation: Biomagnifies from bacteria to top aquatic predators.
  • Fish-eating fish like shark, swordfish, tuna have higher levels.
  • Larger or older fish and those in acidic waters have higher levels.
• Human Exposure: Primarily through consumption of fish from top of the food chain.

Biological Impact

Human Health Effects

• Absorption: Readily absorbed by the gastrointestinal tract; transported across the body, including placenta.
• Developmental Effects: Linked to developmental deficits in children and cardiovascular diseases in adults.
• Autoimmunity: Possible autoimmune effects in sensitive individuals.
• Historical Poisoning Events: Minamata disease in Japan, Grassy Narrows in Canada, Basra poison grain disaster in Iraq.

Effects on Fish and Wildlife

• Impact on Ecosystems: Diminished reproductive success in fish, birds, and mammals.
• Public Health Concerns: Advisories issued to limit fish consumption and mercury exposure.

Public Policy and Regulation

• Fish Consumption Advisories: To limit exposure, especially for vulnerable populations (pregnant women, children).
• Regulatory Actions: Measures to reduce mercury emissions, balancing with economic impacts.
• Global Impacts: Mercury levels in ocean tripled since industrial revolution, affecting fish and human exposure.

Additional Considerations

• Co-exposure Mitigation: Omega-3 fatty acids and selenium may reduce some methylmercury toxicity.
• Ongoing Research: Understanding mercury's environmental cycling and effects on health and ecosystems continues to evolve.