Mercury pollution: Which form is associated with entering water and building up in fish, harming the nervous system?

• A. Methylmercury
• B. Elemental mercury
• C. Mercury sulfide
• D. Mercury vapor

Answer: (A)

Methylmercury is the form that readily enters water and becomes highly concentrated in fish, leading to harm to the nervous system. In aquatic systems, inorganic mercury can be transformed by bacteria into methylmercury, a compound that is both highly soluble and readily absorbed by organisms. It binds strongly to proteins and tissues, and because it travels easily through cell membranes, it crosses the blood–brain barrier and the placenta, causing neurotoxic effects, especially in developing fetuses and young children. This combination of bioavailability and biomagnification means top predator fish can carry high methylmercury levels, which is why consuming contaminated fish poses a risk for nervous-system damage. The other forms—elemental mercury, mercury sulfide, and mercury vapor—do not accumulate in fish to the same extent or are associated with exposure routes (inhalation or insolubility) that don’t explain the water-to-fish bioaccumulation pattern described.