Mercury cycle illustration

Mercury cycles from Earth’s crust to the air to the ocean and back to land. In the ocean, top predator fish such as tuna and swordfish contain high levels of monomethyl mercury, a highly toxic form of the element mercury that causes trembling, erratic behavior, and birth defects. WHOI geochemist Carl Lamborg is trying to identify where in the ocean mercury gets converted to monomethyl mercury.
Caption from Oceanus magazine, Vol. 48, No. 2, pg. 24:
Mercury (Hg) cycles from Earth to atmosphere to ocean and back to Earth. In the ocean, mercury is converted to monomethylmercury (MMHg), a neurotoxin that moves up the food chain and becomes highly concentrated in tuna, swordfish, and other fish that people eat. (Illustration by Jack Cook, © Woods Hole Oceanographic Institution)