Significant amounts of mercury are directly released from the earth's crust by the process of degassing. Both natural and manmade emissions are modified by biological processes into forms more directly harmful to human beings. Mercury is somehow less toxic in its volatile form, mercury-zero, than in organic compounds like methylmercury or inorganic salts (mercury-two).

Common bacteria of the soil and water have adapted to the presence of mercury. They have developed methods to detoxify its organic compounds and salts to the elemental form of mercury zero. Mercury zero, however, is volatile, and thus can spread throughout the environment through secondary biological mechanisms. Once it reaches inland aquatic environments, mercury zero can again accumulate and be transformed into methylmercury, the toxic form that bioaccumulates in fish, animals, and humans. This toxic transformation can occur from any of three causes:

• the photochemical (abiotic) action of sunlight
• the methylcobalamin (a hydrocarbon compound), excreted by bacteria
• the plants of aquatic ecosystems.

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