Mercury cycle

In the atmosphere, mercury vapor (Hg^o) is slowly converted by oxidative processes to Hg2⁺, which is then returned to the earth's surface by rainfall, where it accumulates in soils and in surface waters. Some of the mercury load is then converted back into mercury vapor (Hg^o) and returned to the atmosphere. However, another fraction of the mercury load (Hg2⁺) is washed into rivers, streams, and eventually the ocean where it accumulates in aquatic sediments. It is here that inorganic mercury is converted to methylmercury (MeHg) by microorganisms living in the sediments by a process called methylation.