The widespread occurrence of mercury (Hg) in the environment and, especially in fish has been highlighted because of the harmful effects of mercury compounds on the health of humans and other animals. It is mainly the methylated form of mercury (methyl mercury) that accumulates in aquatic food webs. Exposure to methyl mercury, occurring mainly through fish consumption, can affect the nervous, reproductive, and immune systems of vertebrates, including fish, birds, and humans.

It is well known that certain bacteria and archaea have the capacity to transform inorganic mercury to methylmercury. The transformation takes place under the oxygen limited conditions typically found in wetlands and sediments, but we still do not know how mercury methylation is controlled and influenced by the prevailing environmental conditions. Now a new study from Uppsala and Umeå University demonstrates that the molecular composition of the sediment organic matter plays a central role for mercury methylation.