Bioremediation of mercury and the importance of bacterial mer genes

Mercury exists naturally in small amounts in the environment as the sixteenth rarest element on earth. However, its level is rising due to industrialization and other anthropogenic activities such as the burning of coal and petroleum products, the use of mercurial fungicides in agriculture and the papermaking industry, and mercury catalysts in industries. Mercury-resistant bacteria harbor the mer operon in their genome. The mer operon includes certain functional genes along with promoter, regulator, and operator. The most common functional genes are merA and merB, which code for mercuric ion reductase and organomercurial lyase, respectively. The lyase is responsible for reducing highly toxic organomercurial compounds such as methylmercury and phenyl mercuric acetate into almost nontoxic volatile elemental mercury with the help of the enzyme reductase. When acting together in bacteria, merA and merB confer broad-spectrum mercury resistance. However, merA alone confers bacterial resistance to a narrow spectrum of inorganic mercury. This review discusses the importance of mercury-resistant bacteria harboring both merA and merB as potential agents in mercury bioremediation at highly polluted mercury-contaminated sites.

► Mercury pollution is a severe environmental threat due to rapid industrialization.
► Mercury enters in the food chain in the form of methylmercury through biomagnification.
► Bacteria showing organomercurial resistance harbor merA and merB gene.
► merA and merB together confers broad spectrum mercury resistance.
► Prospects of mercury bioremediation by using mercury resistant bacteria are discussed.