Phytoremediation of Mercury

Mercury is an element      Mercury (Hg) is a silvery metallic liquid as toom temperature.  Nautral sources of Hg occur from outgassing of the earth's crust through volcanoes and evaporation from the ocean.  It can be found in familiar items such as lightbulbs, batteries, thermometers, pesticides, paint and some dental fillings (amalgams).  It is also sometimes used as a catalyst in chemical reactions or in gold extraction procedures.  In nature, mercury exists in several forms:  1) as ionic salts in either the mercurous (I) or mercuric (II) states, 2) as an organometallic compound such as methyl mercury, or 3) as elemental mercury Hg(0) in either liquid or vapor phase.

Mercury in the Environment      Mercury is believed to be transported throughout the environment by two cycles.  On a global scale, Hg(0) vapor circulates through the earth's atmosphere from land sources to the oceans (3).  Researchers believe that the global amount of Hg has increased by a factor of 2-5 since the advent of industry (3).  This amounts to a total estimate of approximately 10,000 tons of mercury being released worldwide into the environment from both man-made and natural sources.  The second cycle occurs on a local scale and involves methylation of atmospheric mercury, which is deposited into bodies of water, by methanogenic bacteria to form methyl mercury.  This compound is somewhat soluble in water and is taken up by organisms and concentrations are "biomagnified" in animals such as fish, which are higher up in the food chain (3).

The Problem:  Mercury is toxic to many organisms      Because many animals, including humans, can potentially feed on contaminated fish, shellfish, or sea mammals, contamination poses an immediate health threat.

	Mercury is toxic to humans      During the 1950's the first major mercury posioning epidemic occurred in Minamata Bay in Kyushu, Japan.  Residents had cons umed methyl mercury-contaminated fish and shellfish.  The source of contamination was effluent from a chemical manufacturing company, Chisso, which specialized in the production of acetylaldehyde.  Mercury was used as a catalyst in the production process and waste was released into Minamata Bay.  Many families who suffered posioning were associated with the local fishing industry.  Victims experienced ataxia (loss of precise control of movement),  visual problems, loss of hearing and mental confusion.  They became prone
	to shouting and violent behavior which often lead to coma (1).  An estimated 1,435 people have died because of this contamination (4).  Additional epidemics occurred not long afterward in Niigata, Japan due to contaminated seafood (1) and in Iraq due to consumption of seed grain that had been treated with a mercury-containing fungicide (2).  The largest concern, however, is that low levels of mercury exposure is particularly harmful to the fetus.  Infants born to mothers who have been exposed to mercury contamination while or before becoming pregnant have shown a high incidence of mental retardation, ataxia, seizure, sensory disturbance, visual problems, and hearing impairment (1).
		Minamata Monument, Minamata, Japan

Mercury is toxic to animals      During the Minamata Bay mercury posioning, people noticed that residential cats that also consumed contaminated seafood exhibited behaviors similar to posioned humans.  Some cats were reported to show slow, unsteady movement and some ran hysterically causing them to jump into the sea and drown (1).  Additionally, mercury is toxic to fish, shellfish (1), and birds (3).

Mercury is toxic to most plants      Plants that are exposed to mercury accumulate the metal, however drastic decreases in growth are usually observed.  Plants exposed to ionic mercury through the root exhibit reduced growth of shoots and roots.  They also accumulate mercury in the root with slow movement to the shoot.  Tree leaves can trap atmospheric mercury.  It is thought that inorganic mercury may cause changes in root tip cell membrane integrity while methyl mercury may affect organelle metabolism processes that eventually interrup cell membrane integrity (3).

A solution:  Removing methyl mercury from water and soil - Phytoremediation Technologies      Phytoremediation or remedying a contaminated site using plants, is a relatively new area of research.  Mercury-resistant bacteria have been reported to produce enzymes that catalyze two reactions:  1) organomercurial lyase - which removes methyl groups from mercury to create ionic mercury, and 2) mercuric ion reductase which converts ionic mercury to volatile elemental mercury.  Plants engineered to express these genes could have potential for relatively inexpensive clean-up of mercury contaminated sites.  Additionally, many sites that are contaminated with various metals are also contaminated with mercury which may be the most toxic metal and is limiting to growth.  Volatilization of elemental mercury would allow mercury to diffuse out of the plant and into the atmosphere at diffuse and non-toxic concentrations (15).

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