Mercury

Bacteria have a uniquely important functional role in the mercury geobiochemical cycle Prior to 1960 it was generally assumed that discharges of elemental mercury remained relatively inert in the environment. More recent work has demonstrated that inorganic mercury can be methylated by organisms present in the soil and sediment of rivers and lakes. Their most significant contribution is the conversion of inorganic mercury to methylmercury by the insertion of a covalent bond between the carbon and mercury atoms. This reaction enables the mercury to penetrate cell membranes more rapidly and accumulate within the cells by complexing with essential proteins, enzymes, and nucleic acids. These reactions demonstrate that all mercury compounds represent a potential threat to living organisms, since in the presence of bacteria, any compound containing mercury can be transformed to the highly poisonous organic sub-stances which are readily incorporated into living tissue.

At first glance it is difficult to see what advantages the bacteria gain from methylating the mercury atom. A possible explanation is provided by examining the enzymatic reactions which result in the removal of mercury from within the bacteria and from the immediate vicinity. During the billion or so years bacteria have existed on earth, they have been subjected to continued exposure to mercury in the deep oceans as well as in soil and sediment. Over time they have evolved a capacity to produce enzymes capable of modifying the mercury before and after it has penetrated into the bacterial cells. Masses of bacteria in the form of colonies or films abound in soil and sediment, especially in areas where organic material is plentiful. The composition of the bacteria and the structure of their colonies vary greatly depending upon environmental conditions. In bacterial films the initial exposure to toxic mercury compounds can induce cell death in most of the organisms. However, some organisms can resist the poisoning by induction of specific enzymes which alter the nature and solubility of the mercury, enabling the bacteria to withstand the toxic action. These specific enzymes are coded by DNA found in small organelles called plasmids which can be transferred from one bacteria to another and even from one species of bacteria to another. Following repeated exposure to the toxic mercury compound all the bacteria will eventually become resistant due to the presence of these specific induced enzymes. A wide variety of chemical changes such as oxidation, reduction and/or methylation or transformation of mercury compounds can be produced by these enzymes. In addition hydrogen sulfide, which can react with mercury compounds, can be released (especially during anoxic conditions), forming mercuric sulfide (metacinnabar). Some of these reactions are shown in the figure to the right.

The significant aspect of these enzymatic reactions is that the sum total of effects result in the elimination of mercury, not only from the bacterial cell itself but also from the immediate environment surrounding the bacteria. This is accomplished in the presence of an enzyme called mercury reductase that reduces ionic mercury to elemental mercury which is insoluble in water and readily passes towards the water surface and into the atmosphere and away from the bacteria. This enzyme is coded by a gene in the plasmid called MerA which in the presence of mercury can be induced to form the reductase enzyme. Other enzymes such as an isotype of vitamin B12 (methylcobalamin) can also be genetically induced by the presence of ionic mercury. This enzyme methylates ionic mercury, forming dimethylmercury and monomethylmercury. As indicated earlier the dimethylmercury is highly insoluble in water and forms a gas which passes towards the water surface where it decomposes to monomethyl-mercury and methane. Methylmercury can be produced and/or destroyed by microbial processes. Persisting monomethylmercury within the bacterial films will activate the MerB gene to form mercury lyase which demethylates the mercury. This can then be acted upon by mercury reductase to form elemental mercury which passes to the water surface. In addition under anoxic conditions the presence of hydrogen sulfide will result in the precipitation of mercuric sulfide (metacinnabar) which is highly insoluble and precipitates onto the sediment. The end result of all these enzymatic reactions is the formation of either gaseous or precipitated compounds which remove the mercury from the immediate vicinity of the bacterial film. Thus these organisms have, over time, evolved mechanisms which permit their continued existence, even in the presence of high levels of poisonous mercury compounds. The formation of dimethylmercury may be just one of the means of removing the mercury atoms from the environment which surrounds the bacteria. Some of the reactions are summarized in the figure to the right.

California Aspects

Mercury was a key element in the California Gold Rush due to its role in separating precious metals such as gold and silver from contaminants. This method of processing gold and silver has been an important component of human civilization for thousands of years and was used by Romans to purify gold using procedures that are still in use in some parts of the world to this day. In California individual prospectors could start out with a small leather bag of mercury, pan the streams for small nuggets or flecks of gold, separate the gold from the gravel by amalgamating it with mercury, then vaporizing the mercury and pocketing the gold. This provided a very quick and dirty method for an individual prospector to harvest his treasure.

The extensive panning activity along California's streams soon exhausted the readily available gold and led to the development of hydraulic mining about 1862. Hard rock mining and dredging was also initiated during this period. A very significant step in all of these procedures involved the use of large quantities of mercury to dissolve the precious metal and then harvest the gold by retorting the amalgam.

Beginning in 1850 the Coast Range on the west side of the Sacramento Valley provided most of the mercury used in the Gold Rush. Around 300 mines produced approximately 140,000 tons of mercury that supported the California Gold Rush and gold mining activities in Alaska and the whole Pacific Rim. Most of it was transported eastward in 76 pound flasks to the Sierra Nevada and surrounding areas. Due to the inefficient extraction and condensation procedures used in procuring the mercury, and its later use in the amalgamation and separation of the gold and silver, mercury vapor was released into the atmosphere. This resulted in a buildup in the whole Sacramento watershed area. It has remained a major pollution problem for more than a hundred years and will likely continue long into the future. A modified geobiochemical mercury cycle as it applies to the Sacramento Watershed Area is shown to the right.

Mercury is a heavy metal that is unique in that it slowly vaporizes when exposed to the atmosphere and its gaseous elemental form is not water soluble. It becomes water soluble and incorporated into rain drops after being oxidized into ionic mercury (Hg++) in the atmosphere in the presence of ozone and oxygen. The polluted water droplets return the mercury back to earth where it is deposited on soil, some of which is carried by erosion into the waterways. A great deal of the ionic mercury becomes complexed with organic and inorganic particles present in the water. It can also be acted upon by microorganisms to form a variety of organic and inorganic compounds that are eroded into nearby streams and transported into the estuaries of the San Francisco Bay-Delta region. Long after the mercury mining has ceased and gold and silver processing has diminished, mercury and its compounds persist. A permanent legacy of these mercury releases remains not only locally but in snow capped glaciers, peat bogs and sediments located hundreds and even thousands of miles away.

Cache Creek is a tributary of the Sacramento River whose watershed area includes a number of large abandoned mercury mines and a gold mining and processing facility. One of these abandoned mercury mines (the Sulfur Bank Mercury Mine), currently an EPA Superfund site, is still releasing mercury-containing leachate into nearby Clear Lake. This and other mines sites contribute to the contamination of Cache Creek which has been estimated to account for about 50% of the mercury carried into the Bay Delta region each year. During the 1995 high water period it was estimated that approximately 1,000 Kg of mercury, mostly in particulate form was carried downstream into the Delta and Bay areas. (Foe, et al 1997)

Mercury Pollution Considerations