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Importance of Bacteriological Processes

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 contain-ing 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 below.

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 below.

Additional References:

Microbial Transformation of Mercury Species and their Importance in the Biogeochemical Cycle of Mercury
By Franco Baldi
Metal Ions in Biological Systems, Vol.34: Mercury and Its Effects on Environment and Biology, edited by A. Sigel and H. Sigel, 1997
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