الفهرس 
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Abstract
fraction of mercury is found free in solution, except close to the point of its discharge (Jernelöv and Lann, 1970).
Mercury being highly volatile, it is always found in the atmosphere in trace amounts probably as metallic mercury or dimethylmercury, and from the atmosphere mercury returns in rain water.
The cycling of mercury in nature is facilitated by the volatile character of its metallic and methylated forms, and by the easy solubility of mercuric salts (as mercuric chloride) in aquous medis (Yamada et.al.. 1969).
There are three factors aid in the transformation of mercury in nature which are microbs, plants and
animals.
1- Bacteria and viruses can fix mercury and trans- form it biochemically in aquatic environment (Purukawa et al., 1969). Bacterial activity in bottom sediments usually increases the volatilization of ionic mercury, probably because it is biochemically converted to a more volatile compound (Yamada et al., 1969). This mobili- zation of mercury in sedements has been throughly per formed on the methylation of this metal by bacterial.