: to make (something) more attractive usually by putting something on it.
There are several types of mercury. They include metallic mercury, and both inorganic and organic mercury compounds. Many of these forms are found in our environment, however, the most common natural forms include metallic mercury, mercury sulfide, mercuric chloride, and methylmercury (http://www.atsdr.cdc.gov/toxprofiles/tp46.html).
Some microorganisms (bacteria and fungi) and natural processes can change the mercury in the environment from one form to another, usually under anaerobic (oxygen-deficient) conditions. The most common organic mercury compound that these microorganisms and natural processes generate from other forms is methylmercury.