The microbes are single-celled organisms that multiply by simple cell division and derive energy for growth and cell functioning by oxidizing iron and sulfur. Oxi­dation involves the removal of electrons from a substance. In biomining process, the microbes remove electrons from dissolved iron (ferrous iron) converting it to another form of iron (ferric iron); electrons are removed from sulfur converting it to sulfuric acid. They obtain carbon for their cellular bodies from carbon dioxide (CO2) in the atmosphere and also require a sulfuric acid environment to grow. This acidic environment is helpful in growth of these microorganisms but acidity must be less than pH 2.5, which is more acidic than vinegar.

The biomining microorganisms do not cause diseases in humans, animals, or plants. Because their food source is inorganic (sulfur and iron) and because they must live in a sulfuric acid environment, they cannot survive in or on plants and animals. These microbes are conveniently grouped within temperature ranges at which they grow and where they are found in the natural environment:

• Ambient temperature bacteria (mesophiles)

• Moderately-thermophilic (heat-loving) bacteria

• Extremely-thermophilic (heat-loving) bacteria

Ambient temperature bacteria (mesophiles). These cylindrical-shaped biomining bacteria are about 1 im long and V2 im in diameter (1 im is 4/100,000 of an inch). About 1,500 of these bacteria could lay end-to-end across a pin head. They only grow and function from 10 to 40°C (50 to 104°F). If the temperature is too low, these bacteria become dormant. If the temperature exceeds 45°C (113°F), the organisms die as their proteins coagulate similar to cooking an egg. Acidithio — bacillus ferrooxidans belong to this group of bacteria. Others include Leptospir — illum ferrooxidans and species of Ferroplasma.

Moderately-thermophilic (heat-loving) bacteria. These bacteria are similar to the “mesophilic” biomining bacteria, except they are somewhat larger in length — about 2-5 im long and they only grow and perform when the temperature exceeds 40°C (104°F). The moderate thermophiles die when the temperature exceeds 60°C (140°F). Examples of moderate thermophiles are species of Sulfobacillus and Acidithiobacillus caldus.

Extremely-thermophilic Archaea. While similar in size (one micrometer in diameter) to ambient temperature bacteria, Archaea have a different molecular organization. In the tree of life, Archaea occupy the lowest branch and are extant members of an offshoot of primitive microbes. They have a spherical shape and characteristically lack a rigid cell wall, rather the contents of the single cell are enclosed by a membrane. These microbes, nevertheless, are extremely robust growing and performing only at temperatures between 60 (140°F) and 85°C (185°F). Examples of extremely-thermophilic Archaea used in biomining are Acidianus brierleyi, Sulfolobus metallicus and Metallosphaera sedula [8].