The acidophilic archaea "Sulfolobus metallicus" and "Metallosphaera sedula" can tolerate up to 4% of copper and have been exploited for mineral biomining. Between 40 and 60% copper extraction was achieved in primary reactors and more than 90% extraction in secondary reactors with overall residence times of about 6 days. All of these microbes are gaining energy by oxidizing these metals. Oxidation means increasing the number of bonds between an atom to oxygen. Microbes will oxidize sulfur. The resulting electrons will reduce iron, releasing energy that can be used by the cell.