The Selenium Cycle is a biological cycle of selenium similar to the cycles of carbon, nitrogen, and sulpur. Within the cycle, there are organisms which reduce the most oxidized form of the element and different organisms complete the cycle by oxidizing the reduced element to the initial state. In the selenium cycle it has been found that bacteria, fungi, and plants, especially species of Astragalus, metabolize the most oxidized forms of selenium, selenate or selenite, to selenide. It is also thought that microorganisms may be able to oxidize selenium of valence zero to selenium of valence +6.