Autotroph is an organism capable of biosynthesizing all cell material from carbon dioxide as the only carbon source. With respect to energy, autotrophs can obtain it from two sources: (1) photoautotrophs from radiation (sunlight) and (2) chemolithoautotrophs from the oxidation of reduced inorganic substrates. Autotrophs are capable of growth exclusively at the expense of inorganic nutrients, and they are of vital importance in the cycling of inorganic compounds on Earth including methanogens, which produce methane from H2 and CO2, and nitrifiers, which convert ammonia to nitrate. Autotrophs are the source of reduced carbon substrates for the heterotrophs. Autotrophs are key elements of the carbon cycle. For this reason, autotrophic organisms are also called primary producers.