Chemoautotrophs obtain energy by using oxygen or compounds with high oxygen content to oxidize, or take electrons from, sulfur compounds, hydrogen, elemental sulfur, ammonia or metals. They use some of the energy obtained from these oxidation reactions to create organic compounds from reactions with carbon dioxide.
Like photosynthetic organisms such as plants and algae, chemoautotrophs do not obtain their energy by consuming other organisms. Chemoautotrophs are exclusively bacteria, and usually live around hydrothermal vents in the deep ocean. Such environments do not receive any sunlight and so cannot support photosynthesis. In most areas of the deep ocean floor, this means that the only organisms that survive feed on waste and other debris from much closer to the surface.
Around hydrothermal vents, however, chemoautotrophs receiving sources of energy from deep within the earth are the producers for their own ecosystems. Hot thermal vents feature sulfur and other compounds that can be oxidized for a great deal of energy. Cooler, more slow-flowing vents emit more dissolved metals. Metals are a much less efficient source of energy. The bacteria that process these metals, such as iron, grow in mats that rapidly accumulate large amounts of rust. The bacteria must process so much of the metals that the vast majority of the mass of any of these mats is made up of this iron oxide.