For photoautotrophs, the limiting factor for energy production is light. However, not all organisms live in areas where light is abundant. As such, these organisms have evolved alternate methods of producing energy compounds that use chemicals found in their environment. An organism that uses chemicals to produce these energy compounds for itself is called a chemoautotroph. Chemoautotrophs use materials such as hydrogen sulfide (H2S), ammonia (NH4), and iron (Fe2+) to synthesize compounds that can be broken down to release energy for the organism.
While there is some diversity to the organisms in the group, many can be referred to as extremophiles because of where they live. Extremophiles, which are mostly members of the domains Bacteria and Archaea, live in areas that would be inhospitable to most organisms. These areas include hot springs, volcanoes, and deep oceans. One type of extremophile is called a methanogen, which is a type of bacterium that lives inside the intestines of cows and in swampy bogs because it needs an environment that is high in methane. It uses methane as a source for generating its own carbon compounds. Other bacteria live in the roots of plants and form a symbiotic relationship with the plant. These chemoautotrophic bacteria take in nitrogen from the atmosphere and convert it into ammonia, a process called nitrogen fixation. This is important because plants cells cannot use atmospheric nitrogen directly. Nitrogen-fixing bacteria change nitrogen into a form that can be assimilated by the plant. These types of bacteria are often self-sufficient since they need only nitrogen, water, and carbon dioxide to thrive. Nitrogen fixation also benefits the rest of the ecosystem because it makes more nitrogen available to plants.
Other chemoautotrophs oxidize hydrogen sulfide and produce sulfur as a waste product. These bacteria are either free-living in the environment or form symbiotic relationships with other organisms. They are mostly found near deep-water hydrothermal vents, where superheated water and chemicals are being spewed from deep within the earth. Since light cannot penetrate this far down in the ocean, these organisms have adapted to convert sulfur into organic compounds that are used for energy. Sulfur acts as the electron donor as the electrons move into an electron transport chain, which conserves energy as it progresses. Electrons formed in this manner pass through the electron transport chain to generate the protons that are used during chemiosmosis, the production of ATP.