Chemotrophs

Chemotrophs are organisms that obtain their energy from chemical reactions, rather than from sunlight. They acquire energy by oxidizing chemical compounds and reducing another substance as part of redox reactions. This energy supports cellular processes and growth.

Chemotrophs are commonly divided by their electron donors into two main groups. Chemoorganotrophs obtain electrons from organic compounds, such as sugars, alcohols, and organic acids. Chemolithotrophs use inorganic compounds as electron donors, including hydrogen gas, reduced sulfur compounds, ammonia, and ferrous iron. Depending on their carbon source, chemotrophs can be autotrophic, growing from carbon dioxide (CO2) as their carbon source, or heterotrophic, using organic carbon.

Electron acceptors in chemotrophy vary. Many chemotrophs perform aerobic respiration, using molecular oxygen as the terminal electron acceptor to yield energy efficiently. Others use anaerobic respiration, employing alternative acceptors such as nitrate, nitrite, sulfate, ferric iron, or carbon dioxide. Some chemotrophs can also derive energy through fermentation under specific conditions, which does not involve external electron acceptors, but such processes are not strictly chemolitotrophic respiration.

Examples of chemotrophs illustrate their ecological roles. Nitrosomonas and Nitrobacter are chemolithoautotrophs in the nitrogen cycle, oxidizing ammonia and nitrite, respectively. Acidithiobacillus ferrooxidans is another chemolithotroph that oxidizes ferrous iron or reduced sulfur compounds. Escherichia coli and many other soil and gut bacteria are chemoorganotrophs that utilize organic substrates for energy and carbon, often requiring oxygen or other electron acceptors for growth.

Chemotrophs occupy diverse environments and contribute to biogeochemical cycles, organic matter decomposition, and industrial processes. Their metabolism contrasts with phototrophs, which derive energy directly from light.