Chemoautotrophic

Chemoautotrophy is a form of metabolism in which organisms obtain energy by oxidizing inorganic chemical compounds and use that energy to synthesize organic matter from carbon dioxide or bicarbonate. Chemoautotrophs are autotrophs; their carbon source is inorganic, not organic. The energy-yielding reactions involve redox changes in inorganic substrates such as hydrogen gas, reduced sulfur compounds, ferrous iron, or ammonia. The energy is used to fix CO2 via pathways such as the Calvin cycle, the reverse TCA cycle, or other CO2 fixation routes.

Most chemoautotrophs are chemolithotrophs, meaning they extract energy from inorganic electron donors and transfer electrons to various acceptors. Electron acceptors can include oxygen, nitrate, nitrite, ferric iron, sulfate, or other compounds, depending on the organism and environment. Some chemoautotrophs are aerobic, requiring oxygen, while others are anaerobic or microaerophilic. These organisms are common in habitats with limited organic carbon, including deep-sea hydrothermal vents, hot springs, acidic mine drainages, and anoxic sediments, as well as soils and some animal guts.

Representative groups and examples include nitrifying bacteria such as Nitrosomonas (ammonia-oxidizing) and Nitrobacter (nitrite-oxidizing); sulfur-oxidizing bacteria such as Thiomicrospira, Beggiatoa, and Thiobacillus; iron-oxidizing bacteria such as Acidithiobacillus; and hydrogen-oxidizing bacteria such as Hydrogenophaga. Some archaea, including Sulfolobus and related thermoacidophiles, are chemoautotrophs that oxidize reduced sulfur compounds; many fix CO2 through pathways like the reverse TCA cycle or the reductive acetyl-CoA (Wood-Ljungdahl) pathway. Chemoautotrophy contributes to primary production and biogeochemical cycling in environments lacking light or abundant organic carbon.