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Reverse Krebs cycle
The reverse Krebs cycle, also known as reverse TCA cycle (rTCA), or reductive citric acid cycle, is an alternative pathway to Calvin-Benson cycle for Carbon Fixation. It has been found in strict anaerobic or microaerobic bacteria (as Aquificales) and anaerobic archea. It was discovered by Evans, Buchanan and Arnon in 1966 working with the photosynthetic green sulfur bacterium: Chlorobium limicola. The cycle is about the biosynthesis of Acetyl-CoA from two molecules of CO2. The key steps of the reverse Krebs cycle are:


 * Oxaloacetate to malate, using NADH + H+


 * Fumarate to succinate, catalyzed by an oxidoreductase, Fumarate reductase


 * Succinate to succinyl-CoA, an ATP dependent step


 * Succinyl-CoA to alpha-ketoglutarate, using one molecule of CO2


 * Alpha-ketoglutarate to isocitrate, using NADPH + H+ and another molecule of CO2


 * Citrate converted into oxaloacetate and acetyl-CoA, this is an ATP dependent step and the key enzyme is the ATP citrate lyase

This pathway is cyclic due to the regeneration of the Oxaloacetate.

The reverse Krebs cycle is a pathway used by microorganism in anaerobic environments. In particular is one of the most used pathway in hydrothermal vents by the Epsilonproteobacteria. This feature is very important in oceans, otherwise there would be no primary production in aphotic enviroments and that would lead to habitat without life. So this kind of primary production is called Dark Primary Production.

One other important aspect is the symbiosis between Gammaproteobacteria and Riftia pachyptila. These bacteria can switch from Calvin-Benson cycle to rTCA cycle and vice versa, due to the different concentration of H2S in the enviroment.