CO2 + 12 e− + 12 H+ + Pi → TP + 4 H2O An alternative perspective accounts for NADPH (source of e−) and ATP: 3 CO2 + 6 NADPH + 6 H+ + 9 ATP + 5 H2O → TP + 6 NADP+ + 9 ADP + 8 Pi The formula for inorganic phosphate (Pi) is HOPO32− + 2H+. Formulas for triose and TP are C2H3O2-CH2OH and C2H3O2-CH2OPO32− + 2H+ Reverse Krebs cycle The reverse Krebs cycle, also known as reverse TCA cycle (rTCA) or reductive citric acid cycle, is an alternative to the standard 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. In particular, it is one of the most used pathways in hydrothermal vents by the Campylobacterota. This feature is very important in oceans. Without it, there would be no primary production in aphotic environments, which would lead to habitats without life. So this kind of primary production is called "dark primary production". The cycle involves 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+ Oxaloacetate + NADH/H+ -> Malate + NAD+ Fumarate to succinate, catalyzed by an oxidoreductase, Fumarate reductase Fumarate + FADH2 <=> Succinate + FAD Succinate to succinyl-CoA, an ATP dependent step Succinate + ATP + CoA -> Succinyl-CoA + ADP + Pi Succinyl-CoA to alpha-ketoglutarate, using one molecule of CO2 Succinyl-CoA + CO2 + Fd{(red)} -> alpha-ketoglutarate + Fd{(ox)} Alpha-ketoglutarate to isocitrate, using NADPH + H+ and another molecule of CO2 Alpha-ketoglutarate + CO2 + NAD(P)H/H+ -> Isocitrate + NAD(P)+ Citrate converted into oxaloacetate and acetyl-CoA, this is an ATP dependent step and the key enzyme is the ATP citrate lyase Citrate + ATP + CoA -> Oxaloacetate + Acetyl-CoA + ADP + Pi This pathway is cyclic due to the regeneration of the oxaloacetate. The bacteria Gammaproteobacteria and Riftia pachyptila switch from the Calvin-Benson cycle to the rTCA cycle in response to concentrations of H2S. Reductive acetyl CoA pathway The reductive acetyl CoA pathway (CoA) pathway, also known as the Wood-Ljungdahl pathway uses CO2 as electron acceptor and carbon source, and H2 as an electron donor to form acetic acid. This metabolism is wide spread