Carbon fixation is the biological incorporation of inorganic carbon, principally carbon dioxide or bicarbonate, into organic compounds. It supplies carbon for growth and connects metabolism with the global carbon cycle. Although best known as part of photosynthesis, it also occurs in microorganisms that obtain energy from chemical reactions rather than sunlight. Several distinct biochemical pathways perform this conversion. Fixation describes a chemical transformation, not necessarily permanent carbon storage. (academic.oup.com)
Carbon sources and energy
Carbon fixation separates two requirements of biological growth: obtaining carbon and obtaining energy. Autotrophic organisms build their cellular material primarily from inorganic carbon. Photoautotrophs, including plants, algae, and cyanobacteria, use sunlight as their energy source. Chemolithoautotrophic members of the Bacteria and Archaea obtain energy by oxidizing substances such as hydrogen, reduced sulfur compounds, or ammonia. Their carbon-assimilation pathways need not be the same as those of photosynthetic organisms. (openstax.org)
Producing reduced organic carbon requires an energy supply and reducing power. Adenosine triphosphate (ATP) drives energy-requiring reactions, while carriers such as NADPH provide reducing equivalents. Individual carboxylation reactions incorporate inorganic carbon, but sustained autotrophic growth requires an integrated pathway that produces biosynthetic precursors and replenishes its carbon-accepting intermediates. Different pathways therefore have different energetic costs, enzyme requirements, and sensitivities to oxygen. (pmc.ncbi.nlm.nih.gov)
The Calvin–Benson–Bassham cycle
The Calvin–Benson–Bassham cycle, commonly called the Calvin cycle, is the predominant pathway of photosynthetic carbon assimilation. In plants and algae, it operates in the stroma of the chloroplast. Its reactions are conventionally grouped into fixation, reduction, and regeneration. The cycle uses ATP and NADPH supplied by the light-dependent reactions, but its carbon-incorporating reactions do not directly absorb light. Consequently, the older expression “dark reactions” is misleading: it does not mean that the cycle normally operates only at night. (pmc.ncbi.nlm.nih.gov)
During fixation, the enzyme Rubisco adds CO₂ to the five-carbon acceptor ribulose-1,5-bisphosphate, or RuBP. The unstable six-carbon product splits into two molecules of 3-phosphoglycerate. ATP and NADPH subsequently support conversion of these compounds into glyceraldehyde-3-phosphate, a three-carbon sugar phosphate. Most of this material regenerates RuBP, allowing further fixation; a smaller fraction leaves the cycle for biosynthesis. (openstax.org)
For every three CO₂ molecules assimilated, the cycle yields one net glyceraldehyde-3-phosphate while consuming nine ATP and six NADPH. This product contributes to the synthesis of carbohydrates, including glucose, rather than glucose being the direct product of Rubisco’s reaction. The accounting describes the cycle itself, excluding additional costs associated with carbon-concentrating mechanisms or photorespiration. (openstax.org)
C3, C4, and CAM photosynthesis
In C3 photosynthesis, the first stable product of Rubisco-mediated fixation is the three-carbon compound 3-phosphoglycerate. Rubisco can also react with oxygen, initiating photorespiration. This competing process consumes resources and releases some previously assimilated carbon. Carbon-concentrating mechanisms can reduce this competition by increasing CO₂ availability around Rubisco. (assets.openstax.org)
In C4 photosynthesis, initial inorganic-carbon incorporation produces four-carbon compounds. In the usual arrangement, these form in mesophyll cells and move to bundle-sheath cells, where decarboxylation supplies CO₂ to the Calvin cycle. The mechanism separates initial capture and Rubisco activity spatially. In crassulacean acid metabolism (CAM), separation is primarily temporal: plants typically open their stomata at night, store captured carbon in organic acids, and release CO₂ internally during the day. This arrangement limits daytime water loss. Neither C4 nor CAM replaces the Calvin cycle; both supply it with concentrated CO₂. (assets.openstax.org)
Alternative microbial pathways
Microorganisms employ several additional autotrophic pathways. These are not simply variants of the Calvin cycle and may use entirely different carbon-fixing enzymes and intermediates. Established alternatives include:
- The reductive tricarboxylic acid cycle, which uses many reactions related to the citric acid cycle in the opposite overall direction to generate organic precursors.
- The reductive acetyl-CoA pathway, or Wood–Ljungdahl pathway, which combines carbon processed through separate branches to produce acetyl-CoA.
- The 3-hydroxypropionate bicycle, which assimilates inorganic carbon through carboxylation reactions and interconnected reaction sequences.
- The 3-hydroxypropionate/4-hydroxybutyrate and dicarboxylate/4-hydroxybutyrate cycles, which regenerate carbon-accepting intermediates through reactions involving 4-hydroxybutyrate. (academic.oup.com)
The reductive glycine pathway provides another route. Research published in 2020 demonstrated that it supports autotrophic growth in Desulfovibrio desulfuricans. This finding illustrates why inventories of carbon-fixation pathways can change as biochemical functions are experimentally established. Detecting pathway genes indicates metabolic potential, whereas demonstrating growth and tracing inorganic carbon into biomass provide stronger functional evidence. (nature.com)
Ecological significance and experimental study
Carbon fixation underlies much of primary production, supplying organic matter to food webs. In the ocean, photosynthetic plankton incorporate dissolved inorganic carbon into biomass. Some material sinks below surface waters, transferring carbon toward deeper reservoirs. On land, fixed carbon enters vegetation and soils. However, cellular respiration, decomposition, and combustion return substantial quantities to inorganic form. High fixation rates therefore do not, by themselves, demonstrate long-term net carbon removal. (science.nasa.gov)
Researchers trace carbon incorporation using labeled carbon sources and analyze the resulting metabolites. Incorporation of the isotope carbon-13 can reveal pathway activity and carbon flow. Carbon fixation is also studied through engineered enzyme networks: in 2016, researchers demonstrated the synthetic CETCH cycle outside living cells, assembling 17 enzymes into a system that converted CO₂ into organic products. Its operation required supplied cofactors and their regeneration, distinguishing an experimentally functional fixation network from a self-sustaining organism or an ecosystem-scale carbon sink. (pmc.ncbi.nlm.nih.gov)