Biological carbon fixation: pathways, mechanisms, and ecological significance

How it works

Fixing carbon requires enzymes that form new chemical bonds between CO2 and organic molecules, together with energy and reducing equivalents (for example ATP and NADPH in photosynthetic organisms). The best-known sequence of reactions is the Calvin cycle, in which the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) incorporates CO2 into a sugar precursor. Other autotrophic pathways—such as the reverse tricarboxylic acid cycle, the reductive acetyl-CoA (Wood–Ljungdahl) pathway, and several hydroxypropionate routes—use different enzymes and cofactors but accomplish the same overall task of converting inorganic carbon into organic molecules.

Where it occurs

• In most photosynthetic autotrophs (plants, algae and many bacteria), carbon fixation is the central process of primary production, using light energy to drive the chemistry.
• Some chemolithoautotrophic microbes also fix CO2, using chemical energy rather than light.
• Certain heterotrophs do not rely on CO2 as their main carbon source, but many can incorporate CO2 in specific biosynthetic or anaplerotic reactions (for example via carboxylases such as pyruvate carboxylase), so CO2 incorporation is not strictly limited to autotrophs.

Ecological and practical significance

Because carbon fixation supplies the organic carbon that supports food webs, it underpins ecosystems and the global carbon cycle. Understanding and engineering fixation pathways is also a focus of research into improving crop yields, developing biological carbon capture, and creating biotechnological routes to produce fuels and chemicals from CO2.