In biology, carbon fixation is the reduction of inorganic carbon (carbon dioxide) to organic compounds by living organisms. The most prominent example is photosynthesis. Organisms that grow by fixing carbon are called autotrophs—plants for example. Heterotrophs, like animals, are organisms that grow using the carbon fixed by autotrophs. Fixed carbon, reduced carbon, and organic carbon all mean organic compounds.

Photosynthesis uses energy from sunlight to drive an autotrophic carbon fixation pathway.

Oxygenic photosynthesis is used by the chief primary producers—plants, algae, and cyanobacteria. They contain the pigment chlorophyll, and use the Calvin cycle to fix carbon autotrophically.

Somewhere between 3.5 and 2.3 billion years ago, cyanobacteria evolved oxygenic photosynthesis. The process works like this:

The essential innovation is the first step, the dissociation of water into electrons, protons, and free oxygen. This allows the use of water, one of the most abundant substances on Earth, as an electron donor—as a source of reducing power. The release of free oxygen is a side-effect of enormous consequence. The first step uses the energy of sunlight to oxidize water to O₂, and, ultimately, to produce ATP