text: The primary form of inorganic carbon that is fixed is carbon dioxide (CO). It is estimated that approximately 250 billion tons of carbon dioxide are converted by photosynthesis annually.  The majority of the fixation occurs in terrestrial environments, especially the tropics.  The gross amount of carbon dioxide fixed is much larger since approximately 40% is consumed by respiration following photosynthesis. Historically it is estimated that approximately 2×10 billion tons of carbon has been fixed since the origin of life.