<P> Carbon fixation or сarbon assimilation is the conversion process of inorganic carbon (carbon dioxide) to organic compounds by living organisms . The most prominent example is photosynthesis, although chemosynthesis is another form of carbon fixation that can take place in the absence of sunlight . Organisms that grow by fixing carbon are called autotrophs . Autotrophs include photoautotrophs, which synthesize organic compounds using the energy of sunlight, and lithoautotrophs, which synthesize organic compounds using the energy of inorganic oxidation . Heterotrophs are organisms that grow using the carbon fixed by autotrophs . The organic compounds are used by heterotrophs to produce energy and to build body structures . "Fixed carbon", "reduced carbon", and "organic carbon" are equivalent terms for various organic compounds . </P> <P> It is estimated that approximately 258 billion tons of carbon dioxide are converted by photosynthesis annually . The majority of the fixation occurs in marine environments, especially areas of high nutrients . The gross amount of carbon dioxide fixed is much larger since approximately 40% is consumed by respiration following photosynthesis . Given the scale of this process, it is understandable that RuBisCO is the most abundant protein on Earth . </P>

Where does the energy to fix gaseous co2 come from