Autotrophic and heterotrophic CO2 fixation.

a) Example of a typical autotrophic CO2 assimilation cycle (e.g. the Calvin-Benson-Bassham or reductive tricarboxylic acid cycle). ATP and reduction equivalents are provided by photo- or chemosynthesis. b) Heterotrophic CO2 assimilation can occur via linear pathways from a carbon substrate to certain products. If needed, ATP and/or reduction equivalents are generated from the substrate itself. Depending on the particular substrate-product combination, different amounts of CO2 can be assimilated.