The carbon cycle. Numbers are storage as 1015 g or fluxes as 1015 g per year (data from Schlesinger

Carbon is an essential part of all organic molecules, and, as constituents of the atmosphere, carbon compounds such as carbon dioxide, CO2, and methane, CH4, substantially influence global climate. This connection between atmospheric carbon and climate has drawn all nations of the planet into discussions of the ecology of carbon cycling, and is discussed more fully in chapter 23.  Carbon moves between organisms and the atmosphere as a consequence of two reciprocal biological processes: photosynthesis and respiration (fig. 20.5). Photosynthesis removes CO2 from the atmosphere, while respiration by primary producers and consumers, including decomposers, returns carbon to the atmosphere in the form of CO2. In aquatic ecosystems, CO2 must first dissolve in water before being used by aquatic primary producers. Once dissolved in water, CO2 enters a chemical equilibrium with bicarbonate, HCO3?, and carbonate, CO3?. Carbonate may precipitate out of solution as calcium carbonate and may be buried in ocean sediments.