Carbon cycle

Man’s recent release of greenhouse gases like as carbon dioxide CO2 into the atmosphere is generally held to be responsible for global warming. In the absence of man carbon dioxide serves as an atmospheric link among carbon released in the marine and terrestrial environments with methane playing a secondary role. Outside the atmosphere carbon forms a broad range of compounds from simple metal cyanides to complex macromolecules such as lignin, starch, cellulose and nucleic acids. Microorganisms take part in the ultimate conversion of all of these into carbon dioxide or methane and these processes may be broadly divided into the aerobic and anaerobic in Figure 2a. Carbon compounds are broken down either aerobically by respiration or anaerobically through fermentation. The release of carbon dioxide is balanced by the fixation of CO2 during photosynthesis through plants and microorganisms or by a diversity of mechanisms.

                            Figure  .  The biogeochemical cycling of the elements. (a) The carbon cycle. (b) The nitrogen cycle.