Carbon Fixation
Carbon fixation refers to the chemical transformation of simple, inorganic compounds of carbon into more complex forms of organic matter. Examples of the simple compounds include carbon dioxide (CO2) and bicarbonate (HCO3 -), while the more complex forms include calcite (CaCO 3), which is inorganic, and the organic matter of organisms.
In ecosystems, carbon is fixed by autotrophs—organisms that utilize an external energy source to drive the synthesis of CO2 and water (H2O) into simple sugars. Usually, sunlight is the energy source for the fixation reactions, which is referred to as photosynthesis, and the organisms as photoautotrophs. Examples of photoautotrophs include plants, algae, and blue-green algae. Expressed simply, the photosynthetic reaction is:
(1) CO2 + 2H2O light CH 2O + 2O2 + H2O.
In reaction 1, the term CH2O refers to a carbohydrate, which is a primary product of photosynthesis. The molecular oxygen (O 2) is a "waste" product of the photosynthetic reaction and is usually released by the autotroph. Through the many complex reactions of metabolism, the carbohydrate can then be used to synthesize the additional biochemicals needed by autotrophs for survival, such as complex carbohydrates, proteins, and lipids.
Some autotrophs are non-photosynthetic, meaning they utilize energy sources other than sunlight to drive their carbon-fixation reactions. These so-called chemoautotrophs use the stored energy of certain chemicals [usually sulfides such as hydrogen sulfide (H2S) or iron sulfide (FeS2)] to drive chemosynthesis. Chemoautotrophs are the basis of ecosystems that are independent of solar radiation.
In addition to photosynthetic fixations of CO2, inorganic forms of carbon can be fixed by other biotic reactions occurring in aquatic ecosystems. One of these involves the fixation of dissolved carbon dioxide by a series of simple reactions, as follows:
(2) CO2 + H2O ( H2CO3
(3) H2CO3 ( H+ + HCO3-
(4) HCO3- ( H+ + CO3 -2
(5) Ca+2 + 2HCO 3- ( CaCO 3 + CO2 + H2O
In reaction 2, dissolved carbon dioxide combines with water to form carbonic acid (a weak acid). In reaction 3, the carbonic acid dissociates to form hydrogen ion and bicarbonate. In reaction 4, the bicarbonate dissociates to hydrogen ion and carbonate. In reaction 5, calcium ion combines with bicarbonate to form calcite plus carbon dioxide and water. Reaction 5 commonly occurs within the bodies of certain aquatic organisms, which utilize calcite to construct their shells (invertebrate animals) or bones (vertebrates). The calcite is an insoluble mineral, which upon death of the organisms sinks to the floor of the body of water and accumulates in the sediment. Through extremely slow geological processes occurring in deep sediment, the calcite may eventually metamorphose into rocks, such as limestone or chalk.
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