Aquatic Conversions

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•  In aquatic ecosystems carbon is present as dissolved carbon dioxide and hydrogen carbonate ions


carbon ocean

Carbon dioxide dissolves in water and some of it will remain as a dissolved gas, however the remainder will combine with water to form carbonic acid  (CO2 + H2O  ⇄  H2CO3)

  • Carbonic acid will then dissociate to form hydrogen carbonate ions  (H2CO3  ⇄  HCO3 + H+)
  • This conversion also releases hydrogen ions (H+), which is why pH changes when CO2 is dissolved in water (> acidic)

Autotrophs absorb both dissolved carbon dioxide and hydrogen carbonate ions and use them to produce organic compounds 

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•  Animals such as reef-building corals and mollusca have hard parts that are composed of calcium carbonate

    and can become fossilised in limestone


When the hydrogen carbonate ions come into contact with the rocks and sediments on the ocean floor, they aquire metal ions

  • This commonly results in the formation of calcium carbonate and the subsequent development of limestone

Living animals may also combine the hydrogen carbonate ions with calcium to form calcium carbonate

  • This calcium carbonate forms the hardened exoskeleton of coral, as well as forming the main component of mollusca shells
  • When the organism dies and settles to the sea floor, these hard components may become fossilised in the limestone

Summary of Carbon Conversions in Aquatic Environments

ocean conversion table

Overview of Calcium Carbonate Formation in Oceans

calcium carbonate