Dissolved inorganic carbon (DIC) is the sum of the aqueous species of inorganic carbon in a solution. Carbon compounds can be distinguished as either organic or inorganic, and as dissolved or particulate, depending on their composition. Organic carbon forms the backbone of key component of organic compounds such as – proteins, lipids, carbohydrates, and nucleic acids. Inorganic carbon is found primarily in simple compounds such as carbon dioxide, carbonic acid, bicarbonate, and carbonate (CO2, H2CO3, , respectively). Dissolved inorganic carbon (DIC) includes three major aqueous species, CO2, ,, and to a lesser extent their complexes in solution with metal ions. Marine ecosystems Solubility pump Aqueous carbon dioxide reacts with water to form carbonic acid which is very unstable and will dissociate rapidly into hydronium and bicarbonate. Therefore, in seawater, dissolved inorganic carbon is commonly referred to as the collection of bicarbonate, carbonate ions, and dissolved carbon dioxide (CO2, H2CO3, , ). CO2(aq) + H2O H2CO3 + H+ + 2 H+ More than 99% of dissolved inorganic carbon is in the form of bicarbonate and carbonate ions meaning that most of the ocean’s carbon storing ability is due to this chemical reactivity. Sea-air flux of CO2 and the resulting dissolved inorganic carbon is affected by physical processes such as strong winds and vertical mixing, and the biological processes of photosynthesis, respiration, and decomposition. Biological pump Dissolved inorganic carbon is a key component of the biological pump, which is defined as the amount of biologically produced organic carbon flux from the upper ocean to the deep ocean. Dissolved inorganic carbon in the form of carbon dioxide is fixed into organic carbon is produced through photosynthesis. Respiration is the reverse process and consumes organic carbon to produce inorganic carbon. Photosynthesis, and the biological pump, is dependent on the availability of inorganic nutrients and carbon dioxide. Photosynthesis: 6 CO2 + 6 H2O + light → C6H12O6 + 6 O2 Respiration: C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy Oceanographers seek to understand the metabolic state of the ocean, or the efficiency of the biological pump, by estimating the net community production (NCP) which is the gross primary productivity (GPP) minus the community respiration (sum of the respiration of the local autotrophs and heterotrophs). An efficient biological pump increases biological export to the deeper ocean which has been hypothesized to suppress CO2 outgassing in the upper ocean. Carbonate pump The carbonate