Atmospheric Carbon Dioxide

The concentration of atmospheric carbon dioxide has increased in the last 300 years i.e. since the industrial revolution, due to the burning of fossil fuels, which may have a dramatic impact on the global climate. Marine ecosystems such as coral reefs play an important part in determining the ocean-atmosphere exchange of carbon dioxide (CO₂). Coral reefs have been directly impacted by human activities, such as pollution and overfishing, but the increase in atmospheric CO₂ will also seriously impact coral reef health in the future by reducing the corals ability to produce it’s hard calcium carbonate skeleton.

Coral reefs contribute to the ocean carbon cycle through the processes of photosynthesis, respiration, calcium carbonate production and dissolution. Photosynthesis is not undertaken by the coral itself but by the algae which has a symbiotic relationship with it. Algae are autotrophs, organisms that can synthesise all the complex organic molecules they need using only simple inorganic compounds. Coral on the other hand is a heterotroph, an organism that cannot synthesise all the organic molecules it needs using only inorganic compounds; instead they oxidise organic carbon (CH₂O as shown in the equation below) to carbon dioxide to yield their cellular energy. Theses processes control the distribution of CO₂ in seawater, determine the fate and transformations of CO₂, and ultimately the exchange of CO₂ between the ocean and atmosphere.

Photosynthesis: CO₂ + H₂O ® CH₂O + O₂

Respiration: CH₂O + O₂ ® CO₂ + H₂O

Calcium carbonate formation: CO₂(aq) + H₂O « H₂CO₃

H₂CO₃^- « H⁺ + HCO₃^2-

HCO₃^- « H⁺ + CO₃^2-

Ca²⁺ + CO₃^2- ® CaCO₃(s)

In coral reef systems, hard coral calcification produces CO₂, the result is that the coral reef can act as a source of CO₂ to the atmosphere. In contrast, algae photosynthetically fix CO₂ into organic matter, potentially removing CO₂ from seawater. The result is that the coral reef can act as a sink of CO₂ from the atmosphere. In a coral reef system, the balance of calcification and algal photosynthesis alters the fate of CO₂, as either stored in the ocean or remaining in the atmosphere, with a potential impact on global climate. Biological complexity on the reef thus contributes to a potential climate feedback that is poorly understood at present. So strange as it seems, as coral reef health declines and algal biomass on the reef increases, the potential for coral reefs to remove CO₂ from the atmosphere increases.