Coral reefs: physical structure

The existence of a coral reef framework creates a three- dimensional, topographically complex habitat and the high levels of species (and hence genetic) diversity on coral reefs would not be possible without this physical structure. Corals are the main builders of the reef framework through the accumulation of limestone by a process known as calcification, but a diversity of other organisms, encrusting coraline algae, foraminifera, molluscs, and echinoderms are also needed in the building of the reef. Coral calcification is intimately related to the internal symbiosis with the zooxanthellae and overall reefs precipitate half of the calcium delivered to the sea each year.

 

However, while zooxanthellae photosynthesis certainly enhance coral calcification rates and are responsible for forming most of the solid coral reef framework, the way in which this operates is yet to be fully determined. Photosynthesis raises the pH which provides more carbonate ions for deposition and zooanthellae are also believed to remove inorganic chemical inhibitors. Energy for active calcification may be provided to the coral by the zooxanthellae but only in shallow water corals where the fixation of carbon exceeds their own respiratory needs. What is certain is that all the goods and services of the reef are directly or indirectly dependent on the reef building corals and their zooxanthellae.

This symbiosis requires sufficient light and good water circulation, and generally exists in a rather narrow range of water temperature, with low nutrient and sedimentation loads. As a result corals are sensitive to environmental factors which perturb any of these factors.

 

For example, corals at individual localities appear to have become adapted to quite narrow temperature regimes and exposure to warmer water is often sufficient to cause bleaching, which is the appearance caused by the expulsion of zooxanthellae by coral. It is not evidence of the death of the coral, but could lead to death if the triggering condition for the expulsion is not improved.