school energy&environment

Microscopic builders

The barrier reef or simply “reef” is one of the ecosystems with the highest number of species in the world. It is a wide and massive calcareous formation of animal origins with myriad colours and shapes. Responsible for the building of this biome are madreporic anthozoa, also known as the ‘builder corals”.
Corals or madrepores are composed of small polyps of differing sizes (from few mms to a few cms) surrounded by a calcareous calyx called “corallite” which looks different according to the species. Each polyp hosts unicellular algae called “zooxanthellae”, which give them a brown-greenish colour.  This particular combination is called “”mutualism”, i.e. both species benefit from living together. The algae, through the photosynthesis of the chlorophyll, supply the polyp with energy in the form of sugars, produce oxygen and remove carbon dioxide (which could form carbonic acid and damage the calcareous skeleton of the polyps). In return, the polyp offers protection to the microscopic and numberless algae. Every square centimetres of the madrepore can contain up to one million zooxanthella algae.
The reefs are composed of calcium carbonate (CaCO₃) used by the coral polyps to build their own Coral formations develop mostly between the water surface and a depth of thirty metres. They need three environmental conditions to grow:

• a mean water temperature above 20°C in winter
• constant salinity
• plenty of light.

Only in these specific conditions can the corals grow and reproduce. Some species (such as the brain coral) grow between 5 and 25 mms a year, others (such as the antler coral) grow much quicker, up to 10-20 cm a year.
The reef is an ever-growing ecosystem since new polyps grow on the old ones that die and so the surface area is always composed of live corals.