How Reefs are Built

Coral reefs grow through a process of increasing their calcareous skeleton mass and their overlying living tissue. The term "calcification" refers to the fact the process in which calcium builds up to form a solid mass. The coral skeleton is composed of aragonite, which is calcium carbonate (CaCO₃) in a fibrous, crystalline form, where it must be noted that calcite, the common form of CaCO₃, is not present anywhere in the skeleton. Algae also deposit aragonite or a soluble calcite form with a high magnesium content in the terms of the reefs. Heinz A. Lowenstam of California Institute of Technology showed in Bermuda that some calcareous organisms deposit less soluble calcite during colder seasons and more soluble aragonite during warmer seasons. The mechanism that controls the seasonal CaCO₃ deposition variation is so far unknown.

Zooxanthellae are required in the process of building up the reef’s skeleton. Zooxanthellae speed up the calcium carbonate precipitation. This causes new layers of skeleton to be formed. There are cations of calcium Ca²⁺ and anions of bicarbonate HCO₃^- in the coral mucus. The two ions react and produce Ca(HCO₃)_2.:

Ca2+ + 2HCO3- Ca(HCO3)2

from which calcium carbonate and carbon acid originate:

Ca(HCO3)2 CaCO3 + H2CO3

Calcium carbonate crystallize on existing aragonite crystals. In this way of a new skeletal layer forms.

Carbonic acid resolves itself into carbon dioxide and water:

H2CO3 CO2 + H20

Coral polyps absorb calcium ions from seawater and are transferred by both diffusion and an active pump mechanism that ends at the area being calcified. This is important because calcium is used in cell metabolism for regulation. The calcium concentrations must be low or else it will interfere with coral tissue functions. The concentration of free calcium ions is lower in corals than in seawater because the calcium ions stick to membranes or organic molecules. Lothar Böhm at the University of the West Indies in Jamaica showed through his studies that calcium has a high turnover rate.

The process in which the skeleton is made takes place during the day with great intensity. At night skeleton growth abates and slows down. The growth depends also on the season of the year, age and shape of corals. Young corals grow much faster than old corals, sphere corals grow slower than corals which have branched shape. A fact of particular interest is that corals are able to change their shape during their life to aid in sunlight retrieval. Temperature is also very important, for corals grow best in temperatures between 25-27^oC.

Factors Influencing Calcification

It seems pretty obvious that light is important in coral calcification because corals need light for photosynthesis. It makes sense that growth decreases when there are drugs present that inhibit photosynthesis. As a matter of fact, corals grow 14 times faster under light than in the dark. For example, calcium uptake is highest at mid-day on a clear sunny day. On a cloudy day, the rate is 50% lower, and in darkness, the rate is 90% lower. The calcium deposition rate rapidly decreases as depth increases.

Why Do These Factors Depend on Zooxanthellae?

There are few relationships as close and tight as that between coral and zooxanthellae. As one might suspect, it is crucial to take zooxanthellae into account when looking at how reef calcification occurs.