One further comment. It has been suggested in the scientific literature that corals do not always get enough amino acids from their zoox, and that aspartic acid can be one that is especially important in this regard.
here's a blurb I wrote a while back:
Some organisms may benefit from specific organic materials in the water column that they cannot make for themselves, or do not get enough of in their particulate diet, and that are not used simply as an energy source. These may include, for example, toxins that they absorb from the water column and that thereby can provide some benefit by making the organishm toxic to predators.
Another example is aspartic acid (a natural amino acid). It is readily taken up by certain corals, which rapidly incorporate it into proteins that may play an important role in calcification. The relationship between certain amino acids and calcification in corals was briefly mentioned in a previous article on the mechanism of calcification and will be discussed in detail in a future article. A brief explanation of how and why aspartic acid and certain other amino acids and organic materials may be involved in calcification is included below.
Organic molecules are known to play a substantial role in the formation of calcium carbonate in many organisms, including abalone shells and other mollusk shells. These materials can be proteins, glycoproteins, mucopolysaccharides, and phospholipids (and likely others that have not yet been identified). They help to induce the nucleation and growth of aragonite and are often referred to as the "organic matrix" because much of the corals' skeleton is composed of these organic materials.
In the case of corals, relatively little information is available about what, exactly, these organic materials are doing. The structures of some of these proteins contain an unusually large amount of aspartic acid residue. These amino acids are capable of binding to calcium, but whether that is a critical function or not has not been established. Here is some speculation about what these organics might be doing with respect to calcification:
...They may help control the concentration of free calcium in the coral, and thereby help control the rate of precipitation of calcium carbonate.
...They may control the location of crystal growth by binding free calcium and ferrying it to the location where the coral wants precipitation to take place.
...They may bind to the aragonite crystal face and thereby control the rate of precipitation.
...They may bind to the aragonite crystal face and thereby prevent precipitation in places where the coral does not want the skeleton to grow.
...They may bind to the aragonite crystal face and thereby inhibit binding of magnesium, phosphate, or other ions that are known to inhibit the growth of calcium carbonate crystals.
Regardless of the mechanisms involved, the need for these organics in calcification is easily verified. Allemand, et al have studied the role of such materials in Stylophora pistillata. Interestingly, they find that inhibitors of protein synthesis reduce the rate of calcification considerably. For example, reducing protein synthesis by 60-85% reduced calcification by 50%. Inhibiting glycoprotein synthesis yielded a similar result. These results did not arise from reduced metabolism, but rather were the effects of specifically reducing only protein and glycoprotein synthesis. The most important conclusion in their paper may be that the rate of skeletogenesis may be limited more by the rate of biosynthesis and exocytosis of organic matrix proteins than by calcium deposition.
Interestingly, the apparently large need for a particular amino acid (aspartic acid) to synthesize these proteins is satisfied by external sources, rather than by either the coral itself or its zooxanthellae. For this reason, some aquarists add aspartic acid, or commercial preparations containing it, to their aquaria. Whether there is a clear benefit to that addition remains to be established.
