Coral calcification

Buchsbaum Pearse, V., and Muscatine, L. Role of symbiotic algae (Zooxanthellae) in coral calcification. Biol. Bull. 141, 350-363 (1971). 

Marubini, F. and Atkinson, MJ. (1999) Effects of lowered pH and elevated nitrate on coral calcification. Marine Ecology Progress Series, 188, 117—121. 

Langdon, C., Takahashi, T., Sweeney, C., Chipman, D., Goddard, J., Marubini, F., Aceves, H., Barnett, H., and Atkinson, M.J., Effect of calcium carbonate saturation state on the calcification rate of an experimental coral reef, Global Biogeochem. Cy., 14, 639-654,2000. 

Biological calcification processes are widely distributed in nature. They can be found in microorganisms, in plants, in the animal kingdom and in humans. Under physiological conditions, the results of mineral deposition in biological systems can be represented by the formation of bones, teeth and shell material as well as coccoliths, corals, pearls etc. The variety of biomineralisates can best be expressed by the fact that approximately 128,000 species of molluscs636 are known. The majority of them (Conchifera) form shells of different kinds of size and shape as well as of color. 

If corals are kept in darkness and in the presence of carbonic anhydrase inhibitors, calcification will still continue but at a much lower rate. The following conclusions can be drawn from these data ... 

Goreau, T. F., and Goreau, N. I. The physiology of skeleton formation in corals III. Calcification rate as a function of colony weight and total nitrogen content in the reef coral Manicinaareolata (Linnaeus). Biol. Bull. Ill, 420-429 (1959). 

Figure 21.2 Nutrient availability changes the relationship between tnicroalgal symbionts and coral hosts (adapted from Dubinsky and Stambler, 1996 Furnas, 2003). Under low nutrient conditions zooxanthellae translocate photosynthate to corals for growth, calcification and reproduction. Under higher nutrients, zooxanthellae increase in abundance and keep more of the photosynthate for themselves translocating less to host coral for metabolic needs.

Two of these studies also looked at boron incorporation into the coral skeleton and observed that it also appears to be, at least in part, related to temperature (Sinclair et al., 1998 Fallon et al., 1999). The fact that at least four elements follow a seasonal pattern related to temperature suggests that elemental incorporation in coral skeletons is linked to calcification and is not simply driven by a thermodynamic temperature effect. If this applies generally, than all of the coral metal paleothermometers will have to be applied with attention to the possibihty of distortions caused by growth factors. 

Shallow marine environments include coral and algal reefs as well as other bioherms and many favour calcification by benthic fauna. Stromatolites and stromatolitic environments are also typical shallow marine formations. The shallow marine carbonate environment may be subdivided into more or less agitated waters with dominantly benthic fauna, calm shallow areas with carbonate muds (e.g. Bahama Banks) with ooids as typical forms of deposits and reef areas with their complicated patterns of calcification and deposition (Bathurst, 1975 Kinsey and Davies, Chapter 2.5). 

Anatomical aspects. The skeletons of corals are formed of crystals of aragonite in an organic matrix. Calcification begins when the free-swimming larva attaches to the substratum. The larva then metamorphoses into a polyp which continues to deposit extracellularly CaCOs and organic matrix through the secretory activity of the calicoblastic epithelium. The epithelium is a layer of cells at the base of the polyp adjoining the skeletal area at which deposition takes place. 

Calcification mechanisms. The mechanisms of CaCOs deposition have not been clearly defined. Two major questions are involved first, the relation of the organic portion of the skeleton to the initiation and control of crystal growth and, second, the role of the symbiotic photosynthetic zooxanthellae, the algae which grow within the tissue of hermatypic corals (Chapman, 1974). Another persistent question has been the site of crystal initiation, whether it is entirely extracellular or whether it is partly intracellulfir as well (Muscatine, 1971). 

Calcification rates in corals are discussed also in Chapter 2.5. 

The extracellular formation of crystals of CaC03 by epithelia is the most common type of skeleton-forming system and occurs in corals (Vandermeu-len, 1975), molluscs (Wilbur, 1964), some annelids (see p. 84) brachiopods (Williams, 1971), and arthropods (Travis, 1970). In the arthropods, elaborate cellular extensions penetrate the mineralized carapace and are important in the calcification process. Formation of a mineralized skeleton in these taxa involves the movement of Ca and HCOj across a layer of cells from the body fluid (absent in corals). Nucleation and crystal growth take place in an organic milieu secreted by the epithelium. 

The CO2/HCO3 system involved in calcification can be considered in a somewhat similar manner. The question as to whether the carbonate ion of the mineral is derived from extracellular fluids originating in the external medium or whether it arises from intracellular metabolic CO2 has been examined in corals (Pearse, 1970), molluscs (Campbell and Speeg, 1969 Wheeler et ah, 1975), and arthropods (Greenaway, 1974c) (see pp. 74, 80, 87). It may be that both sources contribute to the skeletal carbonate. The significance of determining the intracellular and extracellular sources of carbonate is not simply that it enables one to draw up a balance sheet of input or output but that it serves as an indication of the participation of cellular function in mineralization processes. 

The enzyme has been found in corals (Goreau, 1959), annelids (Clark, 1975), crustaceans (Costlow, 1959), molluscs (Wilbur, 1972, for references), and echinoderms (Heatfield, 1970). In all cases, sulfanilamide inhibitors of the enzyme added to the medium reduced the rate of calcification at least 50%, indicating that catalysis by carbonic anhydrase is required for the normal rate of mineralization. In theory, the reactions should occur without enzyme catalysis and this appears to be true. Various mechanisms of action have been suggested (e.g., Goreau, 1959 Istin and Girard, 1970b) but the exact role of carbonic anhydrase remains uncertain. 

Barnes, D.J. and Taylor, D.L., 1973. In situ studies of calcification and photosynthetic carbon fixation in the coral Montastrea annularis. Helgol. Wiss. Meeresunters., 24 284-291. 

Chalker, B.E. and Taylor, D.L., 1975. Light enhanced calcification, and the role of oxidative phosphorylation in calcification of the coral Acropora cervicornis. Proc. R. Soc. London, Ser. B., 190 323—331. 

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