T. F. GOREAU 279 



cium, and the lower for that constituent which was diluted the 

 most during its passage through the cells, e. g. carbonate. This sug- 

 gests that the reservoir of intracellular carbonate available for ex- 

 change with absorbed exogenous carbonate is much greater than 

 the internal pool of freely exchangeable calcium, and that the tissue 

 calcium turnover rates must therefore be much higher than those 

 of carbonate. In previous experiments, we have demonstrated that 

 the exchangeable calcium in corals is indeed maintained at a low 

 level in corals (5, 7 ) . The simultaneous introduction of isotopically 

 labelled calcium and carbon makes it possible to assess the relative 

 sizes of the pools of exchangeable endogenous calcium and carbon 

 by the principle of dilution volumes in a situation where no isotopic 

 equilibration has occurred. Under these conditions, our calculated 

 transfer rates indicate that the internal pool of carbon available for 

 exchange with exogenous carbonate being deposited into the skel- 

 eton is about two to fifteen times greater than the amount of ex- 

 changeable calcium. 



SUMMARY AND CONCLUSIONS 



1. Coral reefs are tropical shallow water communities where 

 intensive biological calcification occurs, resulting in net accumula- 

 tion of limestone into the sediments. Photosynthesis appears to 

 be in some way essential to reef formation. The most important 

 reef-building organisms are calcareous algae and coelenterates, cor- 

 als included. All reef-building coelenterates without exception con- 

 tain symbiotic zooxanthellae. Corals without zooxanthellae grow 

 slowly and never play a significant role in the building of reefs. 



2. The zooxanthellae do not themselves calcify, but their 

 presence results in a very powerful enhancement of calcification 

 in the coral host as soon as photosynthesis begins. We have shown 

 that stimulation of growth by light requires zooxanthellae since this 

 efi^ect does not occur in reef corals from which zooxanthellae are 

 removed, nor does it occur in ahermatypic corals which never have 

 algal symbionts. Of three calcareous algae tested, two calcified 

 much faster in light than in darkness, and in one the efl^ect was re- 

 versed. 



