T. F. GOREAU 273 



in concentration of some substrate common to photosynthesis and 

 calcification. In previous papers (3, 4) we advanced the working 

 hypothesis that acceleration of CaCOg deposition would occur if 

 algal photosynthesis were to remo\e COo from the system and cause 

 the equilibrium reaction 



T 

 Ca(HCO,)., ^ CaCO, + H,CO, 



i 



to go to the right. Although the evidence for this is fairly pursua- 

 sive, other mechanisms may also be involved. Some of these will 

 be discussed below. 



In principle, the rate of CaCO.; production could be stimulat- 

 ed in at least two ways : directly through control of the steady state 

 bicarbonate concentration in the tissues as shown above, or indi- 

 rectly by augmenting the supply of free energy available for active 

 calcium transport through an increase in the rate and efficiency of 

 cellular metabolism. In the discussion below, we will consider some 

 of the possible indirect mechanisms. The onset of photosynthesis 

 by the zooxanthellae immediately produces a rise in the intracel- 

 lular oxygen concentration which may result in some increase in 

 the rate and efficiency of metabolism in the coral. Thiel (15) 

 and Yonge (19), among others, have already emphasized the 

 probable importance of in situ production to the coral, but no spe- 

 cific mechanisms were proposed. There is at present no information 

 on the relation between the pO^. of the medium and the rate of 

 coral growth. Nearly all hermatypic corals are net oxygen producers 

 during the day, and the water circulating in the growing parts of 

 the reef is as a rule supersaturated with oxygen (8, 10, 11, 12, 13) 

 so that the dependence of calcification on oxygen would be diffi- 

 cult to measure in these organisms. In two ahermatypic corals 

 lacking zooxanthellae {Tuhastrca and Asirangia) we observed no 

 significant changes in calcium deposition rates under conditions 

 where the oxygen saturation of the medium varied between fifty 

 and one hundred and twenty two per cent, suggesting that calcifica- 

 tion rates in these corals are relatively independent of oxygen con- 

 centration within the limits tested. 



