280 THE BIOLOGY OF HYDRA : 1961 



3. There is a rough correlation between calcification rate and 

 specific photosynthetic rate as measured by the organic productivity. 

 The highest calcification and productivity rates were noted in the 

 calcareous algae, but in one of these we observed a very strong 

 reduction of CaCO.j deposition in the light in the presence of a 

 very high rate of photosynthesis. Calcification and primary pro- 

 ductivity rates in three hermatypic coelenterates with zooxanthel- 

 lae are on the average about sixty per cent lower than in the 

 calcareous algae. Their slowest calcification rates were observed in 

 the ahermatypic corals that have no zooxanthellae. 



4. Under the conditions of our experiments, it was found that 

 labelled calcium was deposited up to seventeen times faster than 

 labelled carbonate. This discrepancy may be the result of very 

 large diflFerences in the amount of exchangeable endogenous car- 

 bon in relation to the amount of calcium available for exchange, 

 the former being very much larger than the latter so that intracel- 

 lular dilution of the absorbed C^^ was much greater than that of Ca"* '. 



5. Several mechanisms linking photosynthesis and calcifica- 

 tion are discussed. CaCOy production may be enhanced: (1) 

 through removal of CO2 from the calcification site by photosyn- 

 thesis and/or carbonic anhydrase; (2) from stimulation of coral 

 metabolism by photosynthesis of the zooxanthellae, which in turn 

 increases the amount of energy available for active calcium and 

 carbonate transport through the tissues into the skeleton. There 

 is no evidence that metabolic efficiency in reef corals is increased 

 by augmenting the oxygen supply over and above that already 

 available from the environment. The zooxanthellae probably exert 

 their effect by speeding up the rate with which metabolic waste 

 products are removed from the vicinity of the host's cells since the 

 algae require as raw material for photosynthesis those very inor- 

 ganic substances that the coral must get rid of. Rapid removal of 

 these from the host cells must set up strong local concentration 

 gradients resulting in a large increase of metabolic efficiency, thus 

 making more free energy available for a CaCO.^ secretion. 



6. Photosynthesis plays a double role vis a vis the reef: it in- 

 creases the free energy of the community through primary produc- 

 tion and it produces in corals and algae the optimum physiological 

 conditions necessary for rapid and efficient secretion of calcium car- 



