276 Oxygen and Carbon Dioxide 



7.13) and the extraordinary variety and beauty of the reef community 

 will never be forgotten by anyone who has walked on a reef at low 

 tide, or, better, has explored one with a diving helmet. 



These spectacular coral formations are the result of equilibria in 

 the circulation of Ca"^* and C03= ions that are dependent upon physical 

 chemical, and biological agents. In the words of Redfield (1941): 

 "The calcium cycle involves not only exchanges between land, sea, 

 and the bottom. It includes the consideration of the metabolic cycle, 

 the CO2 cycle, the temperature cycle, and the cycle of movement of 

 water between the surface and the depths. One could not find a 

 better example of the necessity of considering the environment as a 

 whole as a unified system." 



In the foregoing discussion of the ecological influences of oxygen 

 and carbon dioxide we have observed that these "sister" materials, 

 reciprocally involved in photosynthesis and respiration, exhibit a 

 great contrast in the circumstances of their occurrence and in their 

 variability. Oxygen is generously abundant in the atmosphere, 

 whereas the chief reservoir of available carbon dioxide is in the ocean. 

 We have found that in certain habitats either of these substances may 

 be so reduced in amount by the physiological processes of the in- 

 habitants as to curtail the life activities of the organisms themselves. 

 We have also reviewed the particularly intriguing further complica- 

 tions brought about by the fact that carbon dioxide reacts with other 

 materials in the environment to influence the equilibria involving pH 

 and CaCOs. All of these relations with oxygen and carbon dioxide 

 are thus further striking demonstrations of the mutual interde- 

 pendence of the organism and its environment. 



