Carbon Dioxide 259 



In the foregoing review of oxygen we have seen that this hfc- 

 giving material is sufficiently abundant in most major habitats, but 

 that it becomes critically scarce in certain special situations. Most 

 land organisms find enough oxygen in their habitats, and the great 

 majority of marine animals are so adapted that the amount of oxygen 

 in sea water ordinarily meets their requirements, although its con- 

 centration is far less than in the atmosphere. As green plants photo- 

 synthesize and grow, they produce more oxygen than they consume, 

 and hence run short of oxygen only in rare instances. But we have 

 seen that in fresh water and under certain conditions on land and in 

 the sea, available oxygen may become seriously reduced, with the 

 result that whole populations may be killed unless they have means 

 of escape, or are capable of anaerobic existence. The distribution of 

 aerobic plants and animals is sharply curtailed at the margins of 

 habitats permanently devoid of oxygen, and within these areas the 

 relatively few groups of completely anaerobic organisms hold un- 

 disputed domain. 



CARBON DIOXIDE 



A consideration of carbon dioxide as an ecological factor is of vital 

 concern first of all because it is one of the essential ingredients of 

 the photosynthetic reaction. This material thus provides the carbon 

 source necessary for the growth of all green plants and indirectly for 

 all other organisms. Carbon dioxide also influences other features of 

 the environment. Quite in contrast with oxygen, carbon dioxide 

 combines chemically with the water medium itself, and forms car- 

 bonic acid. Through this reaction it influences the hydrogen ion 

 concentration (measured as pH), and it forms compounds with 

 calcium and other elements of ecological importance. Carbon di- 

 oxide affects the respiration of animals and, in combination with 

 calcium, takes part in the formation of their bones and shells. 



When animals and plants respire, organic matter is oxidized and 

 carbon dioxide is produced. This process is the reverse of photo- 

 synthesis as indicated in the equations at the beginning of the chapter. 

 Carbon dioxide is also released when dead organic matter is decom- 

 posed by the oxidizing or fermenting activities of microorganisms. 

 On the other hand, carbon dioxide is withdrawn from the environment 

 by the growth of green plants and by lime secretion. Thus, carbon 

 dioxide, like oxygen, enters the living complex in an absolutely vital 

 way. Since the abundance of carbon dioxide in many situations is 

 strongly affected by living organisms, we may definitely list this ma- 



