262 Oxygen and Carbofi Dioxide 



dustrial sources. The supply is increased by carbon dioxide released 

 from the ocean and from respiration and decay. Carbon dioxide is 

 withdrawn from the atmosphere by the photosynthesis of the vegeta- 

 tion. The existing balance between supply and consumption provides 

 a very low concentration of carbon dioxide in the air, but, if the 

 minute amount of this essential gas were not present, all life on land 

 would come to a stop. The lives of terrestrial organisms thus depend 

 upon a very slender thread. In former geologic epochs the abun- 

 dance of atmospheric carbon dioxide may have been considerably 

 greater or smaller with the consequent possibility of significantly in- 

 creased or decreased photosynthesis. At present we realize that car- 

 bon dioxide is a general limiting factor in the sense that plant growth 

 might go on at a higher pace in the world as a whole if our atmosphere 

 contained more of this gas. However, since carbon dioxide is essen- 

 tially uniform over the earth's surface, it does not cause significant 

 diflFerences in the distribution and growth of the terrestrial vegetation. 



Carbon Dioxide in the Aquatic Environment 



The amount of carbon dioxide present in the medium is crucially 

 important for aquatic organisms just as it is for terrestrial forms, but 

 the circumstances of the occurrence of carbon dioxide in the water are 

 much more complex. Carbon dioxide is readily soluble in water, but 

 since the concentration of carbon dioxide in the atmosphere is so low 

 (0.3 cc per liter), water in equilibrium with it will hold only about 

 0.5 cc per liter free CO2 at 0°C, or 0.2 cc per hter at 24°C, in simple 

 solution. Actually, the total CO2 in most natural waters is consider- 

 ably larger because additional amounts are present in the form of 

 carbonate and bicarbonate ions. Sea water at a salinity of 35%o 

 normally contains a total of about 47 cc CO2 per liter, or roughly 150 

 times the concentration in the air. Taking into account the relative 

 volumes of the ocean and of the atmosphere, it has been estimated 

 that more than 50 times as much CO2 exists in the seas as in the air 

 (Rubey, 1951). The ocean is thus the great reservoir for the world's 

 supply of available CO2 and this marine source tends to regulate the 

 amount in the air— just the reverse of the situation with oxygen. 



Reactions of Carbon Dioxide in Water. Dissolved CO2 combines 

 with H2O to form carbonic acid, and this dissociates as follows: 



CO2 + H2O ^ H2CO3 ^ H+ -f HCO3- ^ H+ + C03= 



The amount of CO2 in simple solution plus that in the form of H2CO3 

 is called the free CO2. The amount of the CO2 in the bicarbonate 



