CHAPTER XII 



THE CIRCULATION OF CARBON DIOXIDE IN NATURE 



1. Introduction ; Organized Ferments and Enzymes ; Races of Ferment 

 Organisms ; Fermentation of Alcohol and of Acids ; Optical Activity 

 and Fermentation. 



The ultimate source, from which directly or indirectly all organisms 

 draw their carbon, is the carbonic acid of the atmosphere. In the circula- 

 tion of this gas bacteria play a not less important part than they do in the 

 circulation of nitrogen. Animals, it is well known, cannot assimilate C0 2 ; 

 they are dependent for their carbon entirely upon plants. Among these again 

 only the chlorophyll-bearing forms and the red, brown, and blue-green algae 

 are able, with the energy of the absorbed sunlight, to fix the atmospheric 

 CO a *. From it the plant builds up the organic compounds, with or without 

 nitrogen, that constitute the physical basis of all life upon the earth. 

 Since this assimilating and fixing of carbonic acid is going on without 

 ceasing, there is an evident necessity, if terrestrial life is to endure, for the 

 continual return of carbon in the form of C0 2 to the atmosphere, and such 

 restitution does as a matter of fact take place. A certain amount is given 

 back at once by the plants themselves in the process of respiration, and 

 another portion is being continually poured out into the air in the breath 

 of man and animals. All the remaining carbon fixed in the tissues of 

 plants and animals is finally liberated at the death of the organism by the 

 activity of bacteria and other microbes. Those tissue elements which 

 contain nitrogen as well as carbon undergo putrefaction, and the carbon 

 escapes as C0 2 . The innumerable non-nitrogenous constituents on the 

 other hand — carbohydrates, glucosides, alcohols, organic acids and fats — are 



* The prototrophic saltpetre bacteria constitnte the sole and only exception to this rule. 



