70 CHEMICAL ENVIRONMENT 



the production of CO2 by anaerobes as well as by aerobes. 

 Enzymes are probably responsible for this action. A 

 carbohydrate is usually though not always essential for 

 the growth of anaerobes and serves them as the best source 

 of energy. 



Nitrogen Metabolism.— Nitrogen is the characteristic ele- 

 ment of living material. Protoplasm is a colloidal chemical 

 system in unstable equilibrium and nitrogen is responsible for 

 this instability. No other of the commoner elements is 

 brought into combination with such difficulty, nor is so readily 

 liberated when combined (all commercial explosives are 

 nitrogen compounds). Bacteria, like other forms of proto- 

 plasm, require nitrogen. More marked peculiarities are 

 shown by bacteria with reference to the sources from which 

 they derive their nitrogen than for carbon. Some can even 

 combine the free nitrogen of the air and furnish the only 

 natural means of any importance for this reaction. Some few 

 forms (the nitrite and nitrate formers, Chapter XI) obtain 

 their energy from the oxidation of inorganic nitrogen com- 

 pounds, ammonia and nitrites respectively, and not from 

 carbon. These latter bacteria use carbon from carbon dioxide 

 and carbonates as building material and not as a source of 

 energy. A great many bacteria can secure their nitrogen 

 from nitrates but some are restricted to organic nitrogen. 

 Many bacteria obtain their carbon from the same organic 

 compounds from which their nitrogen is derived. 



Sulphur Metabolism.— Sulphur serves mainly as a consti- 

 tuent of protein compounds in the protoplasmic structure. 

 In some of the sulphur bacteria it is a source of energy, since 

 either free sulphur or H2S or various thiosulphates are 

 oxidized by them. Some of these bacteria can obtain their 

 carbon from CO2 or carbonates, for structural purposes, not 

 for oxidation, and their nitrogen from nitrates or ammonium 

 salts. 



Whether the iron bacteria, belonging to the genus Creno- 

 thrix of the higher, thread bacteria, use this element or its 

 compounds as sources of energy is still a disputed question. 

 The evidence is largely in favor of this view. 



Free hydrogen has been shown to be oxidized by some 

 forms which obtain their energy in this way. 



