78 CHEMICAL ENVIRONMENT 



through a shifting or rearrangement of the atoms in the 

 compounds used as food the oxidation of carbon is brought 

 about. Enzymes are probably responsible for this action. 

 Carbon dioxide is produced by anaerobes as well as aerobes, 

 and frequently in amounts readily collected. A carbohy- 

 drate is usually though not always essential for the growth 

 of anaerobes and serves them as the best source of energy. 



Nitrogen is the characteristic element of living material. 

 Protoplasm is a chemical substance 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). Bac- 

 teria, like other forms of protoplasm, 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 oxida- 

 tion of inorganic nitrogen compounds, ammonia and nitrites 

 respectively, and not from carbon. These latter bacteria 

 use carbon from carbon dioxide and carbonates. 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 serves mainly to supply this element in the 

 protoplasmic structure. In some of the sulphur bacteria it 

 is a source of energy, since either free sulphur or H2S is 

 oxidized by them. Some of these bacteria can obtain their 

 carbon from CO2 or carbonates, 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. 



