GENERAL FOOD RELATIONSHIPS— METABOLISM 69 



Fermentation, next chapter). But there is no doubt of 

 its formation and it has been determined by a few investi- 

 gators. On account of this use of carbon, bacteria require 

 relatively large amounts of this element. One group of 

 bacteria concerned in the spontaneous heating of coal 

 seems to be able to use free carbon from this material both 

 for building purposes and to furnish energy. Another 

 group is said to be able to oxidize marsh gas, CH4, and derive 

 energy from this oxidation and also use this gas as a source 

 of carbon for building purposes. The nitrite, nitrate and 

 sulphur bacteria mentioned later utilize carbon dioxide 

 and carbonates as their carbon supply for building purposes 

 only. Their energy is derived from the oxidation of, nitro- 

 gen or sulphur compounds respectively. Thus these latter 

 elements may be called the "respiratory elements" of these 

 classes of bacteria. One kind of bacterium has been 

 described which uses carbon monoxide, oxidizing a part to 

 furnish energy and reserving a part as a source of carbon 

 for building. With the exception of these few kinds of 

 bacteria which are able to use carbon in the relatively simple 

 inorganic forms just mentioned, bacteria are dependent 

 on carbon from organic compounds both for respiratory and 

 for building purposes and cannot use CO2 for the latter 

 purpose as green plants do. 



Oxygen Metabolism.— The oxygen requirement is high 

 partly on account of its use in respiration. The energy which 

 bacteria require in their life processes is, so far as known, all 

 derived from oxidation. Oxygen is also largely used for build- 

 ing purposes, since it is one of the elements necessary in the 

 formation of most of the compounds present in the bacterial 

 cell, as has been pointed out in Chapter VII. Aerobic bac- 

 teria are compelled to obtain part of their oxygen from the 

 surrounding atmosphere, though they do obtain part from 

 combination. The free oxygen is probably used for oxidative 

 purposes to furnish energy. Anaerobic bacteria are depen- 

 dent on so-called "molecular respiration" for their energy. 

 That is, through a shifting or rearrangement of the atoms 

 in the compounds used as food, some of the elements are 

 oxidized by the oxygen present. One evidence of this is 



