CHAPTER III. 



THE METABOLISM OF THE SULPHUR BACTERIA. 



The Food Requirements and Sources of Energy of the 

 Sulphur Bacteria. 



General Considerations. — The physiological status of any 

 microbe is determined when its nitrogen and its carbon sources 

 of supply are known. The protoplasm of the sulphur bacteria, 

 hke that of all other organisms, is built up of the elements 

 C, H, O, N, S, P, and of these, definite provision must be made 

 in all preparations of bacterial cultures for the supply of nitro- 

 gen and carbon. Of the other elements, hydrogen and oxygen 

 are present in water as well as in other substances in the 

 nutrient media, and the remainder are required in such 

 small quantities that definite provision for their supply is 

 not necessary. In addition, of course, the supply of mineral 

 salts must be taken into consideration, but as the differences 

 among bacteria in this respect are very small they play no 

 part in making physiological distinctions. 



The sources of energy of an organism are also a matter for 

 consideration. Autotrophic organisms like the green plants 

 must build up the complex protoplasmic molecule from such 

 simple substances as carbon dioxide, mineral salts and water ; 

 and the necessary energy is obtained from the light of the sun. 

 The presence of chlorophyll in green plants enables them to 

 " capture " the energy contained in light, and to utilize it 

 for their metabolic purposes. Thus carbon- assimilation takes 

 place and carbohydrates are built up from the carbon dioxide 

 and water of the atmosphere. So far as is known, the energy 

 of the sun is not directly responsible for the further building 

 up of carbohydrates into the still more complex proteins, the 



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