THE BACTERIA AND VIRUSES 



361 



Green Plant 



>-Plant Protein 



A 



Leguminous Plants 

 Root-nodule bacteria 



->.Animal Protein 



Excretory 

 products 



Humus 



Nitrates • 



o 



Nitrites 



Denifrifyine 

 -^Bacter,a-^-*'^'TROGEN 



Azotobacter 

 Clostridium 



/V/ 



tro 



So 



On^ 



Saprophytic 

 Bacteria 



■* ^*^ Ammonia' 

 Fig. 349. — Diagram illustrating the circulation of Nitrogen in nature. 



have obtained in very early geological ages, when the atmosphere was so 

 densely charged with water vapour as to be opaque, and before other 

 organisms had appeared. The amount of energy obtained in this way does 

 not compare with the amounts available from sunlight, so that photo- 

 synthetic organisms have a very great advantage over chemosynthetic ones. 

 Moreover, photosynthesis represents a general gain of energy from outside 

 this planet, while chemosynthesis is a mere exchange of energy between 

 existing terrestrial systems. The soil Bacteria mentioned above form a very 

 important section of this group, though the energy outcome of the reactions 

 involved is not large. The energy release in the oxidation of potassium nitrite 

 to nitrate, for example, is only 22 calories per gram-molecule of nitrite 

 oxidized. 



Other reactions which occur are the oxidation of Hydrogen to water, of 

 methane and of Carbon monoxide to Carbon dioxide (especially in bogs) 

 and of Hydrogen sulphide to water. 



The latter is characteristic of the colourless Sulphur Bacteria, abundant in 

 natural waters, which play an important part in the purification of sewage. 

 The Sulphur is usually deposited in granules within the cells, but some of 

 it is further oxidized to sulphuric acid. This process is much more energetic 

 than nitrification, the yield being 62 calories per gram-molecule of Hydrogen 

 sulphide and 141 calories per gram-molecule of Sulphur oxidized. 



Viruses * 



It has long been known that some types of " variegation," that is to say, 

 the yellow mottling seen on the leaves of many plants, could be communicated 



* A further treatment of Virus Diseases will be found in Volume IV. 



