ASSIMILATION OF CARBON BY PLANTS 183 



trophic, by respiration. Between photosynthesis, or the synthe- 

 sis of organic substances using the energy of Hght, and respiration, 

 i.e., the catabohc processes that have been emphasized in relation 

 to the higher plants, the contrast is not so distinct in the nitrifying 

 bacteria. The only means of providing the energy necessary 

 for their metabolism is the oxidation of substances that are not 

 constituents of their protoplasm but such reduced inorganic 

 substances as ammonia and nitrous acid. 



The nitrifying bacteria are very important in the general 

 cycle of nitrogen in nature. These bacteria are present in all 

 soils, except those that are waterlogged. Almost all of the 

 nitrates that exist in natural conditions are the product of their 

 activity. A peculiarity of these organisms is the strictly coordi- 

 nated work of both groups of the nitrite and the nitrate bacteria, 

 which are so closely interdependent that it was only with diffi- 

 culty that Vinogradsky succeeded in separating them. 



Vinogradsky also discovered the physiology of another group 

 of organisms that obtain their energy from the oxidation of 

 inorganic substances, the sulphur bacteria. The oxidation of 

 hydrogen sulphide by these organisms proceeds according to the 

 following equation: 



H2S -h 2O2 = H2SO4 + 115 cal. 



Just as in the case of nitrifying bacteria, the energy released 

 is used for the various vital processes, and particularly for the 

 decomposition of carbon dioxide. These bacteria therefore do 

 not require organic substances for their nutrition. However, 

 they differ from the nitrifying bacteria in that they are not 

 injured by organic compounds in the nutrient medium. 



The sulphur bacteria represent a fairly diverse group of 

 organisms. They contain single and filamentous forms. Some 

 of them possess a purplish-red pigment, bacteriopurpurin, and 

 have been given the name of ''purple bacteria." Engleman 

 suggested that this pigment substitutes for chlorophyll and 

 participates in the photodecomposition of carbon dioxide. 

 Light, which generally suppresses the development of bacteria, 

 exerts a favorable influence upon the purple bacteria. The 

 physiology of the purple bacteria is not quite clear as yet. 

 According to recent investigations, the reduction of CO2 is closely 

 interrelated with the oxidation of H2S according to the equation 



