100 PHYSIOLOGICAL ACTIVITIES 



suggested by Armsby as a considerable source of nitrogenous 

 food, and this suggestion appears possible. 



The liberation of nitrogen from nitrates or nitrites, either 

 as free nitrogen or as ammonia, is spoken of as "denitrifica- 

 tion," though this term was formerly applied to such libera- 

 tions from compounds of nitrogen generally, even from pro- 

 teins. 



Certain bacteria may also reduce sulphates and other 

 sulphur compounds to H2S, a phenomenon frequently 

 observed in sewage and likewise of importance in the soil. 

 It is possible that phosphates may be similarly reduced.^ 

 Further and more careful study of the reducing actions of 

 bacteria is needed. 



OXroATION. 



As has been stated in discussing the respiration of bac- 

 teria (Chapter VIII) most of these organisms gain their 

 energy through the oxidation of carbon in various forms, 

 chiefly organic, so that CO2 is a product of the activity of 

 nearly all bacteria. Some few oxidize CO to CO2, others 

 CII4 and other paraffins to CO2 for this purpose. One class 

 of bacteria even oxidizes H in small amounts for its energy 

 and uses the carbon dioxide of the air or traces of organic 

 carbon in the air as a source of carbon for ''building" pur- 

 poses. 



One of the familiar oxidations of organic carbon is that 

 of the acetic acid bacteria in the making of vinegar. These 

 oxidize the alcohol which results from the action of yeast 

 to acetic acid according to the formula CH3CH2OH + 02 = 

 CHsCOOH+HsO (see Fig. 60). 



Of the various phenomena of oxidation due to bacteria, 

 the formation of nitrites and nitrates has the greatest prac- 

 tical importance, since it is by this means that the ammonia 

 which results from the decomposition of animal and vege- 

 table tissue and waste products is again rendered available 

 to green plants as food in the form of nitrates. Practically 

 all the nitrates found in Nature, sometimes in large quanti- 



^ Dr. H. H. Green, of Pretoria, South Africa, has isolated from "cattle 

 dips" a bacterium that reduces arsenates to arsenites. 



