CHAPTER VII 

 Bacteria Reducing Nitrates and Sulfates 



General classification of nitrate reducing bacteria. A large number of 

 organisms, including numerous bacteria and actinomyces, fungi, 

 yeasts and higher plants, but especially the first two groups, are capable 

 of reducing nitrates to nitrites; this often serves as the first step in the 

 process of assimilation of nitrate nitrogen. Some organisms, chiefly 

 fungi and certain bacteria, but also higher plants, are capable of reduc- 

 ing the nitrate to ammonia. However, only specific bacteria are cap- 

 able, under certain conditions, of reducing the nitrate and the nitrite 

 to elementary nitrogen and oxides of nitrogen, in which form the nitro- 

 gen escapes into the atmosphere. Under anaerobic conditions, the 

 nitrate and nitrite may serve as sources of oxygen for these bacteria, 

 which enables them to oxidize the available sources of energy. 1 The 

 last process is usually referred to as complete or direct denitrification and 

 the bacteria concerned in this process are spoken of as denitrifying 

 bacteria. These bacteria can be further subdivided into (a) those which 

 use as a source of energy inorganic substances, notably sulfur, and (6) 

 those that use organic carbon compounds as sources of energy. Com- 

 plete denitrification is generally favored by the presence of nitrate, 

 suitable sources of energy (usually carbon compounds), absence of free 

 oxygen and proper reaction. 



The bacteria, which reduce nitrates only to nitrites or to ammonia, 

 but not to nitrogen gas (elementary form and oxides), may be best 

 spoken of as nitrate reducing bacteria, reserving the term denitrifying 

 bacteria for the other organisms. 



Organisms reducing nitrates to nitrites. The reduction of nitrates in 

 the soil has been demonstrated in the first part of the 19th century. 



1 Weissenberg, H. Studien uber Denitrifikation. Arch. Hyg. 30:279-290. 

 1897; Jensen, H. Das Verhaltnis der denitrifizierenden Bakterien zu einigen 

 Kohlenstoffverbindungen. Centrbl. Bakt. II, 3: 622-627, 689-698. 1897; Bei- 

 trage zur Morphologie und Biologie der Denitrifikationsbakterien. Ibid. 4: 

 401-411, 449-460. 1898; Pakes, W. C. C, and Jollyman, W. H. The collection 

 and examination of the gases produced by bacteria from certain media. Jour. 

 Chem. Soc. I, 79: 322-329. 1901. 



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