Chapter XI 



THE NITROGEN CYCLE IN THE SEA 



Considerable interest has centered around nitrogen compounds be- 

 cause available nitrogen often limits the productivity of the sea. The 

 origin, mode of formation, and fate of ammonia, nitrite, and nitrate is a 

 time-honored problem, many aspects of which remain unsolved. Some 

 workers believe that most of the fixed nitrogen available for plant nutri- 

 tion enters the ocean from the atmosphere or from land drainage, whereas 

 others contend that available nitrogen is derived primarily from the de- 

 composition and transformation of nitrogenous compounds in the sea. 

 Only the microbiological aspects of the problem can be reviewed here. 



Ammonia production : — Some ammonia is excreted by animals as a 

 disintegration product of nitrogenous materials, but more ammonia is lib- 

 erated from nitrogenous compounds undergoing bacterial decomposition. 

 The ammonia may be utilized directly by phytoplankton (ZoBell, 1935) 

 or it may be oxidized to nitrite or nitrate. Schreiber (1927) found that 

 there was little to choose between ammonia, nitrate, nitrite, and glycine 

 as sources of nitrogen for bacteria-free cultures of Carteria. Braarud 

 and FoYN (1931) noted that cultures of Chlamydomonas could use 

 glycine, alanine, and asparagine, although amino acids were used less 

 efficiently than either ammonia or nitrate. While amino acids may be 

 utilized directly by certain plants, they are readily decomposed with the 

 liberation of ammonia by bacteria. 



There is ample evidence from the investigations of Waksman and 

 Renn (1936), OsTROFF and Henry (1939), and others that most simple 

 nitrogenous compounds and many complex ones are attacked by marine 

 bacteria. If the nitrogen present exceeds the requirements of the bac- 

 teria, ammonia is usually liberated. However, if carbohydrates or other 

 oxidizable non-nitrogenous carbon compounds are present in excess, the 

 nitrogen from decomposing nitrogenous material may be converted di- 

 rectly into bacterial cell substance and thus not appear immediately as 

 free ammonia (Waksman and Carey, 19356; Waksman and Renn, 1936). 

 In such circumstances, the nitrogen will be liberated as ammonia chiefly 

 after the death and decomposition of the bacteria. 



Under ordinary conditions, ammonia production accompanies the 

 bacterial decomposition of marine organic matter. The increased bac- 

 terial activity which follows the storage of sea water in glass receptacles 

 in the dark results in increased bacterial multiplication and ammonia 

 production (Keys et al., 1935; Waksman and Carey, 1935a; ZoBell and 

 Anderson, 1936a). Mixed net plankton suspended in sea water was 

 found by voN Brand et al. (1937) to be decomposed by bacteria in the 

 dark with the evolution of ammonia. The rapid liberation of ammonia 

 from copepods being decomposed by bacteria in sea water was observed 

 by Waksman et al. (1938). 



The bacterial fermentation of urea is another important source of 

 ammonia in the sea: 



(NH2)2CO -I- H2O = 2 NH3 + CO2 



