Chapter XI 



— 153 



The Nitrogen Cycle 



dinoflagellate plankton from Woods Hole and Vineyard Sound gave rise 

 to nitrite when inoculated into ammoniacal sea-water solutions. Copepod 

 tows which Carey collected off the Continental Shelf at a depth of loo 

 meters gave rise to ammonia and traces of nitrite. She concluded that 

 nitritiers occur primarily in bottom deposits, and that, by vertical mixing 

 of water, they may be brought into the plankton layer. 



While there is hydrographic evidence that some ammonia and nitrite 

 is produced on the sea floor, data accumulated in recent years suggest that 

 most ammonilication occurs in the surface layers of water. Rakestraw's 

 (1936) studies on the occurrence of nitrite in the sea indicate that nitrifi- 

 cation likewise is primarily a surface phenomenon confined largely to the 

 topmost 200 meters of water. Since nitrifying bacteria are seldom found 



NITROGEN 

 MGM./L. 

 0.3H 



0.2- 



^m monia 



0.1 



10 



15 2 2 5 



30 35 40 45 

 DAYS 



60 



Fig. II. — Changes in the ammonia, nitrite, and nitrate content of sea water enriched 

 with mixed marine plankton (from von Brand et a!., 1937). 



in the open ocean in the topmost 200 meters of water where most of the 

 nitrite appears to be formed, the problem of nitrification in the sea must 

 be regarded as unsolved in spite of the wealth of information on the sub- 

 ject. It is the present author's behef that, besides the soil-like nitrifier 

 which has been commonly found in bottom deposits and near land, there 

 are other marine nitrifying organisms which have escaped detection. 



In their experiments on the formation of nitrite and nitrate from 

 ammonia liberated from decomposing plankton (Fig. 11), voN Brand 

 et al. (1937) were unable to isolate nitrifying bacteria. The nitrifiers 

 found in marine materials by voN Brand et al. (1942) were inhibited by 

 temperatures as low as 5° C. It will be recalled that over 80 per cent of the 

 sea floor is colder than 5° C. Although there are many shallow, well- 

 oxygenated marine bottoms where nitrifiers analogous to soil Nitrosomonas 

 could be active according to modern concepts of their cultural require- 

 ments, in many bottoms where nitrifiers have been demonstrated, the low 

 temperature, low oxygen tension, low oxidation-reduction potential, and 

 relatively high organic content would be expected to retard or inhibit 

 nitrification. 



Nitrifying bacteria indistinguishable from the specialized nitrite- 

 forming flora of soils are commonly found in fresh-water lakes. Such 

 bacteria were found in mud from Lake Mendota by Williams and McCoy 

 (1935)- 



Oxidation of nitrite to nitrate: — According to Cooper (1937&), the 

 oxidation of nitrite to nitrate is accompanied by a decrease of free energy, 

 A F°298, of 18,000 gram calories. Like the oxidation of ammonia, there- 

 fore, the reaction requires only activation: 



NO2- -f- 1/2 OaCgas) = NO3- -t- 18,000 cal. 



