v] THE SOURCES OF FOOD 137 



economic products that is taken from the North Sea 

 in the same time is only about 30 millions of kilograms. 

 The excess must therefore be removed for the sea 

 does not appear to be getting richer in these 

 compounds. No deposits containing nitrogen are 

 formed in the same way as calcareous deposits are 

 laid down, but it has been suggested that ammonia 

 is given off from the surface of the sea into the 

 atmosphere, and that it is then redeposited on the 

 land in the rainfall. But this is unlikely and it is far 

 more probable that the excess is removed from the 

 sea by the agency of bacteria. 



Denitrifying bacteria have long been known from 

 cultivated land, and they have also been discovered 

 in the sea near to the land. They are organisms 

 which appear to live under a variety of conditions : 

 some requiring simple carbonaceous food while 

 others must receive more complex substances. Some 

 of them can act in the absence of oxygen while others 

 require this gas. They obtain their nitrogen food 

 from nitrates, nitrites and ammonia, and their life- 

 process with respect to these substances is one of 

 deoxidation, for they can reduce the nitrate to nitrite, 

 the nitrite to ammonia, and the latter substance to 

 free nitrogen. The free nitrogen so formed passes 

 into solution in the sea- water, but the latter already 

 contains as much as it can hold, so the nitrogen then 

 passes back again into the atmosphere. Now it has 



