196 BRITISH FISHERIES 



compounds. Then the nitrifying bacteria come 

 into action and resolve these organic nitrogenous 

 substances into, first of all, ammonia, then nitrous 

 acid, and finally nitric acid.^ But another set of 

 bacteria, the denitrifying bacteria, also exist in the 

 sea, and these reverse the action of the nitrifying 

 bacteria, that is, they convert nitric acid into 

 nitrous, nitrous acid into ammonia, and ammonia 

 into free nitrogen. The latter is then returned to 

 the atmosphere. It is probable that a considerable 

 degree of denitrification takes place in the sea. 

 But the denitrifying bacteria are more active in 

 the warm tropical seas than in the colder polar 

 waters. Therefore more of the nitrogenous 

 compounds are broken down there, and there is 

 less food for diatoms and other organisms which 

 feed in the manner of plants. The plankton is 

 therefore scarcer.^ 



Apparently, then, the atmosphere should be 

 becoming richer in nitrogen because of this 

 denitrification. But leguminous plants (peas, 

 beans, etc.) have the power of taking free nitrogen 

 from the air and utilising this as food. Every 



^ Carbonic acid and water are, of course, also formed in this 

 process, and oxygen is taken up from its solution in the surrounding 

 sea-water. 



^ Apstein {Das Siisswasserplankton, Kiel, 1896) and Brandt 

 (Stoffiwechsel im Meere) showed that, in the fresh-water lakes of 

 Holstein, the plankton was more abundant where there was a large 

 proportion of nitrates than where the proportion of these substances 

 was relatively small. 



