METABOLISM IN THE SEA 195 



nitrogen from the land into the sea every year.^ 

 Animals are unable to use these inorganic nitro- 

 genous substances as food, but the plants can. By 

 the agency of the latter the salts of nitric and 

 nitrous acids and of ammonia are converted into 

 the protoplasmic material of the plant tissue — that 

 is, into a form which can be assimilated by 

 animals. There is thus a perpetual cycle of 

 changes — the nitrogen is taken from the sea in the 

 form of fish food, and it is then assimilated and 

 built up to form the substance of the living body. 

 Then either after death, or during life in the 

 ordinary processes of excretion, this living proto- 

 plasmic substance breaks down and decomposes, 

 and is resolved by putrefactive and other bacteria 

 into simple nitrogen compounds. These find 

 their way into the sea through drains and rivers, 

 and are utilised as food by plants.^ The plants are 

 then eaten, directly or indirectly, by marine 

 animals, among which may be the fishes, and the 

 latter may then be returned to the land. 



An important part is performed by the bacteria 

 of the sea in this cycle of changes. One class of 

 microbes — the putrefactive bacteria — have the power 

 of decomposing organic nitrogenous substance, 

 and by their action the dead body of a fish, 

 for instance, is resolved into simpler organic 



1 Brandt, loc. cit., p. 217. 



2 Diatoms and unicellular plants chiefly, but also by some 

 unicellular animals which feed like plants. 



