272 BACTERIA IN THE SEA [PART III 



(3) The nitrifying bacteria which are able to oxidise the 

 ammonia resulting from putrefactive decomposition, with the result 

 that the nitrogen of the latter compound appears as nitrite or 

 nitrate. 



(4) The nitrogen-fixing bacteria which can use the elementary- 

 nitrogen of the atmosphere as a source of food, converting this into 

 proteid substance. 



(5) The denitrifying bacteria which reverse the action of the 

 nitrogen-fixing bacteria, and reduce nitrate to nitrite, the latter to 

 ammonia, and ammonia to free nitrogen. 



(6) The remarkable sulphur bacteria, the significance of which 

 has not yet been sufficiently investigated. 



In the next chapter I propose to discuss the effect of these 

 various groups of micro-organisms in bringing about the circulation 

 of food matter in the sea. 



