THE BACTERIA IN NATURAL WATERS. 5 



We may take a comparatively simple excretory prod- 

 uct, urea, as an example. Through the activity of an 

 enzyme produced by certain bacteria this compound 

 unites with two molecules of water and is converted into 

 ammonium carbonate, 



/NH 2 

 C0 



While green plants can derive their necessary nitrogen 

 in part, at least, from ammonium compounds it is a well- 

 established fact that this element may be obtained much 

 more readily from nitrates, and it is, therefore, essential 

 as a further step that some means be employed to oxidize 

 the nitrogen. This process of oxidation is known as 

 nitrification, and takes place in a succession of steps, 

 the organic nitrogen being first converted to the form of 

 ammonium salts, and these in turn to nitrites and nitrates, 

 the oxygen used coming from the air. Several groups 

 of organisms are instrumental in bringing about this con- 

 version. It is generally assumed that one group attacks 

 the ammonium compounds and changes them to nitrites, 

 while another group completes the oxidation to nitrates. 

 In the latter form nitrogen is readily taken up by green 

 plants to be built up into the more complex albuminoid 

 substances (organic nitrogen) through the constructive 

 power of chlorophyll. 



This never-ending cycle is illustrated in the accom- 

 panying figure, devised by Sedgwick (Sedgwick, 1889) 

 to illustrate the transformations of organic nitrogen in 



