4 Elements of Water Bacteriology. 



from nitrates, and there are other bacteria which as a 

 further step oxidize the ammoniacal nitrogen to a more 

 available form. 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 more complex albuminoid sub- 

 stances (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 nature, 

 the increasing size and closeness of the spiral on the left- 

 hand side indicating the progressive complexity of organic 

 matter as built up by the chlorophyll bodies of green 

 plants in the sunlight, and the other half of the figure the 

 reverse process carried out largely by the bacteria. In 

 nature there are many short circuits, as, for instance, when 

 dead organic matter is used as food for animals and built 

 up into the living state again without being nitrified and 

 acted upon by green plants; but the complete cycle of 

 organic nitrogen is as indicated on the diagram. 



