86 



Marine Microbiology 

 THE NITROGEN CYCLE 



nitrogen 

 fixation 



denitrification 



nitrate reduction 



• NO: 



=:N0, 



nnineralization assimilation 

 organic N 



Fig. 1. The nitrogen cycle. 



by blue-green algae of the genus Calothrix. Close examination of 

 several intertidal zone areas has shown that Calothrix is also' 

 abundant on rocks at lower levels, but is obscured by the growth 

 of macroalgae found there. Calothrix also appear to be fairly 

 abundant in plankton tows made in inshore waters. 



Terrestrial and fresh-water members of this genus are known 

 to be active fixers of nitrogen. The same has been found to be 

 true for marine isolates, as shown in Table 1. These figures were 

 obtained with algae grown in the synthetic sea water medium 

 given in Table 2. The appearance of a typical nitrogen fixing 

 marine Calothrix is shown in Figure 2. 



In the course of a routine screening of Northern California 

 sea water samples to determine the number of nitrogen fixing 

 organisms therein, pink yeasts of the genus Rhoclotonda fre- 

 quently developed on the plates of "nitrogen-free" media that 

 were used. Since agar always contains some nitrogen, little at- 

 tention was paid to these until similar yeasts were isolated from 

 water samples taken in the Gulf of California at Guaymas and 

 near Friday Harbor, Washington, and shown to grow in strict!)- 

 nitrogen-free liquid media. There is at present, strong presumptive 



