390 Marine Microbiology 



SUMMARY 



Azotobacter found in all the investigated regions all over 

 the water column ("Ov and H2S zones") of the Black Sea, are 

 distributed microzonally. The number of Azotobacter in water 

 depends upon the season of a year. In the open sea the numbers of 

 cells and microcolonies of Azotobacter, per 10,000 cells of phyto- 

 plankton, were hundreds and thousands times more in summer 

 than in winter. A direct dependence is displayed between the 

 content of Azotobacter in sea water and that of the large forms 

 of phytoplankton, but not between the former and the general 

 quantity of phytoplankton. Apparently, the main source of car- 

 bon for Azotobacter in sea water are the moribund cells of large 

 forms of phytoplankton and particles of indigested vegetable 

 food of zooplankton, usually more abundant in summer. The 

 presence of Azotobacter in water of the "H2S- zone" is caused, 

 probably, by the sedimentation of vegetable detritus carrying 

 down Azotobacter. Besides Azotobacter, in shallow waters Clos- 

 tridium has also been found, though it has not been found in 

 water of the deep-water regions. In muds and on PhyUophora the 

 number of Azotobacter is greater than in water. Muds from dif- 

 ferent depths always contained Clostridium in greater numbers 

 than Azotobacter. Wind disturbances on the sediments influence 

 the number of Azotobacter and Clostridium in shallow waters. 

 In water of all the regions of the Black Sea, nitrogen fixing 

 nonsporeforming rods and Spirilla have been found. They domi- 

 nated quantitatively over Azotobacter. The average level of the 

 productivity of nitrogen fixation by Azotobacter was lower than 

 that of Spirillum, and it was approximately the same as in non- 

 sporeforming pigmented rods. Some other microorganisms (non- 

 pigmented nonsporeforming rods, Clostridium and yeast) have 

 shown lower nitrogen fixing ability. In the muds of the Black 

 Sea Azotobacter and Clostridium apparently play a considerable 

 role in the process of nitrogen fixation and Azotobacter,— on the 

 surface of macrophytes too. However, in sea water the leading 

 role belongs, apparently, to some other organisms, and the role of 

 Clostridium in the deep-water regions probably falls off com- 

 pletely. 



