386 Marine Microbiology 



summer seasons. The maximum number of large forms of phyto- 

 plankton in summer reaches 6000 cells in 1 ml of water. Their 

 biomass forms about 70 per cent of the biomass of all summer 

 forms of phytoplankton mainly at tJie expense of warm- water 

 species: Rhizosolenia calcar-avis and Ceratium tripos (6). Be- 

 sides, more abundant nourishment of zooplankton in summer in 

 the open sea leads to the increase of the content of the remains 

 of poorly digested vegetable food in sea water (1, 2, 5). The 

 maximum numbers of Azotobacter in sea water in summer often 

 reach more than 2380 cells and microcolonies per 1 liter. The 

 comparison of the data on the quantitative distribution of Azoto- 

 bacter and phytoplankton gives an opportunity to suppose that 

 moribund cells of large forms of phytoplankton and large pieces 

 of indigested vegetable food of zooplankton are the main source 

 of carbon for Azotobacter in sea water. If one proceeds from this 

 premise, then the vertical distribution of Azotobacter in water 

 of the deep-water regions of the sea at different seasons is ex- 

 plained rather easily. Vegetable detritus settling at different 

 speeds can carry Azotobacter to deeper layers of water and to the 

 bottom. Apparently the occurrence of Azotobacter all over the 

 water column (including the "H2S-zone") is due to this reason. 

 There as the observations show, it can keep its vitality. 



The microscopy of the 640 initial cultures (series of flasks 

 with the Fiodorov medium, inoculated with different volumes of 

 sea water of each sample) has shown that from 160 samples of 

 the water, Azotobacter has been found in 122 samples, and micro- 

 organisms different from Azotobacter and Clostridium (more 

 often nonsporeforming rods and Spirilla) in 148 samples. The 

 latter two forms prevailed over Azotobacter quantitatively both 

 in natural conditions in sea water, according to the data of the 

 method of titres, and in the Azotobacter medium inoculated with 

 sea water (the results of microscopy of the inoculations). They 

 were isolated in cultures much easier than Azotobacter . One is 

 left with the impression that in sea water, Azotobacter has serious 

 competitors. This is confirmed by the fact that in most of the 

 inoculations containing Azotobacter (83 samples of water from 

 122) cells of unhealthy or dead Azotobacter in appearance pre- 

 dominated, while in the same inoculations, nonsporeforming rods 



