Distribution and Ecology of Azotohacter in the Black Sea 385 



determined by the method of titrcs after careful and repeated 

 microscopy of the inoculations. The Fjodorov liquid medium was 

 used for aerobes and Vinogradsky medium for anaerobes; both 

 contained 50 per cent sea water. The water was inoculated 

 in the following quantities. 100, 10, 1 and 0.1 ml., the sediment 

 and thallus of algae-10; 1; 0.1 and 0.01 g. The method of titres 

 does not give any possibility of estimating the absolute numbers 

 of cells of heterotrophic bacteria (4), though it gives the possi- 

 bility of characterizing the number of cells and microcolonies. 

 The washings from algae were used for a direct microscopic 

 count of the general number of the bacteria. From fresh thallus 

 Phyllophora the impresses of the growth of bacteria in natural 

 conditions were made on slides. From water and sediment on 

 elective media, about 50 strains of Azotohacter, 30 strains of Clos- 

 tridium and more than 100 strains of some other microorganisms 

 have been isolated. Nitrogen fixing ability of the strains was 

 determined by the micromethod of Kjeldahl. 



RESULTS 



It follows from Table 1, that Azotohacter, found in all the 

 regions over the whole water column "O^-and H2S zones" of the 

 Black Sea, is distributed microzonallv. This is explained, ob- 

 viously, as in the case of other heterotrophic bacteria (4) by 

 the microzonal distribution of organic matter in water. The num- 

 ber of Azotohacter in water depends upon the season of a year. 

 The comparative analysis of the distribution of Azotohacter and 

 phytoplankton in water of the deep-water regions of the Black 

 Sea has shown that in winter, in 10,000 cells of phytoplankton 

 only few individuals occur (on the surface horizon, tens) and in 

 summer hundreds to thousands and at a depth of 175-200 m 

 tens of thousands of cells and microcolonies of Azotohacter. 

 Between the general quantitv of phytoplankton and the quantity 

 of Azotohacter there is no direct dependence. Probably most of 

 the vegetating species of phytoplankton are not connected by 

 symbiotic relationships with Azotohacter. However, a direct de- 

 pendence is displayed between the content of Azotohacter in sea 

 water and the content of large forms of phytoplankton. The num- 

 ber of the latter in winter 1956 was considerablv less than in 



