Significance of Microorganisms of Upper Sediment Layer 713 



soluble compounds (without sugars and cellulose) from 18 to 

 37.7 per cent and tlie lignin - humus complex, from 47 to 63 per 

 cent. The composition or organic substance of the North Caspian 

 sediments is very close to that of tlie Azov Sea and Taganrog 

 Bay. 



The composition of organic substance of sediments is such 

 that its decomposition may take place mostly with the help of bac- 

 teria. Therefore we inevitably deal with bacterial processes when 

 considering the process of regeneration of bio-microelements 

 from sediments. 



The uppermost silt layer situated at the sediment water 

 interface is most active with respect to the processes in question. 

 As far as the rate of regeneration of bio-microelements in the silt 

 layer is concerned, it should be noted that the upper layer con- 

 sumes from 1.5 to three times more oxygen than those situated 

 somewhat lower. Such an upper layer does not exceed, as a rule, 

 1-1.5 cm and is formed in tlie Azov Sea in fifteen to thirty years. 



The sediment layer bordering on water is most active also 

 with respect to bacterial processes. The number of bacteria in 

 silt distinctly decreases with depth but at a depth of 1 m it be- 

 comes stable and does not exceed 100-200 million per g of wet 

 silt. 



The most distinct reduction in number of bacterial cells is 

 observed at a depth of 25-30 cm from the sediment surface. Tlie 

 number of cells amounts to 1 x 10^ per g on the interface with 

 water, whereas it does not exceed 4 x 10^ at a depth of 30 cm. The 

 accumulation of bacteria on the sediment surface is often so great 

 that it forms a dense bacterial film. In 1938, Butke\ich, 

 noted a dense film of bacterial origin in tlie North Caspian Sea. 

 The film was of protective value for the water column; Butkevich 

 thought that it absorbed the gases HjS, CO.., N2, CH-,, H- which 

 were formed in the silt. The recent investigation carried out in 

 the North Caspian Sea in 1951-1952 (9) showed that despite the 

 absence of dense bacterial film, the upper sediment layer is \evy 

 rich in bacteria and contains representatixcs of such physiological 

 groups as those that oxidize methane and hydrogen, as well as 

 those of putrefactive and sulphurating type. The latter two are of 

 considerable importance for biogen regeneration; puti-ef active 



