ZoBeU 



^94 



Marine Microbiology 



There is no evidence that either numbers or kinds of bacteria in bottom 

 deposits are influenced by seasonal cycles except near shore or in very 

 shallow water where bottom conditions are influenced by turbulence, 

 storms, run-off, insolation, etc. In deep water, where environmental con- 

 ditions are monotonously constant, one might expect no detectable sea- 

 sonal fluctuations in the microbial population, but in shallow water, which 

 is subject to cyclic changes caused by meteorological conditions and sea- 

 sonal cycles in organic productivity, one might anticipate and does find 

 seasonal changes in the bottom flora. This is especially true of lakes in 

 the temperate zone which undergo a seasonal overturn. Even here, 

 though, seasonal changes in the bottom flora are not as pronounced as are 

 changes in the microbial population of the overlying water. 



Relatively small numbers of bacteria are found in coarse sand which 

 occurs along the coast and in shallow water. However, the bacterial pop- 

 ulation is more closely related to the character of the sediments than to 

 their distance from land, because, as a rule, sand contains fewer bacteria 

 than sediments consisting chiefly of smaller particles regardless of their 

 topographical location. Table XXVIII shows the average number of 

 bacteria found in different types of bottom deposits. The particle size 

 and nitrogen content of the sediments is adapted from data by Trask 

 (1932). The water content was determined by drying the samples for 

 24 hours at 105° C. 



Table XXVIII. — -Average nitrogen, water, and bacteria content of sediment samples from 

 the Channel Island region (from ZoBell, igjSa): — 



The greater abundance of bacteria found in finer sediments is attrib- 

 uted primarily to the higher organic content, although a multiplicity of 

 other interrelated factors are involved. At the mud-water interface, the 

 sorting action in sedimentation tends to segregate particles of similar size, 

 so the tendency is for any free-floating bacteria to be deposited with other 

 particles of colloidal dimensions. Likewise bottom-dwelling animals which 

 ingest bacteria are more abundant in coarse sediments, such as sand or 

 silt, than in clay. Finally, small particles offer much more surface area 

 than larger particles, and solid surfaces enhance the multiplication and 

 survival of bacteria (ZoBell, 1943Z)). 



According to Reuszer (1933), the distribution of bacteria in marine 

 mud is directly correlated with the organic content. In the topmost lay- 

 ers at the mud-water interface it was found that the bacterial population 

 was more dependent upon the degree of decomposition of the organic 

 matter than upon the total quantity of organic matter present. Much of 

 the organic content of bottom deposits consists of material which is fairly 

 refractory to bacterial decomposition, being designated as "marine 

 humus" by Waksman (1933). Marine humus is more resistant to attack 

 than freshly deposited plant and animal residues (Waksman and Hotch- 

 Kiss, 1937), but in the presence of free oxygen it is slowly decomposed, as 

 shown experimentally by the liberation of carbon dioxide and ammonia. 



