Chapter V — 87 — Distribution in the Sea 



In their "Elements of Water Bacteriology," Prescott and Winslow 

 (1931) state that chief among the factors influencing the diminution of 

 bacteria in surface waters appear to be sedimentation, the activity of 

 other microorganisms, light, temperature, food supply, and perhaps more 

 obscure conditions such as osmotic pressure. 



Baier (1935), RuBENTSCHiK ct al. (1936), Taylor (1940), and others 

 Stress the importance of sedimentation as a factor which limits the mi- 

 crobial population of lakes, reservoirs, and bays. 



Sedimentation is not the prime cause of the paucity of bacteria in the 

 open ocean, but it helps to explain the vertical distribution of bacteria. 

 Here sedimentation occurs more slowly due to a dearth of suspended par- 

 ticles. Most of the suspended particles in the open ocean are diatoms, 

 dinoflagellates, and other plankton organisms equipped with special flota- 

 tion adaptations which retard or prevent sinking. It is with such solid 

 particles that the bacteria in the open ocean are associated. Following 

 the death of the supporting plankton organisms, the organic tissues of the 

 latter are gradually decomposed, leaving skeletal remains of greater den- 

 sity. As the density increases and the flotation mechanisms disintegrate, 

 the skeletal remains sink faster and faster towards the bottom. The rate 

 of sinking of such organisms or their remains at different depths probably 

 approximates the reciprocal of the curves which represent their abundance 

 at different depths (see Figure 7 on page 67), assuming that the organ- 

 isms are not completely decomposed. 



Of course, large numbers of plankton organisms are devoured by pred- 

 atory animals and some may be completely decomposed by bacteria. 

 However, the remains of numerous plankton organisms sink to the sea 

 bottom, carrying with them attached bacteria. The numbers of bacteria 

 found in the sea at different depths are directly proportional to the quan- 

 tity of particulate material, regardless of whether the latter is living or 

 dead. The paucity of particulate matter at depths exceeding 200 meters 

 is believed to be ascribable primarily to the accelerated rate of sedimenta- 

 tion when the remains of organisms reach this depth. A similar view is 

 expressed by Lloyd (1930) who observed a progressive decrease in the 

 bacterial population with depth in the Clyde Sea, paralleling the sinking 

 of suspended matter. 



Effect of organic matter : — The concentration of organic nutrients in 

 sea water is very low, being near or below the threshold of the require- 

 ments for many kinds of bacteria. According to Krogh (1934), sea water 

 contains from 4 to 5 mgm. of total organic matter per liter, or, about 

 1/5000 as much as ordinary garden soil. An appreciable portion of the 

 organic content of sea water is highly refractory to attack by bacteria. 

 Keys et al. (1935) estimated that only 10 to 15 per cent of the total or- 

 ganic content of sea water is utilized by bacteria after storage for several 

 days at 21° C. Waksman and Carey (19356) found that 50 per cent of 

 the total organic content of sea water is readily subject to decomposition 

 by bacteria, the rest being very resistant to bacterial attack. About 

 60 per cent of the organic matter decomposed by bacteria is completely 

 oxidized to carbon dioxide and water, and 40 per cent is converted into 

 bacterial cell substance or intermediate products of metabolism. S.milar 

 results have been reported by Waksman and Renn (1936). 



That organic matter is a factor which limits the multiplication of bac- 

 teria in sea water has been established by the experiments of Waksman 



