Chapter V — 79 — Distribution in the Sea 



2,270:1 for bacteria in phytoplankton tows (Waksman et al., 1933c). 



On the sea floor the direct influence of animals is more pronounced than 

 that of plants. Dead animals, like dead plants, provide food for sapro- 

 phytic bacteria. Large numbers of bacteria are also often associated with 

 living marine animals, but it is doubtful whether this association materi- 

 ally influences the distribution of bacteria in the sea except in localized 

 regions. Many kinds of marine animals ingest and digest bacteria as a 

 source of food (page 173). Such animals may tend to restrict the bac- 

 terial population of the sea within certain limits. The interrelations of 

 bottom fauna and bacteria have been investigated by Mare (1942). 



Animals which promote the precipitation and sedimentation of bac- 

 teria probably play an important role in restricting the bacterial popula- 

 tion. Lamellibranch moUusks, certain tunicates, sponges, coelenterates, 

 and other ciliated mucus feeders filter large quantities of water, removing 

 from it suspended material including bacteria. Sea mussels of average 

 size studied by Fox et al. (1937) propelled from 2.2 to 2.9 liters of water 

 per hour through their gill chambers removing all suspended matter. 

 Some of the material removed from the water is swallowed by the mussel 

 and the rest is rejected as strands of pseudofeces. Fox and CoE (1943) 

 have demonstrated the ability of the California mussel to remove from 

 sea water very fine, uncentrifugable, colloidal material such as Congo 

 Red or boiled ''soluble starch" particles which are considerably smaller 

 than bacteria. In experiments conducted by ZoBell and Landon (1937), 

 the California mussel reduced the bacterial population from 200 million 

 bacteria per ml. of sea water to less than 10,000 per ml. in two hours. 

 DoDGSON (1928) has reviewed the earlier literature on the importance of 

 mussels, oysters, and other animals in the purification of water. 



The early literature on the role of predatory protozoans and other 

 bacteria-feeders upon the self-purification of natural water has been 

 reviewed by Muller (191 26), Prescott and Winslow (1931), and by 

 Baler (1935). There is clear-cut evidence that in polluted water which 

 may contain millions to billions of bacteria per ml., protozoans and 

 micro-crustaceans thrive at the expense of the bacterial population. This 

 is illustrated by the experiments of Purdy and Butterfield (1918) in 

 which the bacterial population of polluted water containing no Para- 

 moecia remained fairly constant, but when Paramoecia were present the 

 bacterial population decreased while the Paramoecia increased. 



A stage is reached when there are no longer enough bacteria present to 

 provide a livelihood for the predatory animals. A survey of the literature 

 fails to reveal any instance in which protozoans or other bacterivorous 

 organisms have reduced the bacterial population below several thousand 

 per ml. as determined by plating procedures. Since the bacterial popula- 

 tion of sea water is generally in the order of magnitude of hundreds per 

 ml., it is reasonable to conclude that bacterivorous animals are not a major 

 factor in restricting the number of bacteria found in the open sea. In 

 polluted water or bottom deposits, however, which support large bacterial 

 populations, bacteria may constitute an important part of the dietary of 

 small animals, and the predatory activities of such animals may be an 

 important ecological factor. 



Waksman (1937) relates that the significance attached to protozoans 

 as a factor controlling the microbial population of the soil has been consid- 

 erably modified in recent years. In many cases the protozoans are actu- 

 aUy beneficial, as shown by the observations of Cutler and Crump (1929), 



