ZoBeU 



— 80 



Marine Microbiology 



Meiklejohn (1930), and others. Although protozoans consume appre- 

 ciable numbers of bacteria, Cutler and Crump (1935) believe that the 

 activities of protozoans in the soil keep the bacteria at a level of maxi- 

 mum efficiency. 



The significance of the relationship between the fauna of lakes and the 

 bacterial population is emphasized by Baier (1935) who has reviewed 

 the rather extensive literature on the importance of bacteria as food for 

 animals. Baier's appraisal of the relative role played by bacteria as food 

 for different classes of animals in different types of lakes is summarized 

 in Table XXII. 



Table XXII. — Relative imparlance of bacleria as food for different classes of predacious 

 animals in different kinds of lakes (from Baier, 1935): — 



Further details on the relationship of marine bacteria to flora and fauna 

 are given in Chapter XIV. 



The antagonistic effects of microorganisms : — Bacteria and allied 

 microorganisms indigenous to the marine environment help to make the 

 sea uninhabitable to exotic or adventitious species, and the indigenous 

 species may have some antagonistic effect on each other. The chief way 

 in which this may occur is by alteration of the food supply or by reduction 

 of its concentration to levels below the minimal requirements of other 

 organisms. Certain microorganisms produce specific toxic substances 

 which inhibit the growth of other organisms in the immediate vicinity or 

 destroy them. There are several other ways in which microorganisms are 

 mutually antagonistic, a relationship which is designated as antibiosis in 

 contradistinction to symbiosis. 



Waksman (1937) writes: ''When two organisms are capable of utiliz- 

 ing the same nutrients, but are differently affected by environmental con- 

 ditions (reaction, air supply, temperature), the one organism that finds 

 conditions more suitable develops more rapidly and thus depresses the 

 other." This, together with the production of toxic substances, explains 

 why certain fungi and bacteria are capable of growing in practically pure 

 culture even in a non-sterile environment. Some workers regard the toxic 

 or growth-inhibiting substances as "protective metabolic products pro- 

 duced by microorganisms in their struggle for existence." 



A classical example of a toxic or antagonistic substance is penicillin 

 which is produced by a soil fungus, Penicillium notatmn. Penicillin in ex- 

 tremely dilute concentrations inhibits the growth of bacteria. The pro- 

 duction by actinomycetes of toxic substances which limit the development 

 of soil bacteria has been reported. A strain of Pseudomonas fluorescens 

 isolated from water by Lewis (1929) produced in nutrient media a toxin 



